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Fast Retrospectively Gated Quantitative Four-Dimensional (4D) Cardiac Micro Computed Tomography Imaging of Free-Breathing Mice
Abstract
We sought to demonstrate retrospectively gated dynamic 3D cardiac micro computed tomography (CT) of free-breathing mice.
Five C57Bl6 mice were scanned using a cone-beam scanner with a slip-ring-mounted flat-panel detector. After the injection of an intravascular iodinated contrast agent, projection images were acquired over the course of 50 seconds, while the scanner rotated through 10 complete rotations. The mouse respiratory and electrocardiogram signals were recorder simultaneously with image acquisition. After acquisition, the projection images were retrospectively sorted into projections belonging to different cardiac time points, occurring only during expiration.
Dynamic 3D cardiac images, with isotropic 150-microm voxel spacing, were reconstructed at 12-millisecond intervals throughout the cardiac cycle in all mice. The average ejection fraction and cardiac output were 58.2+/-4.6% and 11.4+/-1.3 mL/min, respectively. The measured entrance dose for the entire scan was 28 cGy. Repeat scans of the same animals showed that intrasubject variability was smaller than intersubject variability.
We have developed a high-resolution micro computed tomography method for evaluating the cardiac function and morphology of free-breathing mice in acquisition times shorter than 1 minute.
... Some applications, such as studies of skeletal development (Guldberg et al., 2004), may accept lower resolution but require larger FoV, that is, imaging the whole mouse or rat using the larger field of view offered by flat-panel-based systems (Bartling et al., 2007a,b;Drangova et al., 2007;Missbach-Guentner et al., 2007). As an example of bone imaging using micro-CT in Figure 8, we present images from a longitudinal study of bone healing in a model of large lesion in rat femur. ...
... Alternatively, rapid 3D phenotyping of cardiovascular development in mouse embryos has been achieved by ex vivo micro-CT with iodine staining (Degenhardt et al., 2010). In vivo micro-CT-based cardiac morphological and functional imaging in mice and rats has been reported using both PG and RG approaches (Badea et al., 2005(Badea et al., , 2008c; Bartling et al., 2007a,b;Detombe et al., 2008;Drangova et al., 2007;Nahrendorf et al., 2007;Song et al., 2007). Using both approaches, quantitative estimates can be made for ejection fraction, stroke volume, and cardiac output on the basis of left ventricle (LV) volume measurements in the 4D (3D þ time) micro-CT data sets. ...
... Cardiac micro-CT requires the use of contrast agents to be able to discriminate between the myocardium and the blood. In most of the proposed studies, blood pool contrast agents have been used; although with faster scanning available with the flat-panel-based systems (Bartling et al., 2007a,b;Drangova et al., 2007), this may be optional. For example, recently, a method for imaging cardiac perfusion in the mouse has been proposed using a scan protocol that consists of repetitive injections of conventional low-molecular-weight contrast media within several consecutive scans (Sawall et al., 2012). ...
Preclinical imaging has a critical role in phenotyping, in drug discovery, and in providing a basic understanding of mechanisms of disease. Translating imaging methods from humans to small animals is not an easy task. The purpose of this work is to review high-resolution computed tomography (CT) also known as micro-CT for small-animal imaging. We present the principles, the technologies, the image quality parameters, and the types of applications. We show that micro-CT can be used to provide not only morphological but also functional information such as cardiac function or vascular permeability. Another way in which micro-CT can be used in the study of both function and anatomy is by combining it with other imaging modalities, such as positron emission tomography or single-photon emission tomography. Compared to other modalities, micro-CT imaging is usually regarded as being able to provide higher throughput at lower cost and higher resolution. The limitations are usually associated with the relatively poor contrast mechanisms and the radiation damage, although the use of novel nanoparticle-based contrast agents and careful design of studies can address these limitations.
... If the specimen to detector distance and source to specimen distance are similar, as in most rotating gantry systems, penumbra blurring becomes comparable to the focal spot size. Rotating gantry systems operating at moderate magnifications for dynamic, in vivo micro-CT imaging include [30] and [31]. To allow the use of a high fluence X-ray source with a large focal spot, the specimen can be placed close to the detector, limiting spatial resolution, but enabling very short exposures for in vivo cardiorespiratory imaging in rodents. ...
... retrospectively), the projections are sorted into different subsets corresponding to different cardiac and respiratory phases. With this protocol, the scan time can be shortened to 50 s, when using a slip ring gantry [30]. However, the irregular angular distribution causes streaking artifacts in the FBP-based reconstructed images. ...
... Some applications, such as studies of skeletal development [99], may accept lower resolution but require a larger field of view, i.e. imaging the whole mouse or rat using a flat-panel detector [30,100,101]. The study of cancelous bone requires higher resolution, and is often performed ex vivo using excised bones. ...
Micron-scale computed tomography (micro-CT) is an essential tool for phenotyping and for elucidating diseases and their therapies. This work is focused on preclinical micro-CT imaging, reviewing relevant principles, technologies, and applications. Commonly, micro-CT provides high-resolution anatomic information, either on its own or in conjunction with lower-resolution functional imaging modalities such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT). More recently, however, advanced applications of micro-CT produce functional information by translating clinical applications to model systems (e.g. measuring cardiac functional metrics) and by pioneering new ones (e.g. measuring tumor vascular permeability with nanoparticle contrast agents). The primary limitations of micro-CT imaging are the associated radiation dose and relatively poor soft tissue contrast. We review several image reconstruction strategies based on iterative, statistical, and gradient sparsity regularization, demonstrating that high image quality is achievable with low radiation dose given ever more powerful computational resources. We also review two contrast mechanisms under intense development. The first is spectral contrast for quantitative material discrimination in combination with passive or actively targeted nanoparticle contrast agents. The second is phase contrast which measures refraction in biological tissues for improved contrast and potentially reduced radiation dose relative to standard absorption imaging. These technological advancements promise to develop micro-CT into a commonplace, functional and even molecular imaging modality.
... Despite these challenges, multiple preclinical imaging modalities are currently available to measure murine cardiac function. Successful studies have been previously reported for both MR [7][8][9][10][11][12] and microCT [13][14][15][16][17]. Fewer studies in the mouse, however, have explored SPECT as a potential tool for evaluating left ventricular function [18][19][20][21]. ...
... The mean EF and CO values we obtained in both study groups were comparable to what is reported elsewhere [8][9][10][17][18][19]. Our results for end-diastolic volume (EDV) and endsystolic volume (ESV) are similar to those obtained by Dawson et al. [9] in a study comparing 3D echocardiography with MR for evaluation of LV function. ...
... Our study also showed good internal agreement between calculated EDV and ESV values in the two modality groups. There were differences, however, in our calculated EDV and ESV values compared with some prior studies [3,8,14,17] which may have resulted from differences in injected contrast agent volume and anesthetic conditions. In prior experiments, we have observed that injected volumes > 300 μL result in hemodynamic consequences including reduced EF and increased EDV and ESV (unpublished). ...
The objective of this study was to compare a new generation of four-dimensional micro-single photon emission computed tomography (microSPECT) with microCT for the quantitative in vivo assessment of murine cardiac function.
Four-dimensional isotropic cardiac images were acquired from anesthetized normal C57BL/6 mice with either microSPECT (n = 6) or microCT (n = 6). One additional mouse with myocardial infarction (MI) was scanned with both modalities. Prior to imaging, mice were injected with either technetium tetrofosmin for microSPECT or a liposomal blood pool contrast agent for microCT. Segmentation of the left ventricle (LV) was performed using Vitrea (Vital Images) software, to derive global and regional function.
Measures of global LV function between microSPECT and microCT groups were comparable (e.g., ejection fraction = 71 ± 6 % microSPECT and 68 ± 4 % microCT). Regional functional indices (wall motion, wall thickening, regional ejection fraction) were also similar for the two modalities. In the mouse with MI, microSPECT identified a large perfusion defect that was not evident with microCT.
Despite lower spatial resolution, microSPECT was comparable to microCT in the quantitative evaluation of cardiac function. MicroSPECT offers an advantage over microCT in the ability to evaluate simultaneously myocardial radiotracer distribution and function, simultaneously. MicroSPECT should be considered as an alternative to microCT and magnetic resonance for preclinical cardiac imaging in the mouse.
... 4 Retrospective gating is faster and simpler to implement, but it generates projections with an irregular angular distribution, which can result in reconstructed volumes afflicted by streaking artifacts. 5 In both strategies, once the projections are acquired, images of the heart at several phases of the cardiac cycle can be reconstructed separately. However, many of the features in these images are preserved over time, and there has been growing interest in exploiting this redundancy in the reconstruction process to improve image quality and reduce scan time and radiation dose. ...
... 18 However, because the projection acquisition and the physiological processes of the animal are in general uncorrelated, the angular distribution of the projections in each set will be irregular, and this results in streaking artifacts in the images reconstructed with FBP. 5 To overcome this problem, we generate synthetic sets of projections with a regular angular distribution by interpolating the projections in each set, using the following method. ...
... where w(r, σ ) = exp − r σ 2 (5) and σ is the standard deviation of the Gaussian. In bilateral filtration, however, the contribution is also weighted by the difference in the values of the neighboring voxel and the modified voxel: ...
X-ray CT imaging of dynamic physiological processes entails the
reconstruction of volumetric images of objects with x-ray attenuation
properties that vary over time and energy. We show how the same
algebraic model can be used to represent both temporal and spectral
information. This model enables the formulation of algorithms capable of
recovering information in either dimension. These dimensions can also be
combined to develop algorithms that recover both dimensions
simultaneously. We present such an algorithm, describe its
implementation, and test it in simulations.
... Recent advances in micro-CT technologies have enabled in vivo CT imaging of the beating mouse heart using either synchrotron 3 or high-power clinical x-ray tubes. 4, 5 Sera et al. 3 reported high-resolution cardiac micro-CT images at 10 ms temporal resolution and 12 m voxel size using monochromatic synchrotron radiation. It provides exquisite details of the mouse heart, but requires access to remote synchrotron facilities. ...
... By removing the underlying physiological drive to breath, ventilation makes it difficult to obtain true physiological parameters from a pulmonary physiology perspective. Drangova et al. 5 employed a commercial micro-CT scanner ͑Locus Ultra, GE Healthcare, London, Ontario, Canada͒ with a slip-ring gantry and an x-ray tube running at 4 kW ͑80 kV and 50 mA͒ to collect cardiac images of free-breathing mice at 12 ms temporal resolution and 150 m voxel size. The spatial resolution is limited primarily by the blurring from the source penumbra and the fast gantry motion. ...
... The spatial resolution is limited primarily by the blurring from the source penumbra and the fast gantry motion. [5][6][7] In the experiments of Sera 3 and Badea, 4 images were acquired with prospective gating, while in Drangova's experiment, 5 retrospective gating was used. The retrospective gating technique can image the entire cardiac cycle in a single scan, but it unavoidably requires redundant projections/exposures 5 to minimize the missing view artifact for any phase in the cardiac cycle. ...
HTML Purpose: The modern day x-ray tube has undergone little change since its invention in the early 1900's. It has been recently demonstrated that carbon nanotubes may serve as effective field emitters in a cold cathode x-ray source, with specific advantages including precise x-ray exposure control to potentially reduce x-ray exposure and increase system resolution. Furthermore, there is the potential to create micro-focus x-ray sources. Our goal was to develop a micro computed tomography CT system based on this new nanotube based x-ray source for applications in small animal imaging. Methods and Materials: A carbon nanotube based x-ray source was implemented with a triode gating structure, tungsten target and beryllium window. Maximum tube voltage was 40 kV with a current range of 0 to 500 microamps. A Hamamatsu C7921 detector was fixed opposite the x-ray source, with 50 x 50 micron pixel resolution and 1056 by 1056 pixels. Normal C57BL/6 strain, eight-week-old mouse carcasses were imaged. The animal was rotated in this setup with a rotary stage connected to a computer controlled stepper motor. (CT) reconstructions were performed offline with a modified fan-beam algorithm on 480 x-ray transmission images at 36kVp and 100 microampere current. Results: CT images were reconstructed for a 5 x 5 x 5 cm FOV. Images demonstrated good reproduction of fine bony structure such as the skull, ribs and forepaw digits. Focal spot size of the x-ray source for the acquisition was estimated to be at 200 microns by 1000 microns. Further refinements in the x-ray focal spot size are ongoing. Conclusion: Cold cathode carbon nanotube based x-ray sources offer clear advantages over the traditional tungsten filaments, including precise control of the x-ray source for high-resolution imaging and no need for a mechanical shutter to reduce x-ray dose. Questions about this event email: yueh@alum.mit.edu
... 4 Retrospective gating is faster and simpler to implement, but it generates projections with an irregular angular distribution, which can result in reconstructed volumes afflicted by streaking artifacts. 5 In both strategies, once the projections are acquired, images of the heart at several phases of the cardiac cycle can be reconstructed separately. However, many of the features in these images are preserved over time, and there has been growing interest in exploiting this redundancy in the reconstruction process to improve image quality and reduce scan time and radiation dose. ...
... 18 However, because the projection acquisition and the physiological processes of the animal are in general uncorrelated, the angular distribution of the projections in each set will be irregular, and this results in streaking artifacts in the images reconstructed with FBP. 5 To overcome this problem, we generate synthetic sets of projections with a regular angular distribution by interpolating the projections in each set, using the following method. ...
... where w(r, σ ) = exp − r σ 2 (5) and σ is the standard deviation of the Gaussian. In bilateral filtration, however, the contribution is also weighted by the difference in the values of the neighboring voxel and the modified voxel: ...
Purpose
Micro‐CT is widely used for small animal imaging in preclinical studies of cardiopulmonary disease, but further development is needed to improve spatial resolution, temporal resolution, and material contrast. We present a technique for visualizing the changing distribution of iodine in the cardiac cycle with dual source micro‐CT.
Methods
The approach entails a retrospectively gated dual energy scan with optimized filters and voltages, and a series of computational operations to reconstruct the data. Projection interpolation and five‐dimensional bilateral filtration (three spatial dimensions + time + energy) are used to reduce noise and artifacts associated with retrospective gating. We reconstruct separate volumes corresponding to different cardiac phases and apply a linear transformation to decompose these volumes into components representing concentrations of water and iodine. Since the resulting material images are still compromised by noise, we improve their quality in an iterative process that minimizes the discrepancy between the original acquired projections and the projections predicted by the reconstructed volumes. The values in the voxels of each of the reconstructed volumes represent the coefficients of linear combinations of basis functions over time and energy. We have implemented the reconstruction algorithm on a graphics processing unit (GPU) with CUDA. We tested the utility of the technique in simulations and applied the technique in anin vivo scan of a C57BL/6 mouse injected with blood pool contrast agent at a dose of 0.01 ml/g body weight. Postreconstruction, at each cardiac phase in the iodine images, we segmented the left ventricle and computed its volume. Using the maximum and minimum volumes in the left ventricle, we calculated the stroke volume, the ejection fraction, and the cardiac output.
Results
Our proposed method produces five‐dimensional volumetric images that distinguish different materials at different points in time, and can be used to segment regions containing iodinated blood and compute measures of cardiac function.
Conclusions
We believe this combined spectral and temporal imaging technique will be useful for future studies of cardiopulmonary disease in small animals.
... The images were reoriented to align the long axis of the left ventricle and the aorta in the same plane. As in the studies by Detombe et al (Drangova et al 2007, Detombe et al 2008, we used the left ventricle (LV) to determine the different phases in the cardiac cycle. Using a region growing algorithm, the chamber of the left ventricle was isolated and its volume calculated. ...
... Measurements of the left ventricle systolic and diastolic volumes, along with the ejection fraction and stroke volumes, that we obtained with MVivo Au and our cardiac-gating protocol are similar to those obtained by Detombe et al (Drangova et al 2007, Detombe et al 2008, which suggests the measurement accuracy is similar for our imaging protocol. No significant difference between the nanoparticle and the iodine-based contrast agents can be noticed in the measured values. ...
Although micro-computed tomography (micro-CT) images have high contrast for bone or air, between soft tissues the contrast is typically low. To overcome this inherent issue, attenuating exogenous contrast agents are used to provide contrast enhancement in the vasculature and abdominal organs. The aim of this study is to measure the contrast enhancement time course for a gold nanoparticle blood-pool contrast agent and use it to perform cardiac-gated 4D micro-CT scans of the heart. Six healthy female C57BL/6 mice were anesthetized and imaged after receiving an injected dose of MVivo gold nanoparticle blood-pool contrast agent. Following the injection, we performed micro-CT scans at 0, 0.25, 0.5, 0.75, 1, 2, 4, 8, 24, 48 and 72 h. The mean CT number was measured for 7 different organs. No contrast enhancement was noticed in the bladder, kidneys or muscle during the time-course study. However, it clearly appears that the contrast enhancement is high in both right ventricle and vena cava. To perform cardiac-gated imaging, either the gold nanoparticle agent (n = 3) or an iodine-based (n = 3) contrast agent was introduced and images representing 9 phases of the cardiac cycle were obtained in 6 additional mice. A few typical cardiac parameters were measured or calculated, with similar accuracy between the gold and iodinated agents, but better visualization of structures with the gold agent. The MVivo Au contrast agent can be used for investigations of cardiac or vascular disease with a single bolus injection, with an optimal cardiac imaging window identified during the first hour after injection, demonstrating similar image quality to iodinated contrast agents and excellent measurement accuracy. Furthermore, the long-lasting contrast enhancement of up to 8 h can be very useful for scanning protocols that require longer acquisition times.
... RG with our system is performed by rotating the subject slowly while projections are acquired (Badea et al 2011a). Other groups have implemented RG on a slip-ring, flat-panel-based micro-CT system for cardiac studies in mice (Drangova et al 2007, Bartling et al 2007. Unfortunately, RG produces an irregular angular distribution of projections per phase; thus, when using Feldkamp's reconstruction algorithm, streaking artifacts affect the image quality of the tomographic reconstructions. ...
... Simulating undersampled acquisitions by using only 500 and 250 projections further reduced dose to half and a quarter of this value. The total dose achieved in this work is lower than many published values of dose for cardiorespiratory micro-CT scans; common doses for these scans range from 280 to 500 mGy (Drangova et al 2007, Graham et al 2008, Armitage et al 2012, Maier et al 2014, Brehm et al 2015. The total dose associated with fully sampled fast PG and uncorrelated RG, 170 mGy, represents between 38 to 52 times less than the lethal dose of 6.5-9 Gy, known as LD50/30, for C57BL/6 mice (Williams et al 2010). ...
Micro-CT is widely used in preclinical studies, generating substantial interest in extending its capabilities in functional imaging applications such as blood perfusion and cardiac function. However, imaging cardiac structure and function in mice is challenging due to their small size and rapid heart rate. To overcome these challenges, we propose and compare improvements on two strategies for cardiac gating in dual-source, preclinical micro-CT: fast prospective gating (PG) and uncorrelated retrospective gating (RG). These sampling strategies combined with a sophisticated iterative image reconstruction algorithm provide faster acquisitions and high image quality in low-dose 4D (i.e. 3D + Time) cardiac micro-CT. Fast PG is performed under continuous subject rotation which results in interleaved projection angles between cardiac phases. Thus, fast PG provides a well-sampled temporal average image for use as a prior in iterative reconstruction. Uncorrelated RG incorporates random delays during sampling to prevent correlations between heart rate and sampling rate We have performed both simulations and animal studies to validate these new sampling protocols. Sampling times for 1000 projections using fast PG and RG were 2 and 3 minutes, respectively, and the total dose was 170 mGy each. Reconstructions were performed using a 4D iterative reconstruction technique based on the split Bregman method. To examine undersampling robustness, subsets of 500 and 250 projections were also used for reconstruction. Both sampling strategies in conjunction with our iterative reconstruction method are capable of resolving cardiac phases and provide high image quality. In general, for equal numbers of projections, fast PG shows less errors than RG and is more robust to undersampling. Our results indicate that only 1000-projection based reconstruction with fast PG satisfies a 5% error criterion in left ventricular (LV) volume estimation. These methods promise low-dose imaging with a wide range of preclinical applications in cardiac imaging.
... Even so, in 2005 we successfully performed cardio-respiratory gated 4D micro-CT imaging of the mouse heart (Badea et al. 2005). Since then, several other groups have demonstrated 4D cardiac micro-CT with faster scanning times and lower dose, generally at the expense of spatial resolution (Drangova et al. 2007; Bartling et al. 2007). In spite of these promising results, MRI is considered the gold standard for small animal cardiac imaging (Brau et al. 2004;Epstein 2007). ...
... In spite of these promising results, MRI is considered the gold standard for small animal cardiac imaging (Brau et al. 2004;Epstein 2007). Our group has acquired some of the highest-resolution murine cardiac MR images (Bucholz et al. 2008(Bucholz et al. , 2010; however, acquisition times for 4D micro-CT can be much faster (micro-CT: as little as 50 seconds, Drangova et al. 2007; MR: 31 minutes, Bucholz et al. 2008) when isotropic spatial resolution is required, making CT an ideal alternative for high-throughput phenotyping applications. ...
Micro-CT can play an important role in preclinical studies of cardiovascular disease because of its high spatial and temporal resolution. Quantitative analysis of 4D cardiac images requires segmentation of the cardiac chambers at each time point, an extremely time consuming process if done manually. To improve throughput this study proposes a pipeline for registration-based segmentation and functional analysis of 4D cardiac micro-CT data in the mouse. Following optimization and validation using simulations, the pipeline was applied to in vivo cardiac micro-CT data corresponding to ten cardiac phases acquired in C57BL/6 mice (n = 5). After edge-preserving smoothing with a novel adaptation of 4D bilateral filtration, one phase within each cardiac sequence was manually segmented. Deformable registration was used to propagate these labels to all other cardiac phases for segmentation. The volumes of each cardiac chamber were calculated and used to derive stroke volume, ejection fraction, cardiac output, and cardiac index. Dice coefficients and volume accuracies were used to compare manual segmentations of two additional phases with their corresponding propagated labels. Both measures were, on average, >0.90 for the left ventricle and >0.80 for the myocardium, the right ventricle, and the right atrium, consistent with trends in inter- and intra-segmenter variability. Segmentation of the left atrium was less reliable. On average, the functional metrics of interest were underestimated by 6.76% or more due to systematic label propagation errors around atrioventricular valves; however, execution of the pipeline was 80% faster than performing analogous manual segmentation of each phase.
... It have been shown previously that the motion induced artifacts can be minimized by techniques including physically restricting the motion [13] and retrospective gating. In retrospective gating projections from different phases of the respiratory cycles are taken but only the images belonging to a retrospectively selected phase are reconstructed, which requires a significant increase of the imaging dose [44][45][46]. Prospective gating techniques where the image acquisitions are synchronized with the physiological signals have also been investigated. In most of these studies, animals are intubated and ventilator controlled [10,47,48]. ...
... The first scanner is able to perform micro-CT of mouse heart with prospective gating and collect images at 150m spatial resolution and 10msec temporal resolution. The second scanner is able to perform micro-CT imaging of mouse heart with retrospective gating and collect images at 300m spatial resolution and 12msec temporal resolution [46]. The first scanner produces better image quality and has higher dose efficiency, primarily because of the prospective gating approach. ...
Introduction Carbon Nanotube Field Emission X-Ray Dynamic Micro-CT Based on the CNT X-Ray Technology Conclusion and Future Direction Acknowledgment References
... Retrospectively gated micro-CT. This method has been previously described and used to successfully evaluate cardiac function in both healthy and MI mice (6,7). Images were acquired using a volumetric cone-beam micro-CT scanner (Locus Ultra, General Electric Healthcare, London, ON, Canada) equipped with a flat-panel detector mounted on a slip-ring gantry, enabling continuous acquisition of X-ray projections. ...
... To calculate LV volume, a threshold level that separated the myocardium from the blood pool was identified automatically (18); this was done separately for each scan, since contrast level in the blood changed over the course of the study. As described in prior studies (6,7), automatic segmentation of the LV chamber from the myocardium was achieved using a region-growing approach. Chamber volume was calculated by multiplying the number of voxels defining the LV chamber [ranging between 7,500 (ESV) and 16,000 (EDV) in a healthy mouse] and the voxel volume (3.4 nl). ...
Conductance catheters (CC) represent an established method of determining cardiac function in mice; however, the potentially detrimental effects a catheter may have on the mouse heart have never been evaluated. The present study takes advantage of rapid three-dimensional (3D) microcomputed tomography (CT) to compare simultaneously acquired micro-CT and CC measurements of left ventricular (LV) volumes in healthy and infarcted mice and to determine changes in LV volume and function associated with CC insertion. LV volumes were measured in C57BL/6 mice (10 healthy, 10 infarcted, 2% isoflurane anesthesia) using a 1.4-Fr Millar CC. 3D micro-CT images of each mouse were acquired before CC insertion as well as during catheterization. Each CT scan produced high-resolution images throughout the entire cardiac cycle in <1 min, enabling accurate volume measurements as well as direct visualization of the CC within the LV. Bland-Altman analysis demonstrated that CC measurements underestimate volume compared with CT measurements in both healthy [bias of -18.4 and -28.9 μl for end-systolic (ESV) and end-diastolic volume (EDV), respectively] and infarcted mice (ESV = -51.6 μl and EDV = -71.7 μl); underestimation was attributed to the off-center placement of the catheter. Individual evaluation of each heart revealed LV dilation following CC insertion in 40% of mice in each group. No change in ejection fraction was observed, suggesting the enlargement was caused by volume overload associated with disruption of the papillary muscles or chords. The enlargement witnessed was not significant; however, the results suggest the potential for CC insertion to detrimentally affect mouse myocardium, necessitating further investigation.
... 48 The FDK algorithm has also been employed in animal studies for respiratory and cardiac gated 4D-CBCT reconstruction. 49,50 While this method accurately locates high-contrast structures throughout the breathing cycle, it may suffer from streak artifacts and low image quality unless the scanning process is slowed down with more cone-beam projections acquired in each phase. ...
Cone‐beam CT (CBCT) is the most commonly used onboard imaging technique for target localization in radiation therapy. Conventional 3D CBCT acquires x‐ray cone‐beam projections at multiple angles around the patient to reconstruct 3D images of the patient in the treatment room. However, despite its wide usage, 3D CBCT is limited in imaging disease sites affected by respiratory motions or other dynamic changes within the body, as it lacks time‐resolved information. To overcome this limitation, 4D‐CBCT was developed to incorporate a time dimension in the imaging to account for the patient's motion during the acquisitions. For example, respiration‐correlated 4D‐CBCT divides the breathing cycles into different phase bins and reconstructs 3D images for each phase bin, ultimately generating a complete set of 4D images. 4D‐CBCT is valuable for localizing tumors in the thoracic and abdominal regions where the localization accuracy is affected by respiratory motions. This is especially important for hypofractionated stereotactic body radiation therapy (SBRT), which delivers much higher fractional doses in fewer fractions than conventional fractionated treatments. Nonetheless, 4D‐CBCT does face certain limitations, including long scanning times, high imaging doses, and compromised image quality due to the necessity of acquiring sufficient x‐ray projections for each respiratory phase. In order to address these challenges, numerous methods have been developed to achieve fast, low‐dose, and high‐quality 4D‐CBCT. This paper aims to review the technical developments surrounding 4D‐CBCT comprehensively. It will explore conventional algorithms and recent deep learning‐based approaches, delving into their capabilities and limitations. Additionally, the paper will discuss the potential clinical applications of 4D‐CBCT and outline a future roadmap, highlighting areas for further research and development. Through this exploration, the readers will better understand 4D‐CBCT's capabilities and potential to enhance radiation therapy.
... The advances in new and existing imaging modalities for small animals allow more accurate, high resolution, 3D, longitudinal imaging of the cardiovascular system and provides rapid translation of new knowledge to the clinic [19]. Micro-CT imaging is frequently used for characterization of cardiac function and structure in small animals, and current systems now provide cardio-respiratory gating, to minimize movement interference and discriminate between systolic and diastolic phase [20,21]. The present study shows that micro-CT measurements of LV mass corresponded well with post-mortem assessment of heart weight. ...
Purpose
In this study, we explored the role of apoptosis as a potential biomarker for cardiac failure using functional micro-CT and fluorescence molecular tomography (FMT) imaging techniques in Ercc1 mutant mice. Ercc1 is involved in multiple DNA repair pathways, and its mutations contribute to accelerated aging phenotypes in both humans and mice, due to the accumulation of DNA lesions that impair vital DNA functions. We previously found that systemic mutations and cardiomyocyte-restricted deletion of Ercc1 in mice results in left ventricular (LV) dysfunction at older age.
Procedures and Results
Here we report that combined functional micro-CT and FMT imaging allowed us to detect apoptosis in systemic Ercc1 mutant mice prior to the development of overt LV dysfunction, suggesting its potential as an early indicator and contributing factor of cardiac impairment. The detection of apoptosis in vivo was feasible as early as 12 weeks of age, even when global LV function appeared normal, underscoring the potential of apoptosis as an early predictor of LV dysfunction, which subsequently manifested at 24 weeks.
Conclusions
This study highlights the utility of combined functional micro-CT and FMT imaging in assessing cardiac function and detecting apoptosis, providing valuable insights into the potential of apoptosis as an early biomarker for cardiac failure.
... However, the 4D cardiac CT is challenging in mice due to their rapid heart rates (200-800 beats per minute [31]) and small heart size (∼5 mm diameter), thus requiring both high temporal and high spatial resolution [27]. To achieve high temporal resolution and reduce the cardiac motion artifacts, the prospective gating schemes [4,18,23] with ECG gating and the retrospective gating schemes [6,17,22,38] are usually used in 4D micro-CT. Although the retrospective gating scheme is much faster than the classical prospective gating scheme, it leads to non-uniform distribution of projection data, therefore reconstructed images are affected by streaking artifacts when analytical reconstruction algorithms are used [5]. ...
BACKGROUND: Micro-computed tomography is important in cardiac imaging for preclinical small animal models, but motion artifacts may appear due to the rapid heart rates. To avoid the influence of motion artifacts, the prospective ECG gating schemes based on an X-ray source trigger have been published for cardiac imaging. However, due to the lack of pulsed X-ray exposure modes, high-resolution micro-focus X-ray sources do not support source triggering in most cases.
OBJECTIVE: To develop a fast-cardiac multiphase acquisition strategy using prospective ECG gating for micro-focus X-ray tubes with a continuous emission mode.
METHODS: The proposed detector-trigger-based prospective ECG gating acquisition scheme (DTB-PG) triggers the X-ray detector at the R peak of ECG, and then collects multiple phase projections of the heart in one ECG cycle by sequence acquisition. Cardiac multiphase images are reconstructed after performing the same acquisition in all views. The feasibility of this strategy was verified in multiphase imaging experiments of a phantom with 150 ms motion period and a mouse heart on a micro-focus micro-CT system with continuous emission mode.
RESULTS: Using a high frame-rate CMOS detector, DTB-PG discriminated the positions of the motion phantom well in 10 different phases, and was also able to distinguish the changes in the cardiac volume of the mouse in different phases. The acquisition rate of DTB-PG is much faster than other prospective gating schemes, as demonstrated by theoretical analysis.
CONCLUSIONS: DTB-PG combines the advantages of prospective ECG gating strategies and the X-ray detector-trigger mode to suppress the motion artifacts, achieve ultra-fast acquisition rates, and relax hardware limitations.
... Using these signals postsampling (i.e., retrospectively), the projections are sorted into different subsets corresponding to different cardiac and respiratory phases. With this protocol, the scan time can be shortened to w50 s [59]. However, the irregular-angular distribution causes streaking artifacts in the FBP-based reconstructed images (see Fig. 4.2). ...
Micro X-ray computed tomography, also known as micro-CT, is nowadays a commonly used, cost-effective, and noninvasive three-dimensional imaging modality applied in preclinical research. Micro-CT provides high-resolution anatomic information, and it is used either on its own or in conjunction with lower-resolution functional imaging modalities such as positron emission tomography and single photon emission computed tomography. In this chapter, we review the basic physics of micro-CT systems and their components, image reconstruction algorithms, basic protocols, and some of the commonly used applications for this technology. New developments in spectral micro-CT imaging will further improve the usefulness of micro-CT in acquiring functional and molecular information. In the long term, preclinical micro-CT systems could serve for testing nanoparticles that show promise in the field of theragnostics (therapy and diagnostics).
... The radiation dose applied to the animals during CT imaging is often seen as a limitation, especially for serial examinations. Previously, cardiac-gated CTA studies mainly investigated on pre-clinical flat-panel or micro-CT scanners reported a dose of ~90 mGy (32)(33)(34)(35)(36). Using a clinical-grade 256-slice CT scanner with retrospective ECG-gating, we achieved a mean dose of ~25 mGy, which cannot be expected to significantly affect the health of animals, even if applied serially. ...
Background:
Non-invasive computer tomography (CT)- and magnetic resonance (MR)-based cardiac imaging still remains challenging in rodents. To investigate the robustness of non-invasive multimodality cardiac imaging in rabbits using clinical-grade CT and MR scanners.
Methods:
A total of 16 rabbits (2.7-4.0 kg) serially underwent cardiac-gated imaging using a clinical-grade 256-row CT and a 1.5 Tesla MR-scanner at baseline and at 4-month follow-up (16±1 weeks). Image analysis included image quality (5-grade scale), left ventricular (LV) volumes, LV stroke volume, LV diameters, LV wall thickness and ejection fraction (LVEF).
Results:
Cardiac MR (CMR) and CT angiography (CTA) provide images with an overall good image quality (excellent or good quality: CMR 82% vs. CTA 78%, P=0.68). Linear regression analysis demonstrated a good correlation of all diameters (diam.) and volumes (vol.) as assessed by CTA and CMR (diam.: r=0.9, 95% CI: 0.8-0.9; vol.: r=0.8, 95% CI: 0.6-0.9; P<0.0001 for both). CTA-based volumetric analysis revealed slightly higher LVEF values as compared to CMR (CTA: 64%±1%, CMR: 59%±1%, P=0.002). Analysis of inter-/intra-observer agreement demonstrated excellent agreements for diameters (CMR: 98.5%/98.7%; CTA: 98.2%/97.4%) and volumes (CMR: 99.9%/98.8%; CTA 98.7%/98.7%). Finally, serial CMR- and CTA-based assessment of cardiac diameters and volumes delivered excellent intersession agreements of baseline versus follow-up data (diam.: CMR: r=0.89; CTA: r=0.92; vol.: CMR: r=0.87; CTA: r=0.96, P<0.0001 for all).
Conclusions:
Multimodality non-invasive assessment of cardiac function and aortic hemodynamics is feasible and robust in rabbits using clinical-grade and MR and CT scanners. These imaging modalities could improve serial cardiac assessment for disease monitoring in preclinical settings.
... gating Notes refs Minifocus tubes 30-200 μm Continuous • Temporal resolution limited by the max. frame rate of X-ray detectors 47,66 Medical tubes 300-800 μm >5 ms • • Only with low magnification/ Sub-optimal spectral quality for small animal imaging 48,67 Carbon nanotube field emission X-ray tubes >100 μm >100 μs www.nature.com/scientificreports www.nature.com/scientificreports/ ...
Accurate dynamic three-dimensional (4D) imaging of the heart of small rodents is required for the preclinical study of cardiac biomechanics and their modification under pathological conditions, but technological challenges are met in laboratory practice due to the very small size and high pulse rate of the heart of mice and rats as compared to humans. In 4D X-ray microtomography (4D μCT), the achievable spatio-temporal resolution is hampered by limitations in conventional X-ray sources and detectors. Here, we propose a proof-of-principle 4D μCT platform, exploiting the unique spatial and temporal features of novel concept, all-optical X-ray sources based on Thomson scattering (TS). The main spatial and spectral properties of the photon source are investigated using a TS simulation code. The entire data acquisition workflow has been also simulated, using a novel 4D numerical phantom of a mouse chest with realistic intra- and inter-cycle motion. The image quality of a typical single 3D time frame has been studied using Monte Carlo simulations, taking into account the effects of the typical structure of the TS X-ray beam. Finally, we discuss the perspectives and shortcomings of the proposed platform.
... Fast respiratory gating is critical to deal with artifact from a mouse's respiration to accurately segment the tumor and while it has been available [38][39][40], it is increasingly more available in small animal micro CT software upgrades. Representative micro CT slices of increased tumor progression in mice exposed to CIH compared to Sham are shown in Fig 2A. ...
Background
Epidemiological data suggests that obstructive sleep apnea (OSA) is associated with increased cancer incidence and mortality. We investigate the effects of cyclical intermittent hypoxia (CIH), akin to the underlying pathophysiology of OSA, on lung cancer progression and metastatic profile in a mouse model.
Methods
Intrathoracic injection of Ad5CMVCre virus into a genetically engineered mouse (GEM) KrasG12D+/-; p53fl/fl; myristolated-p110αfl/fl-ROSA-gfp was utilized to induce a solitary lung cancer. Male mice were then exposed to either CIH or Sham for 40–41 days until harvest. To monitor malignant progression, serial micro CT scans with respiratory gating (no contrast) was performed. To detect spontaneous metastases in distant organs, H&E and immunohistochemistry were performed.
Results
Eighty-eight percent of injected Ad5CMVCre virus was recovered from left lung tissue, indicating reliable and accurate injections. Serial micro CT demonstrated that CIH increases primary lung tumor volume progression compared to Sham on days 33 (p = 0.004) and 40 (p<0.001) post-injection. In addition, CIH increases variability in tumor volume on day 19 (p<0.0001), day 26 (p<0.0001), day 33 (p = 0.025) and day 40 (p = 0.004). Finally, metastases are frequently detected in heart, mediastinal lymph nodes, and right lung using H&E and immunohistochemistry.
Conclusions
Using a GEM mouse model of metastatic lung cancer, we report that male mice with solitary lung cancer have accelerated malignant progression and increased variability in tumor growth when exposed to cyclical intermittent hypoxia. Our results indicate that cyclical intermittent hypoxia is a pathogenic factor in non-small cell lung cancer that promotes the more rapid growth of developing tumors.
... Numerous research groups have demonstrated the usefulness of microCT for quantification of cardiac morphology and global ventricular systolic indices, such as stroke volume, ejection fraction (EF), and cardiac output. [6][7][8][9] To our knowledge, however, the use of microCT to assess other key functional parameters of systolic emptying or diastolic filling has not been previously reported. The assessment of diastolic function is of paramount importance because systolic function is commonly preserved in cardiovascular disease, par-ticularly during its early manifestation, and improvement in diastolic function correlates with prolonged survival. ...
Background:
The use of microscopic computed tomography to assess the key functional parameters of systolic emptying or diastolic filling in small animals has not been previously reported. The aim of the study was to test whether microscopic computed tomography can assess the dynamics of both left ventricle and right ventricle (RV) diastolic filling and systolic emptying in an experimental model of pulmonary arterial hypertension Methods and Results: The Wistar-Kyoto rats were injected subcutaneously with the VEGF (vascular endothelial growth factor)-receptor inhibitor SU5416 (20 mg/kg body weight) and were then exposed to chronic hypoxia (10% oxygen) for 21 days (SU5416-hypoxia) followed by normoxia for an additional 2 weeks. Thereafter, multiphase cine cardiac images were acquired using a microscopic computed tomography scanner in conjunction with a blood-pool iodinated contrast agent. Examination of the 3-dimensional images of SU5416-hypoxia rats confirmed the presence of severe pulmonary arterial hypertension. Functional parameters that describe the dynamics of ventricular systolic ejection and diastolic filling were calculated. RV peak ejection rate was significantly decreased ( P<0.03) in SU5416-hypoxia rats compared with controls. RV peak filling rate had a significant decrease compared with controls ( P<0.03), particularly in the early phase of diastole ( P<0.03). This was accompanied by increased time to peak filling rate ( P<0.03) and total filling time ( P<0.06). Spearman analysis between microscopic computed tomography RV diastolic indices and invasively derived RV end-diastolic pressure indicated excellent correlation.
Conclusions:
We developed a method that allows rapid and accurate assessment of cardiac functional indices and that paves the way for more extensive preclinical cardiovascular research.
... Image reconstruction techniques for the collection of 4D micro-CT data, where the fourth dimension is time, have been applied to other areas of science, e.g. clinical X-ray CT imaging (Drangova et al., 2007;Song et al., 2007;Badea et al., 2004Badea et al., , 2011. Nevertheless, fluid flow through porous media is a unique problem. ...
... , we consider a second form of functional CT imaging, temporal CT. There are several forms of temporal CT, including dynamic perfusion imaging, 82, 97, 98 respiratory imaging,89 and cardiac imaging,74,99,100 all of which can be performed in small animals.Furthermore, all of these temporal CT reconstruction problems benefit greatly from sparsity-constrained iterative reconstruction techniques like the ones we have discussed.12,70,74 In this work, we focus on cardiac micro-CT, dealing with respiratory imaging in enough depth to perform reliable cardiac reconstruction in the presence of respiratory motion. ...
X‐ray CT is widely used, both clinically and preclinically, for fast, high resolution, anatomic imaging; however, compelling opportunities exist to expand its use in functional imaging applications. For instance, spectral information combined with nanoparticle contrast agents enables quantification of tissue perfusion levels, while temporal information details cardiac and respiratory dynamics. Common implementations of spectral and temporal (spectro‐temporal) CT discretely sample the time points and energies to be reconstructed, proportionally increasing acquisition time and ionizing radiation dose with data dimensionality. Here, we propose and develop an integrated framework for spectro‐temporal CT data acquisition, reconstruction, and analysis which drastically reduces the sampling time and radiation dose associated with spectro‐temporal CT imaging. Specifically, we exploit the latent, gradient sparse and low rank structure of spectro‐temporal CT data sets to recover their full dimensionality from highly undersampled projection measurements. We achieve reliable, high fidelity results through a novel combination of hierarchical projection sampling, the split Bregman optimization method, and piecewise‐constant kernel regression. The integrated framework generalizes to arbitrary spectral and temporal CT reconstruction problems, while maintaining or even improving upon the sampling time and radiation dose associated with anatomic imaging protocols. We believe that this integrated framework will serve as the basis for a new generation of routine, functional CT imaging protocols.
... However, imaging the mouse thorax can be problematic, given motion blur resulting from the rapid heart and respiratory rates. Continuous imaging while recording the respiratory and cardiac cycle allows for retrospective image selection, 24 but involves a high radiation dose, often representing a substantial fraction of the lethal dose for a small animal. As such, the ability to perform longitudinal scanning is limited. ...
A new imaging technology has emerged that uses carbon nanotubes ( CNT ) as the electron emitter (cathode) for the X‐ray tube. Since the performance of the CNT cathode is controlled by simple voltage manipulation, CNT ‐enabled X‐ray sources are ideal for the repetitive imaging steps needed to capture three‐dimensional information. As such, they have allowed the development of a gated micro‐computed tomography ( CT ) scanner for small animal research as well as stationary tomosynthesis, an experimental technology for large field‐of‐view human imaging. The small animal CT can acquire images at specific points in the respiratory and cardiac cycles. Longitudinal imaging therefore becomes possible and has been applied to many research questions, ranging from tumor response to the noninvasive assessment of cardiac output. Digital tomosynthesis ( DT ) is a low‐dose and low‐cost human imaging tool that captures some depth information. Known as three‐dimensional mammography, DT is now used clinically for breast imaging. However, the resolution of currently‐approved DT is limited by the need to swing the X‐ray source through space to collect a series of projection views. An array of fixed and distributed CNT ‐enabled sources provides the solution and has been used to construct stationary DT devices for breast, lung, and dental imaging. To date, over 100 patients have been imaged on Institutional Review Board‐approved study protocols. Early experience is promising, showing an excellent conspicuity of soft‐tissue features, while also highlighting technical and post‐acquisition processing limitations that are guiding continued research and development. Additionally, CNT ‐enabled sources are being tested in miniature X‐ray tubes that are capable of generating adequate photon energies and tube currents for clinical imaging. Although there are many potential applications for these small field‐of‐view devices, initial experience has been with an X‐ray source that can be inserted into the mouth for dental imaging. Conceived less than 20 years ago, CNT ‐enabled X‐ray sources are now being manufactured on a commercial scale and are powering both research tools and experimental human imaging devices. WIREs Nanomed Nanobiotechnol 2018, 10:e1475. doi: 10.1002/wnan.1475
This article is categorized under: Diagnostic Tools > Diagnostic Nanodevices
Diagnostic Tools > In Vivo Nanodiagnostics and Imaging
... Over the past several years MicroCT has become the modality many researches considered for characterization of cardiac structure and function in small animals [26][27][28][29]30 . However, the instrumentation used in the prior work was either custom built or no longer commercially available. ...
The use of Micro-Computed Tomography (MicroCT) for in vivo studies of small animals as models of human disease has risen tremendously due to the fact that MicroCT provides quantitative high-resolution three-dimensional (3D) anatomical data non-destructively and longitudinally. Most importantly, with the development of a novel preclinical iodinated contrast agent called eXIA160, functional and metabolic assessment of the heart became possible. However, prior to the advent of commercial MicroCT scanners equipped with X-ray flat-panel detector technology and easy-to-use cardio-respiratory gating, preclinical studies of cardiovascular disease (CVD) in small animals required a MicroCT technologist with advanced skills, and thus were impractical for widespread implementation. The goal of this work is to provide a practical guide to the use of the high-speed Quantum FX MicroCT system for comprehensive determination of myocardial global and regional function along with assessment of myocardial perfusion, metabolism and viability in healthy mice and in a cardiac ischemia mouse model induced by permanent occlusion of the left anterior descending coronary artery (LAD).
... One of the advantages of the new generation of FPDs for micro-CT application is certainly the increased response stability and better data-transfer performance, allowing to acquire tomographic images of full-size mice in very short times; several commercial micro-CTs, such as Quantum GX (PerkinElmer, USA), IRIS (Inviscan Sas, Strasbourg, France) and Tomoscope (CT Imaging GmbH, Erlangen, Germany), allow whole body imaging in less than 10 s. Moreover, high frame rates (420 fps) allow image acquisition modalities that were previously impossible, such as retrospectively ECG-gated 3D cardiac imaging [80]. ...
... In cardiac clinical CT, for instance, radiographic acquisition is gated by the electrical activity of the heart as an external indicator of movement phase 11 . The same technique has been extended to image cardiac and respiratory movements in mice, achieving 12 ms temporal resolution and 150 mm voxel spacing in a clinical CT scanner 12 , and 17 ms temporal resolution and 20 mm voxel spacing at a synchrotron facility 13 . Impressive as these results are, each prospective or retrospective gating technique published so far relies on the acquisition of an external signal that is then used for gating. ...
Visualizing fast micrometer scale internal movements of small animals is a key challenge for functional anatomy, physiology and biomechanics. We combine phase contrast tomographic microscopy (down to 3.3 μm voxel size) with retrospective, projection-based gating (in the order of hundreds of microseconds) to improve the spatiotemporal resolution by an order of magnitude over previous studies. We demonstrate our method by visualizing 20 three-dimensional snapshots through the 150 Hz oscillations of the blowfly flight motor.
... Some of them have been reported to be useful for structural and functional analysis [12] [13]. Moreover, recent report presented a more accurate imaging of functional changes using four dimensional (4-D) imaging of CT [14] [15]. In conclusion, micro-CT imaging is useful for accurate and detailed analysis of the whole heart function in both normal rats and rats with myocardial infarction. ...
Objective:
High resolution image is required for accurate measurement of cardiac function for the analysis of detailed regional function especially in a small animal.
Methods:
Left ventricular function of rat hearts was measured using micro-computed tomography (micro-CT) with administration of contrast agent in a rat with normal heart as well as rats with mild and severe myocardial infarction (MI). Following the CT acquisition, the hearts were sectioned for pathological evaluation.
Results:
The volume plot per each disk of the normal heart revealed that contraction force
peaked at the middle of the heart. In the heart with mild infarction, the volume plot curve clearly demonstrated that infarction was located only at the apex of the heart, whereas severe infarction was disturbed in larger area. The left ventricular ejection fraction of the normal, mild MI, and severe MI hearts were 68.6%, 40.0%, and 16.4%, respectively. In addition, volume analysis in severe MI demonstrated ventricular dilatation, although that in mild MI did not show any change in the ventricular volume. Histological results were consistent with the CT measurement.
Conclusions:
Micro-CT provided accurate measurement of cardiac function in rats, which is especially useful for the analysis of small animals with heterogeneous dysfunction of the heart.
... The mouse heart is nearly 3000 times smaller than a human's and has an R-R interval one-tenth as long, which demands high spatial and temporal resolution for effective imaging. Despite these challenges, several groups have demonstrated successful cardio-respiratory gated 4D micro-CT imaging of the mouse heart (1)(2)(3). In addition to high-resolution scanning, cardiac micro-CT imaging requires contrast agents that remain in the bloodpool. ...
Noninvasive small animal imaging techniques are essential for evaluation of cardiac disease and potential therapeutics. A novel preclinical iodinated contrast agent called eXIA 160 has recently been developed, which has been evaluated for micro-CT cardiac imaging. eXIA 160 creates strong contrast between blood and tissue immediately after its injection and is subsequently taken up by the myocardium and other metabolically active tissues over time. We focus on these properties of eXIA and show its use in imaging myocardial infarction in mice. Five C57BL/6 mice were imaged ~2 weeks after left anterior descending coronary artery ligation. Six C57BL/6 mice were used as controls. Immediately after injection of eXIA 160, an enhancement difference between blood and myocardium of ~340 HU enabled cardiac function estimation via 4D micro-CT scanning with retrospective gating. Four hours post-injection, the healthy perfused myocardium had a contrast difference of ~140 HU relative to blood while the infarcted myocardium showed no enhancement. These differences allowed quantification of infarct size via dual-energy micro-CT. In vivo micro-SPECT imaging and ex vivo triphenyl tetrazolium chloride (TTC) staining provided validation for the micro-CT findings. Root mean squared error of infarct measurements was 2.7% between micro-CT and SPECT, and 4.7% between micro-CT and TTC. Thus, micro-CT with eXIA 160 can be used to provide both morphological and functional data for preclinical studies evaluating myocardial infarction and potential therapies. Further studies are warranted to study the potential use of eXIA 160 as a CT molecular imaging tool for other metabolically active tissues in the mouse. Copyright © 2014 John Wiley & Sons, Ltd.
... Robust respiratory and cardiac signals are required for lung [12] and heart [13,14] imaging in rodents. The monitoring signals provided by the PRA receiver were adequate for animal monitoring and gating. ...
Advances in laboratory imaging systems for CT, SPECT, MRI, and PET facilitate routine micro-imaging during pre-clinical investigations. Challenges still arise when dealing with immune-compromised animals, biohazardous agents, and multi-modality imaging. These challenges can be overcome with an appropriate animal management system (AMS), with the capability for supporting and monitoring a rat or mouse during micro-imaging. We report the implementation and assessment of a new AMS system for mice (PRA-3000 / AHS-2750, ASI Instruments, Warren MI), designed to be compatible with a commercial micro-CT / micro-SPECT imaging system (eXplore speCZT, GE Healthcare, London ON). The AMS was assessed under the following criteria: 1) compatibility with the imaging system (i.e. artifact generation, geometric dimensions); 2) compatibility with live animals (i.e. positioning, temperature regulation, anesthetic supply); 3) monitoring capabilities (i.e. rectal temperature, respiratory and cardiac monitoring); 4) stability of co-registration; and 5) containment. Micro-CT scans performed using a standardized live-animal protocol (90 kVp, 40 mA, 900 views, 16 ms per view) exhibited low noise (+/-19 HU) and acceptable artifact from high-density components within the AMS (e.g. ECG pad contacts). Live mice were imaged repeatedly (with removal and replacement of the AMS) and spatial registration was found to be stable to within +/-0.07 mm. All animals tolerated enclosure within the AMS for extended periods (i.e. > one hour) without distress, based on continuous recordings of rectal temperature, ECG waveform and respiratory rate. A sealed AMS system extends the capability of a conventional micro-imaging system to include immune-compromised and biosafety level 2 mouse-imaging protocols.
... Dynamic imaging with micro-CT, such as in cardiopulmonary and perfusion applications[1], often requires the use of scanning protocols that challenge the constraints of traditional reconstruction algorithms based on filtered backprojection (FBP)[2], and violation of these constraints can result in artifacts in the reconstructed data[3]. Iterative algorithms, in which the reconstructed volume is refined by repeatedly comparing simulated reprojections from the reconstructed volume with the original projections, present an alternative to FBP with the potential for overcoming its limitations[4]. ...
Dynamic imaging with micro-CT often produces poorly-distributed sets of projections, and reconstructions of this data with filtered backprojection algorithms (FBP) may be affected by artifacts. Iterative reconstruction algorithms and total variation (TV) denoising are promising alternatives to FBP, but may require running times that are frustratingly long. This obstacle can be overcome by implementing reconstruction algorithms on graphics processing units (GPU). This paper presents an implementation of a family of iterative reconstruction algorithms with TV denoising on a GPU, and a series of tests to optimize and compare the ability of different algorithms to reduce artifacts. The mathematical and computational details of the implementation are explored. The performance, measured by the accuracy of the reconstruction versus the running time, is assessed in simulations with a virtual phantom and in an in vivo scan of a mouse. We conclude that the simultaneous algebraic reconstruction technique with TV minimization (SART-TV) is a time-effective reconstruction algorithm for producing reconstructions with fewer artifacts than FBP.
Synchrotron X-rays can be used to obtain highly detailed images of parts of the lung. However, micro-motion artifacts induced by such as cardiac motion impede quantitative visualization of the alveoli in the lungs. This paper proposes a method that applies a neural network for synchrotron X-ray Computed Tomography (CT) data to reconstruct the high-quality 3D structure of alveoli in intact mouse lungs at expiration, without needing ground-truth data. Our approach reconstructs the spatial sequence of CT images by using a deep-image prior with interpolated input latent variables, and in this way significantly enhances the images of alveolar structure compared with the prior art. The approach successfully visualizes 3D alveolar units of intact mouse lungs at expiration and enables us to measure the diameter of the alveoli. We believe that our approach helps to accurately visualize other living organs hampered by micro-motion.
As rubber materials are used for damping, clarifying the relationship between the loss factor and microstructure would help develop high-performance damping materials. Although nondestructive observations using X-ray computed tomography (CT) under repetitive deformation have been reported, no observations have been reported at the submicron order that capture low-strain deformation, such as vibration exposure. The internal deformation behavior of materials with different loss factors has not yet been evaluated. This study proposes a dynamic X-ray CT method for specimens under tensile amplitudes, directly evaluating the internal deformation behavior of materials under dynamic conditions. The proposed 4D-CT has an excitation of 1 Hz and a spatial resolution of 0.5 μm. The local strain was obtained from X-ray CT at each phase, and the deformation behavior was evaluated. The results revealed that the peak of the local strain amplitude distribution curve decreased and the distribution widened as fine particles were mixed.
Due to the availability of commercial laboratory systems and the emergence of user facilities at synchrotron radiation sources, studies of microcomputed tomography or microCT have increased exponentially. MicroComputed Technology provides a complete introduction to the technology, describing how to use it effectively and understand its results. The first part of the book focuses on methodology, covering experimental methods, data analysis, and visualization approaches. The second part addresses various microCT applications, including porous solids, microstructural evolution, soft tissue studies, multimode studies, and indirect analyses. The author presents a sufficient amount of fundamental material so that those new to the field can develop a relative understanding of how to design their own microCT studies. One of the first full-length references dedicated to microCT, this book provides an accessible introduction to field, supplemented with application examples and color images.
Although discovered more than 100 years ago, X-ray source technology has evolved rather slowly. The recent invention of the carbon nanotube (CNT) X-ray source technology holds great promise to revolutionize the field of biomedical X-ray imaging. CNT X-ray sources have been successfully adapted to several biomedical imaging applications including dynamic micro-CT of small animals and stationary breast tomosynthesis of breast cancers. Yet their more important biomedical imaging applications still lie ahead in the future, with the development of stationary multi-source CT as a noteworthy example.
Goal:
We investigate the signal and noise performance of an x-ray microtomography system that incorporates a complementary metal-oxide-semiconductor flat-panel detector as a projection image receptor.
Methods:
Signal and noise performance is analyzed in the Fourier domain using modulationtransfer function (MTF), noise-power spectrum (NPS), and noiseequivalent number of quanta (NEQ) with respect to magnification and different convolution kernels for image reconstruction.
Results:
Higher magnification provides lower NPS, and thus higher NEQ performance in the transaxial planes from microtomography. A window function capable of smoothing the ramp filter edge to below one-half of the Nyquist limit results in better performance in terms of NPS and NEQ. The characteristics of convolution kernels do not affect signal and noise performance in longitudinal planes; hence, MTF performance mainly dominates the NEQ performance. The signal and noise performances investigated in this study are demonstrated with images obtained from the contrast phantom and postmortem mouse.
Conclusion:
The results of our study could be helpful in developing x-ray microtomography systems based on flat-panel detectors.
Conventional X-ray sources face limitations due to their reliance upon thermionic emission for electron generation. A recently developed X-ray source avoids this problem by using carbon nanotubes (CNTs) as a cathode material for field emission of electrons instead of a heated tungsten filament. This CNT X-ray source is built compactly and is capable of high flux and excellent temporal resolution, and it is well suited for a variety of biomedical imaging applications. Here, we discuss the design of a micro-computed tomography system employing a CNT field-emission X-ray tube and its applications for live small-animal imaging in preclinical studies of human illness such as cancer and cardiac disease.
Aim:
To assess the accuracy of ECG-gated micro (µ)-SPECT in a mouse myocardial infarction (MI) model in comparison to 3D-echocardiography. Animals, methods: In a mouse (Swiss mice) MI model we compared the accuracy of technetium-99m sestamibi (99mTc-sestamibi) myocardial perfusion, electrocardiogram (ECG) gated µSPECT to 3D-echocardiography in determining left ventricular function. 3D-echocardiography and myocardial perfusion ECG-gated µSPECT data were acquired in the same animal at baseline (n = 11) and 7 (n = 8) and 35 (n = 9) days post ligation of the left anterior descending coronary artery (LAD). Sham operated mice were used as a control (8, 6 and 7 mice respectively). Additionally, after day 35 µSPECT scans, hearts were harvested and 2,3,5-triphenyl-2H-tetrazolium chloride (TTC) staining and autoradiography was performed to determine infarct size.
Results:
In both infarcted and sham-operated mice we consistently found comparable values for the end-diastolic volume (EDV), end-systolic volume (ESV) and ejection fraction (EF) obtained by 3D-echocardiography and ECG-gated µSPECT. Excellent correlations between measurements from 3D-echocardiography and ECG-gated µSPECT were found for EDV, ESV and EF (r = 0.9532, r = 0.9693 respectively and r = 0.9581) in infarcted mice. Furthermore, comparable infarct size values were found at day 35 post MI by TTC staining and autoradiography (27.71 ± 1.80% and 29.20 ± 1.18% with p = 0.43).
Conclusion:
We have demonstrated that ECG-gated µSPECT imaging provides reliable left ventricular function measurements in a mouse MI model. Obtained results were comparable to the highly accurate 3D-echocardiography. This, in addition to the opportunity to simultaneously image multiple biological processes during a single acquisition makes µSPECT imaging a serious option for studying cardiovascular disease in small animals.
This chapter focuses on the ability of small-animal CT to provide information about molecular species and their spatial distribution in tissues. Over the past several decades radionuclide imaging methods have been the mainstay of in vivo molecular imaging by virtue of the variety of biologically active molecules that can be labeled with a radioactive marker. CT image data has been used to provide both attenuation correction of the SPECT and PET images as well as provide the anatomic localization of the radionuclide accumulation. This important contribution of CT to molecular imaging is presented in those chapters directly addressing the radionuclide imaging approaches. Although, the presence of higher atomic weight atomic labels (e.g., iodine) of biologically active tracer molecules can be conveyed by conventional X-ray attenuation-based imaging methods (in milli-molar concentrations as compared to pico-molar concentrations by radionuclide methods), molecular species can be conveyed by non-attenuating aspects of X-ray interaction with matter by virtue of their molecular bonds that are characteristic of polymeric molecules. These non-attenuating X-ray imaging methods are now starting to emerge from the feasibility demonstrations and hence will be explored in some depth in this chapter. © 2014 Springer Science+Business Media New York. All rights are reserved.
This chapter provides the reader with an introductory look into the significance of preclinical imaging relative to human medicine. The concept of translating preclinical research to the clinical realm is presented with a focus on the primary human disease model, the mouse. Here, the term Small Animal Imaging (SAI) is used to describe preclinical imaging of mice. The fundamental operating principles of the various SAI technologies and primary differences with their clinical counterparts are described. First generation and state-of-the-art instruments are reviewed as well as the utility of combining these technologies into multimodality instruments. Considerations in small animal use, such as anesthesia and radiation dose are followed by a brief look at SAI center design. The application of SAI in the areas of cardiology, neurology, and oncology are reviewed and finally, a perspective on the future of SAI is given. This is not meant to be a comprehensive review, but rather a primer for the biomedical student or researcher to become familiarized with the overall field of preclinical imaging. For further information the reader is referred to external sources of literature.
Purpose:
Cardiac
in vivo micro-CT imaging of small animals typically requires double gating due to long scan times and high respiratory rates. The simultaneous respiratory and cardiac gating can either be done prospectively or retrospectively. In any case, for true 5D imaging, i.e., three spatial dimensions plus one respiratory-temporal dimension plus one cardiac temporal dimension, the amount of information corresponding to a given respiratory and cardiac phase is orders of magnitude lower than the total amount of information acquired. Achieving similar image quality for 5D than for usual 3D investigations would require increasing the amount of data and thus the applied dose to the animal. Therefore, the goal is phase-correlated imaging with high image quality but without increasing the dose level.
The purpose of this study was to measure changes in cardiac function as cardiomyopathy progresses in a mouse model of Duchenne muscular dystrophy using 3-D ECG-gated echocardiography. This study is the first to correlate cardiac volumes acquired using 3-D echocardiography with those acquired using retrospectively gated micro-computed tomography (CT). Both were further compared with standard M-mode echocardiography and histologic analyses. We found that although each modality measures a decrease in cardiac function as disease progresses in mdx/utrn–/– mice (n = 5) compared with healthy C57BL/6 mice (n = 8), 3-D echocardiography has higher agreement with gold-standard measurements acquired by gated micro-CT, with little standard deviation between measurements. M-Mode echocardiography measurements, in comparison, exhibit considerably greater variability and user bias. Given the radiation dose associated with micro-CT and the geometric assumptions made in M-mode echocardiography to calculate ventricular volume, we suggest that use of 3-D echocardiography has important advantages that may allow for the measurement of early disease changes that occur before overt cardiomyopathy.
PURPOSE
The goal of this study is to demonstrate the efficacy of using a novel non-contact sensor-gated carbon nanotube (CNT) x-ray micro-CT system to image mice with congenital diaphragmatic hernias.
METHOD AND MATERIALS
5 mice (4 wild type, one CDH) were imaged during peak inspiration and end-exhalation using a non-contact CNT micro-CT system. Micro-CT imaging was gated via non-contact respiratory sensor on an unconstrained animal. Animals were allowed to breathe freely during the imaging under isofluorane anesthesia. Images were reconstructed using isotropic voxels of 77-μm resolution (50 kVp, 400 projections, 30-ms x-ray pulse). Lung volumes were measured from CT images with region-growing techniques and thresholds derived from the surrounding air and soft tissues. Functional parameters such as tidal volume, FRC and minute volume, were calculated for each mouse.
RESULTS
Typical micro-CT imaging techniques usea physical sensor for gating and physical constraints to hold down the animal. This would be ineffective in a congenital diaphragmatic hernia model (CDH) and mouse pups since they are typically too frail or small (<10g), respectively to activate pressure sensors underneath the abdomen. Physical constraints exert pressure on and physically damage internal organs during imaging. Such interventions can mischaracterize the effects of disease processes like CDH. Using a noncontact sensor in conjunction with a CNT micro-CT in these mice allows for more accurate evaluation of lung volumes and physiology. Mean lung volumes of the control mice at peak inspiration and end-expiration (FRC) were 0.78 ± 0.073 ml and 0.641 ± 0.10 ml, respectively. Average tidal volume was 0.14 ± 0.034 ml. The average minute volume was 13.62 ± 5.04 mL/min. The CDH mouse which died, had considerably lower lung volumes than control, right lung 0.11 and left lung (herniated side) 0.044 ml.
CONCLUSION
This study demonstrates the ability to use the noncontact respiratory sensor to gate acquisition of micro-CT images in mice and acquire clinically significant functional data without compromising the integrity of internal structures. This serves as a platform for future study on CDH mouse pups that can yield insight on a fatal congenital disease.
CLINICAL RELEVANCE/APPLICATION
A novel non-contact sensor allows the gated imaging of mouse models with frail pulmonary function.
Background: Micro-computed tomography offers numerous advantages for
small animal imaging, including the ability to monitor the same animals
throughout a longitudinal study. However, concerns are often raised
regarding the effects of x-ray dose accumulated over the course of the
experiment. In this study, we scan C57BL/6 mice multiple times per week
for six weeks, to determine the effect of the cumulative dose on
pulmonary tissue at the end of the study. Methods/Results: C57BL/6 male
mice were split into two groups (irradiated group=10, control group=10).
The irradiated group was scanned (80kVp/50mA) each week for 6 weeks; the
weekly scan session had three scans. This resulted in a weekly dose of
0.84 Gy, and a total study dose of 5.04 Gy. The control group was
scanned on the final week. Scans from weeks 1 and 6 were reconstructed
and analyzed: overall, there was no significant difference in lung
volume or lung density between the control group and the irradiated
group. Similarly, there were no significant differences between the week
1 and week 6 scans in the irradiated group. Histological samples taken
from excised lung tissue also showed no evidence of inflammation or
fibrosis in the irradiated group. Conclusion: This study demonstrates
that a 5 Gy x-ray dose accumulated over six weeks during a longitudinal
micro-CT study has no significant effects on the pulmonary tissue of
C57BL/6 mice. As a result, the many advantages of micro- CT imaging,
including rapid acquisition of high-resolution, isotropic images in
free-breathing mice, can be taken advantage of in longitudinal studies
without concern for negative dose-related effects.
Background: Evaluation of cardiovascular function in mice using micro-CT requires that a contrast agent (CA) be administered to differentiate the blood from the myocardium. eXIA 160, an aqueous colloidal poly-disperse CA with a high concentration of iodine (160mg I/mL), creates strong contrast between blood and tissue with a low injection volume. In this study, the blood-pool enhancement time-course of eXIA 160 is monitored over a 24-hour period to determine its optimal use during cardiac function studies. Methods/Results: 8-second scans were performed (80kVp, 110mA) using the GE Locus Ultra micro-CT scanner. Male mice (black, 22-24g) were injected via tail vein with 5 muL/g body weight eXIA 160 (Binitio Biomedical Inc.). A precontrast scan was performed; following injection, mice were scanned at 15, 30, 45, and 60 minutes, 2, 4, 8, 24, and 48 hours. Overall, the highest contrast in the left ventricle occurred at 5 minutes (687 HU). Uptake of the CA by the myocardium was also observed: myocardial tissue showed increasing enhancement over a 4-hour period, remaining even once the contrast was eliminated from the vasculature. Conclusion: eXIA 160 provided high contrast between blood and myocardial tissue for a period of 30 minutes following injection. Notably, this CA was also taken up by the myocardium and provided continued enhancement when the contrast agent was eliminated from the blood, making LV wall function studies possible. In conclusion, eXIA 160, with its high iodine concentration and targeted tissue uptake characteristics, make it an ideal agent to use when evaluating cardiovascular function in mice.
Background
Micro-computed tomography (micro-CT) offers numerous advantages for small animal imaging, including the ability to monitor the same animals throughout a longitudinal study. However, concerns are often raised regarding the effects of X-ray dose accumulated over the course of the experiment.PurposeTo scan C57BL/6 mice multiple times per week for 6 weeks, in order to determine the effect of the cumulative dose on pulmonary and cardiac tissue at the end of the study.Material and MethodsC57BL/6 male mice were split into two groups (irradiated group = 10, control group = 10). The irradiated group was scanned (80 kVp/50mA) three times weekly for 6 weeks, resulting in a weekly dose of 0.84 Gy, and a total study dose of 5.04 Gy. The control group was scanned on the final week. Scans from week 6 were reconstructed and the lungs and heart were analyzed.ResultsOverall, there was no significant difference in lung volume or lung density or in left ventricular volume or ejection fraction between the control group and the irradiated group. Histological samples taken from excised lung and myocardial tissue also showed no evidence of inflammation or fibrosis in the irradiated group.Conclusion
This study demonstrated that a 5 Gy X-ray dose accumulated over 6 weeks during a longitudinal micro-CT study had no significant effects on the pulmonary and myocardial tissue of C57BL/6 mice. As a result, the many advantages of micro-CT imaging, including rapid acquisition of high-resolution, isotropic images in free-breathing mice, can be taken advantage of in longitudinal studies without concern for negative dose-related effects.
A cone beam micro-CT has previously been utilized along with a pressure-tracking respiration sensor to acquire prospectively gated images of both wild-type mice and various adult murine disease models. While the pressure applied to the abdomen of the subject by this sensor is small and is generally without physiological effect, certain disease models of interest, as well as very young animals, are prone to atelectasis with added pressure, or they generate too weak a respiration signal with this method to achieve optimal prospective gating. In this work we present a new fibre-optic displacement sensor which monitors respiratory motion of a subject without requiring physical contact. The sensor outputs an analogue signal which can be used for prospective respiration gating in micro-CT imaging. The device was characterized and compared against a pneumatic air chamber pressure sensor for the imaging of adult wild-type mice. The resulting images were found to be of similar quality with respect to physiological motion blur; the quality of the respiration signal trace obtained using the non-contact sensor was comparable to that of the pressure sensor and was superior for gating purposes due to its better signal-to-noise ratio. The non-contact sensor was then used to acquire in-vivo micro-CT images of a murine model for congenital diaphragmatic hernia and of 11-day-old mouse pups. In both cases, quality CT images were successfully acquired using this new respiration sensor. Despite the presence of beam hardening artefacts arising from the presence of a fibre-optic cable in the imaging field, we believe this new technique for respiration monitoring and gating presents an opportunity for in-vivo imaging of disease models which were previously considered too delicate for established animal handling methods.
In this paper we report the development of a high resolution dynamic micro-computed tomography (CT) scanner with a stationary mouse bed using a compact carbon nanotube (CNT) x-ray tube. The scanner comprises a rotating x-ray tube and detector pair and a stationary and a horizontally positioned small animal bed. The system is optimized for in vivo mouse cardiac imaging. Its performance is evaluated with CT scans of phantoms and free-breathing mice. The modulation transfer function (MTF) at 10% is 5 lp/mm. At single frame acquisition, mouse cardiac micro-CT at 20 msec temporal resolution has been demonstrated by prospectively gating the imaging acquisitions to both respiration and cardiac signals.
We have previously reported the development of a dynamic micro-CT scanner with a stationary mouse bed using a compact carbon nanotube (CNT) field emission x-ray tube and preliminary results on its utility for prospectively gated cardiac imaging. In this paper we report the recent progress in improving the performance characteristics of this scanner. Through optimization of the CNT cathode, the stable emission current has been increased. The output power of the CNT x-ray source has reached ~100W peak power at 100μm focal spot size. The higher flux enables improvement of the x-ray energy spectrum to minimize the beam hardening effect and increasing the system temporal resolution by using shorter x-ray exposure time. The scanner's temporal resolution has been increased to ~10 msec, which is sufficient for high-resolution micro-CT imaging of mouse heart and lung under free-breathing setting. The spatial resolution is maintained at 6.2 lp per mm at 10% system MTF. The nanotube micro-CT scanner's application in mouse cardiac imaging has been demonstrated with high-resolution (80 μm and 15 msec) micro-CT of the mouse heart under free-breathing setting.
Functional imaging of an animals thoracic region requires cardiac and respiratory gating. The information on respiratory motion and ECG required for double-gating are extracted from the rawdata and used to select the projections appropriate for a given motion phase. A conventional phase-correlated reconstruction (PC) therefore uses only a small amount of the total projections acquired. Thus the resulting images comprise a high noise level unless acquired with very high dose, and streak artifacts may occur due to the sparse angular sampling. Here, we are aiming at getting high fidelity images even for relatively low dose values. To overcome these issues we implemented an iterative reconstruction method encompassing a five-dimensional (spatial, cardiac-temporal, respiratory-temporal) edge-preserving filter. This new phase-correlated low-dose (LDPC) reconstruction method is evaluated using retrospectively-gated, contrast-enhanced micro CT data of mice. The scans performed com-prise 7200 projections within 10 rotations over 5 minutes. A tube voltage of 65 kV was used resulting in an administered dose of about 500 mGy. 20 respiratory phases and 10 cardiac phases are reconstructed. Using LDPC reconstruction the image noise is typically reduced by a factor of about six and artifacts are almost removed. Reducing the number of projections available for reconstruction shows that we can get comparable image quality with only 200 mGy. LDPC enables high fidelity low-dose double-gated imaging of free breathing rodents without compromises in image quality. Compared to PC image noise is significantly reduced with LDPC and the administered dose can be reduced accordingly.
A model developed to simulate the behavior of oil and gas accidentally released from deep water is presented. This model presents major modifications to a three-dimensional model developed earlier (Yapa and Zheng, 1997) that simulate the behaviour of oil from under water accidents (shallow water). In deepwater, the ultra-high pressure and cold temperature causes phase changes in gases. These combined with relatively strong currents in some deepwater regions presents extraordinary challenges to modeling jets/plumes from deepwater oil and gas blowouts. The present model incorporates the phase changes of gas, associated changes in thermodynamics and its impact on the hydrodynamics of the jet/plume. Hydrate formation, hydrate decomposition, gas dissolution, non-ideal behavior of the gas, and possible gas separation from the main plume due to strong cross currents are integrated with the jet/plume hydrodynamics and thermodynamics.This paper presents the complete model development and testing of various computational modules with available data. A companion paper presents the comparison of model results with three large-scale field experiments conducted in the Norwegian Sea.
Anesthetics provide sedation and immobility facilitating echocardiography in mice, but influence cardiac function. We studied the effects of intraperitoneal and inhaled anesthetic agents on echocardiographic measurements. Mice were anesthetized with intraperitoneal tribromoethanol (TBE), ketamine-midazolam (K/M), ketamine-xylazine (K/X), or inhaled isoflurane (Isf), and echocardiographic parameters were assessed at 5, 10, 15, and 20 min. In C57BL/6N mice, Isf produced high initial heart rates (HR) that decreased to levels comparable to TBE at 15-20 min (approximately 450 beats/min) and the most stable percent fractional shortening (%FS) and end-diastolic dimension (EDD). With TBE, %FS initially was low, but increased comparable to Isf (approximately 45%) at 15 min. K/M produced similar time trends but lower absolute values compared with TBE for all parameters. K/X produced cardiac depression evidenced by low HR and %FS, and increased EDD. Isf was the most reproducible in repeat studies at 12 days. In C57BL/6J compared with C57BL/6N mice, K/M produced higher HR, and %FS and TBE produced smaller EDD. In conclusion, anesthetic agent, timing of echocardiographic measurements, and genetic background are all critical variables during echocardiography in mice.
The mouse is an important model system for cardiovascular biology, with echocardiography a critical tool for noninvasive measurement of cardiac morphology and function. The feasibility and short-term temporal consistency of repeated echocardiographic measurements in conscious mice has not been previously evaluated. We performed serial 2-dimensional guided M-mode transthoracic echocardiographic measurements at 5- to 10-minute intervals over 60 minutes in conscious mice and in mice treated with 1 of 3 anesthetic regimens: ketamine and acepromazine (n = 14); pentobarbital (n = 14); and ketamine and xylazine (n = 13). Unanesthetized mice received intraperitoneal saline (n = 6) or no injection (n = 7). In sequentially repeated measurements over 1 hour in conscious mice, none of the measured or derived echocardiographic parameters differed from baseline, whereas all 3 anesthetic regimens produced significant, prolonged, and temporally variable decreases in heart rate and fractional shortening. The relationship between heart rate and fractional shortening was not altered by anesthetic choice. Serial echocardiographic assessments of cardiac function, dimension, and mass can be performed with high reproducibility in conscious mice.
To establish fast, high-resolution in vivo cine magnetic resonance imaging (cine-MRI) on a vertical 11.7-T MR system and to investigate the stability of normal and failing mouse hearts in the vertical position.
To optimize the method on a high-field system, various MR-related parameters, such as relaxation times and the need for respiratory gating, were quantitatively investigated. High-resolution cine-MRI was applied to normal mice and to a murine heart failure model. Cardiac functional parameters were compared to matched mice imaged previously on a horizontal MR system.
A T(1) of 1.10 +/- 0.27 seconds and a T(2) of 18.5 +/- 3.9 msec were measured for murine myocardial tissue. A quantitative analysis also proved respiratory gating to be essential for obtaining artifact-free cine images in the vertical position at this field strength. Cardiac functional parameters of mice, obtained within one hour, agreed well with those from previous studies of mice in the horizontal position.
This work shows that MR systems with a vertical bore design can be used to accurately measure cardiac function in both normal and chronically failing mouse hearts within one hour. The increased signal-to-noise ratio (SNR) due to the higher field strength could be exploited to obtain higher temporal and spatial resolution compared to previous studies that were performed on horizontal systems with lower field strengths.
To assess the performance of motion gating strategies for mouse cardiac magnetic resonance (MR) at high magnetic fields by quantifying the levels of motion artifact observed in images and spectra in vivo.
MR imaging (MRI) of the heart, diaphragm, and liver; MR angiography of the aortic arch; and slice-selective 1H-spectroscopy of the heart were performed on anesthetized C57Bl/6 mice at 11.75 T. Gating signals were derived using a custom-built physiological motion gating device, and the gating strategies considered were no gating, cardiac gating, conventional gating (i.e., blanking during respiration), automatic gating, and user-defined gating. Both automatic and user-defined modes used cardiac and respiratory gating with steady-state maintenance during respiration. Gating performance was assessed by quantifying the levels of motion artifact observed in images and the degree of amplitude and phase stability in spectra.
User-defined gating with steady-state maintenance during respiration gave the best performance for mouse cardiac imaging, angiography, and spectroscopy, with a threefold increase in signal intensity and a sixfold reduction in noise intensity compared to cardiac gating only.
Physiological gating with steady-state maintenance during respiration is essential for mouse cardiac MR at high magnetic fields.
Micro-computed tomography(micro-CT) is becoming an accepted research tool for the noninvasive examination of laboratory animals such as mice and rats, but to date, in vivo scanning has largely been limited to the evaluation of skeletal tissues. We use a commercially available micro-CT device to perform respiratory gated in vivo acquisitions suitable for thoracic imaging. The instrument is described, along with the scan protocol and animal preparation techniques. Preliminary results confirm that lung tumors as small as 1 mm in diameter are visible in vivo with these methods. Radiation dose was evaluated using several approaches, and was found to be approximately 0.15 Gy for this respiratory-gated micro-CT imaging protocol. The combination of high-resolution CT imaging and respiratory-gated acquisitions appears well-suited to serial in vivo scanning.
Cardiopulmonary imaging in rodents using micro‐computed tomography (CT) is a challenging task due to both cardiac and pulmonary motion and the limited fluence rate available from micro‐focus x‐ray tubes of most commercial systems. Successful imaging in the mouse requires recognition of both the spatial and temporal scales and their impact on the required fluence rate. Smaller voxels require an increase in the total number of photons (integrated fluence) used in the reconstructed image for constant signal‐to‐noise ratio. The faster heart rates require shorter exposures to minimize cardiac motion blur imposing even higher demands on the fluence rate. We describe a system with fixed tube/detector and with a rotating specimen. A large focal spot x‐ray tube capable of producing high fluence rates with short exposure times was used. The geometry is optimized to match focal spot blur with detector pitch and the resolution limits imposed by the reproducibility of gating. Thus, it is possible to achieve isotropic spatial resolution of 100μm with a fluence rate at the detector 250 times that of a conventional cone beam micro‐CT system with rotating detector and microfocal x‐ray tube. Motion is minimized for any single projection with 10ms exposures that are synchronized to both cardiac and breathing motion. System performance was validated in vivo by studies of the cardiopulmonary structures in C57BL/6 mice, demonstrating the value of motion integration with a bright x‐ray source.
Demonstrate noninvasive imaging methods for in vivo characterization of cardiac structure and function in mice using a micro-CT system that provides high photon fluence rate and integrated motion control.
Simultaneous cardiac- and respiratory-gated micro-CT was performed in C57BL/6 mice during constant intravenous infusion of a conventional iodinated contrast agent (Isovue-370), and after a single intravenous injection of a blood pool contrast agent (Fenestra VC). Multiple phases of the cardiac cycle were reconstructed with contrast to noise and spatial resolution sufficient for quantitative assessment of cardiac function.
Contrast enhancement with Isovue-370 increased over time with a maximum of approximately 500 HU (aorta) and 900 HU (kidney cortex). Fenestra VC provided more constant enhancement over 3 hr, with maximum enhancement of approximately 620 HU (aorta) and approximately 90 HU (kidney cortex). The maximum enhancement difference between blood and myocardium in the heart was approximately 250 HU for Isovue-370 and approximately 500 HU for Fenestra VC. In mice with Fenestra VC, volumetric measurements of the left ventricle were performed and cardiac function was estimated by ejection fraction, stroke volume, and cardiac output.
Image quality with Fenestra VC was sufficient for morphological and functional studies required for a standardized method of cardiac phenotyping of the mouse.
Noninvasive radiologic imaging has recently gained considerable interest in basic and preclinical research for monitoring disease progression and therapeutic efficacy. In this report, we introduce flat-panel volumetric computed tomography (fpVCT) as a powerful new tool for noninvasive imaging of different organ systems in preclinical research. The three-dimensional visualization that is achieved by isotropic high-resolution datasets is illustrated for the skeleton, chest, abdominal organs, and brain of mice. The high image quality of chest scans enables the visualization of small lung nodules in an orthotopic lung cancer model and the reliable imaging of therapy side effects such as lung fibrosis. Using contrast-enhanced scans, fpVCT displayed the vascular trees of the brain, liver, and kidney down to the subsegmental level. Functional application of fpVCT in dynamic contrast-enhanced scans of the rat brain delivered physiologically reliable data of perfusion and tissue blood volume. Beyond scanning of small animal models as demonstrated here, fpVCT provides the ability to image animals up to the size of primates.
Microcomputed tomography (Micro‐CT) has the potential to noninvasively image the structure of organs in rodent models with high spatial resolution and relatively short image acquisition times. However, motion artifacts associated with the normal respiratory motion of the animal may arise when imaging the abdomen or thorax. To reduce these artifacts and the accompanying loss of spatial resolution, we propose a prospective respiratory gating technique for use with anaesthetized, free‐breathing rodents. A custom‐made bed with an embedded pressure chamber was connected to a pressure transducer. Anaesthetized animals were placed in the prone position on the bed with their abdomens located over the chamber. During inspiration, the motion of the diaphragm caused an increase in the chamber pressure, which was converted into a voltage signal by the transducer. An output voltage was used to trigger image acquisition at any desired time point in the respiratory cycle. Digital radiographic images were acquired of anaesthetized, free‐breathing rats with a digital radiographic system to correlate the respiratory wave form with respiration‐induced organ motion. The respiratory wave form was monitored and recorded simultaneously with the x‐ray radiation pulses, and an imaging window was defined, beginning at end expiration. Phantom experiments were performed to verify that the respiratory gating apparatus was triggering the micro‐CT system. Attached to the distensible phantom were 100μm diameter copper wires and the measured full width at half maximum was used to assess differences in image quality between respiratory‐gated and ungated imaging protocols. This experiment allowed us to quantify the improvement in the spatial resolution, and the reduction of motion artifacts caused by moving structures, in the images resulting from respiratory‐gated image acquisitions. The measured wire diameters were 0.135mm for the stationary phantom image, 0.137mm for the image gated at end deflation, 0.213mm for the image gated at peak inflation, and 0.406mm for the ungated image. Micro‐CT images of anaesthetized, free‐breathing rats were acquired with a General Electric Healthcare eXplore RS in vivo micro‐CT system. Images of the thorax were acquired using the respiratory cycle‐based trigger for the respiratory‐gated mode. Respiratory gated‐images were acquired at inspiration and end expiration, during a period of minimal respiration‐induced organ motion. Gated images were acquired with a nominal isotropic voxel spacing of 44μm in 20–25min (80kVp, 113mAs, 300ms imaging window per projection). The equivalent ungated acquisitions were 11min in length. We observed improved definition of the diaphragm boundary and increased conspicuity of small structures within the lungs in the gated images, when compared to the ungated acquisitions. In this work, we have characterized the externally monitored respiratory wave form of free‐breathing, anaesthetized rats and correlated the respiration‐induced organ motion to the respiratory cycle. We have shown that the respiratory pressure wave form is an excellent surrogate for the radiographic organ motion. This information facilitates the definition of an imaging window at any phase of the breathing cycle. This approach for prospectively gated micro‐CT can provide high quality images of anaesthetized free‐breathing rodents.
The goal of this study was to determine if an iodinated, liposomal contrast agent could be used for high-resolution, micro-CT of low-contrast, small-size vessels in a murine model.
A second-generation, liposomal blood pool contrast agent encapsulating a high concentration of iodine (83-105 mg I/mL) was evaluated. A total of five mice weighing between 20 and 28 g were infused with equivalent volume doses (500 microL of contrast agent/25 g of mouse weight) and imaged with our micro-CT system for intervals of up to 240 min postinfusion. The animals were anesthetized, mechanically ventilated, and vital signs monitored allowing for simultaneous cardiac and respiratory gating of image acquisition.
Initial enhancement of about 900 H in the aorta was obtained, which decreased to a plateau level of approximately 800 H after 2 hr. Excellent contrast discrimination was shown between the myocardium and cardiac blood pool (650-700 H). No significant nephrogram was identified, indicating the absence of renal clearance of the agent.
The liposomal-based iodinated contrast agent shows long residence time in the blood pool, very high attenuation within submillimeter vessels, and no significant renal clearance rendering it an effective contrast agent for murine vascular imaging using a micro-CT scanner.
Small animal imaging has recently become an area of increased interest because more human diseases can be modeled in transgenic and knockout rodents. Current micro-CT systems are capable of achieving spatial resolution on the order of 10 mum, giving highly detailed anatomical information. However, the speed of data acquisition of these systems is relatively slow, when compared with clinical CT systems. Dynamic CT perfusion imaging has proven to be a powerful tool clinically in detecting and diagnosing cancer, stroke, pulmonary and ischemic heart diseases. In order to perform this technique in mice and rats, quantitative CT images must be acquired at a rate of at least 1 Hz. Recently, a research pre-clinical CT scanner (eXplore Ultra, GE Healthcare) has been designed specifically for dynamic perfusion imaging in small animals. Using an amorphous silicon flat-panel detector and a clinical slip-ring gantry, this system is capable of acquiring volumetric image data at a rate of 1 Hz, with in-plane resolution of 150 mum, while covering the entire thoracic region of a mouse or whole organs of a rat. The purpose of this study was to evaluate the principal imaging performance of the micro-CT system, in terms of spatial resolution, image uniformity, linearity, dose and voxel noise for the feasibility of imaging mice and rats. Our investigations show that 3D images can be obtained with a limiting spatial resolution of 2.7 line pairs per mm and noise of 42 HU, using an acquisition interval of 8 seconds at an entrance dose of 6.4 cGy.
Pathological manifestations of lung damage after irradiation support the use of death between 11 weeks (80 days) and 23 weeks (160 days) as an assay for pneumonitis in the C3Hf strain of mice. The proportion of deaths and the time to reach a specific percent of lethality were found to be dose and dose per fraction-related. The median survival time and the latency period for the time of occurrence of pneumonitis, were dependent on total biological dose over a narrow range, equivalent to "normalized" doses of 36-41 Gy in 2 Gy fractions. All these criteria entered into the analysis of data. The linear-quadratic isoeffect model gives a good fit for the results over a range of doses from 1.6 to 8 Gy per fraction, that is, with the exclusion of the single dose data. The value of alpha/beta was always approximately 4.0 Gy from three kinds of analyses. The half-time for repair from sublethal injury was approximately 0.75 h. Both of these values are in agreement with the results from other investigators.
The potential advantage for sparing normal tissue damage by hyperfractionation of low-LET radiation may be limited by the repair kinetics of tissues in the irradiated field. Tissues with slow repair kinetics will limit the number of fractions that may be given on the same day. Results are presented for mouse lung treated with a range of doses per fraction using either two or three fractions per day in multiple X-ray fractionation schedules. The results are analysed to determine whether the repair kinetics follow a single exponential function of time. The calculated repair rate (T1/2) was about 1.2 h for two fractions per day of 2 Gy (10F/5d) but slightly less (T1/2 = 0.8 h) for two fractions of 9 Gy (2F/1d). For smaller doses per fraction of 1.1 Gy, given three times per day (39F/13d), the T1/2 was not significantly less (T1/2 = 0.3-0.7 h). For three fractions per day of 1.1 Gy per fraction an unsatisfactory fit is achieved using a single exponential function of time, and a better fit is obtained using two components of repair. The repair kinetics are slow for lung, in comparison to acute reacting tissues (except skin), and may require that 6-8 h (i.e. four or five half-times) should be allowed between fractions on the same day so that more than 95 per cent of the repairable dose is repaired. At present the variation in repair kinetics with doses per fraction between 1.1 and 9 Gy are not significantly different, so no reduction of interfraction interval should be proposed.
The kinetics of repair of sublethal damage in mouse lung was studied after fractionated doses of 137Cs gamma-rays. A wide range of doses per fraction (1.7-12 Gy) was given with interfraction intervals ranging from 0.5 to 24 h. The data were analysed by a direct method of analysis using the incomplete repair model. The half-time of repair (T1/2) was 0.76 h for the pneumonitis phase of damage (up to 8 months) and 0.65 h for the later phase of damage up to 12 months. The rate of repair was dependent on fraction size for both phases of lung damage and was faster after large dose fractions than after small fractions. The T1/2 was 0.6 h (95 per cent c.1. 0.53, 0.69) for doses per fraction greater than 5 Gy and 0.83 h (95 per cent c.1 0.76, 0.92) for doses per fraction of 2 Gy. Repair was nearly complete by 6 h, at least for the pneumonitis phase of damage. To the extent that extrapolation of these data to humans may be valid, these results imply that treatments with multiple fractions per day that involve the lung will not be limited by the necessity for interfraction intervals much longer than 6 h.
A high-resolution laboratory CT scanner has been developed for imaging objects undergoing periodic motion. The scanner comprises an x-ray image intensifier, optically coupled to a linear photodiode array. Gated time-evolved projections of a single slice of the moving object are acquired, reformatted, and reconstructed. The resulting series of CT images shows the object at different phases of its motion cycle. The scanner has an adjustable field of view (FOV) and the resolution can be as high as 3.2 mm-1 (for the 40-mm FOV). The spatial resolution depends on the inherent resolution of the scanner and on the object's velocity. For objects moving at 1 cm s-1, the spatial resolution is reduced by 9% in the direction of motion. The signal intensity in the reconstructed image is linear for materials with attenuation coefficients as high as 1.5 cm-1 (for a 90-kVp x-ray beam), with an average accuracy of +/- 0.02 cm-1. The average accuracy of circumference measurements made from the CT images is +/- 0.3 mm. Lastly, an application of this dynamic CT scanner to imaging excised human arterial specimens under simulated physiological pressure conditions is presented as an example.
Data published previously have shown that repair of sublethal damage in mouse lung proceeds with two significantly different repair half-times of 0.4 h and 4.0 h and that the fast component has approximately four times more weight than the slow component. None of these data, however, were obtained after small dose fractions similar to those used in clinical radiotherapy. The purpose of the experiments presented here was to determine the half-time of the fast component only of repair in mouse lung after doses per fraction of 2.0 Gy. We irradiated the whole thoraces of mice with six equal doses of 2.09 Gy given at intervals ranging from 0 to 45 min. The dose was topped up 24 h later by a range of single doses designed to bring the response, i.e. breathing rate and death from pneumonitis, into the observable range. Data on breathing rate were converted into quantal response data. All data were analyzed by the linear-quadratic model that contains two rates of repair (H.D. Thames et al., Radiother, Oncol. 15, 49-53, 1989). The data showed that the repair rate is very rapid, giving a t1/2 ranging from 0.25 to 0.75 h for breathing rate and mortality, in agreement with our data published previously for higher dose fractions. There were no differences between the t1/2's obtained from the two assays of damage. These data indicate that the half-time of the fast component of repair in mouse lung is approximately 0.4 h after clinically relevant dose fractions.
We have recently developed an iodinated lipid-based contrast agent capable of residing in the blood pool for extended periods of time relative to conventional water-soluble contrast agents. The purpose of this study was to examine the effects of combining this new blood-pool agent (ITG-PEG) with a hepatocyte-selective agent (ITG-LE; Molecular Biosystems) for accurate CT detection of small (< 10 mm) VX2 tumors in rabbit liver.
Preliminary pharmacokinetic analyses were conducted in SD rats (12) by injection of either I-125-labeled ITG-PEG or I-125-labeled ITG-LE followed by subsequent blood collection and quantification of radioactivity. Preliminary CT studies were conducted in both normal (3) and tumor-bearing NZW rabbits (2). Tumor-bearing rabbits were laparotomized and VX2 cells injected directly into the hepatic parenchyma to produce a total of eight focal lesions (2-10 mm diameter). Animals underwent CT scanning 10 days later with multiple techniques including noncontrast and helical i.v. enhanced (600 mg I/kg iohexol), and then 24 hours later using both ITG-PEG and ITG-LE (200 mg I/kg). Tissue density measurements (HU) of liver, tumor, and blood (descending aorta) were acquired in each case for comparison. Tumor morphology was verified by gross pathologic inspection.
Pharmacokinetic analysis in rats as well as CT studies in normal rabbits revealed that ITG-PEG remains in the blood-pool phase for more than 2 hours following i.v. administration. In fact, blood density in normal rabbit obtained with ITG-PEG was 95.1 HU +/- 5.8 at 120 minutes compared to 90.7 HU +/- 6.1 immediately after injection. Although liver enhancement was greater with iohexol (67 HU within 1 minute of injection), than for ITG-PEG/ITG-LE (32 HU, 60 minutes postinjection), liver to lesion ratios favored ITG-PEG/ITG-LE due to significant enhancement of tumor itself with iohexol (+40 HU). Tumor enhancement was minimal with ITG-PEG/ITG-LE. Lesions were subjectively much better defined with ITG-PEG/ITG-LE with sharper edge definition.
In these animal models, a new iodinated lipid-based contrast agent composed of both blood pool and hepatocyte-selective components afforded favorable CT imaging results compared to a conventional urographic agent, albeit at one-third the total iodine dose.
The purpose of this work was to develop high-resolution cardiac magnetic resonance imaging techniques for the in vivo mouse model for quantification of myocardial function and mass. Eight male mice were investigated on a 7-Tesla MRI scanner. High-quality images in multiple short axis slices (in-plane resolution 117 microm2, slice thickness 1 mm) were acquired with an ECG-gated cine sequence. Left ventricular end-diastolic and end-systolic volumes and mass were calculated from segmented slice volumes. There was precise agreement of left ventricular mass determined ex vivo and by MRI. Intraobserver (5%) and interobserver (5%) variability of in vivo MR measurements were low.
A novel lipid emulsion (LE) was developed for hepatoselective delivery of a polyiodinated triglyceride (ITG) with potential for use in CT. This work assessed the effects of mean particle size, total administered dose, and formulation composition on the in vivo biodistribution and imaging profiles of the ITG-LE in rats.
The concentration of radioactivity derived from intravenously administered 125I-ITG-LE was determined as a function of time after injection. CT imaging studies of the abdomen evaluated the extent of hepatic enhancement after administration of ITG-LE.
Mean emulsion particle diameter and total administered dose exerted the greatest effect on ITG-LE biodistribution profiles. In the optimal delivery scenario, >70% of the administered dose localized to the liver 30 minutes after injection. Liver enhancement profiles in CT imaging studies were consistent with biodistribution profiles.
These results suggest that an appropriately formulated and administered dose of ITG-LE provides tissue-selective localization of contrast material for use in CT.
The surgical removal of a primary tumor can result in the rapid growth of metastases. The production of angiogenesis inhibitors by the primary tumor is one mechanism for the inhibition of metastatic tumor growth. The effect of curative radiotherapy to a primary tumor known to make an inhibitor of angiogenesis and the effects on distant metastases has not been studied. We here show that the eradication of a primary Lewis lung carcinoma (LLC-LM), which is known to generate angiostatin, is followed by the rapid growth of metastases that kill the animal within 18 days after the completion of radiation therapy. The right thighs of C57BL/6 mice (n = 25) were injected s.c. with 1 x 10(6) LLC-LM cells. Animals were randomized to one of five groups: no irradiation, 40 Gy in one fraction, 30 Gy in one fraction, 40 Gy in two 20 Gy fractions, or 50 Gy in five 10 Gy fractions. Tumors were clinically eradicated in each treatment group. All of the surviving animals became dyspneic and were killed within 14-18 days after the completion of radiation therapy. Examination of their lungs revealed >46 (range, 46-62) surface metastases in the treated animals compared with 5 (range, 2-8) in the untreated animals. The lung weights had increased from 0.2 g (range, 0.19-0.22 g) in the controls to 0.58 g (range 0.44-0.84) in the experimental animals. The most effective dose regimen was 10 Gy per fraction for five fractions, and serial experiments were conducted with this fractionation scheme. Complete response of the primary tumor was seen in 25 of 35 (71%) mice. The average weight of the lungs in the nonirradiated animals was 0.22 g (range, 0.19-0.24 g) and in the irradiated animals was 0.66 g (range, 0.61-0.70 g). The average number of surface metastases increased from five per lung (range, 2-13) in the control animals to 53 per lung (range, 46-62) in the irradiated animals. Both differences were statistically significant with P < 0.001. If the nontumor-bearing leg was irradiated or the animals were sham-irradiated, no difference in the number of surface metastases or lung weights was observed between the control group and the treated group. Urinary levels of matrix metalloproteinase 2, the enzyme responsible for angiostatin processing in this tumor model, were measured and correlated with the viability and size of the primary tumor. Administration of recombinant angiostatin prevented the growth of the metastases after the treatment of the primary tumor. In this model, the use of radiation to eradicate a primary LLC-LM tumor results in the growth of previously dormant lung metastases and suggests that combining angiogenesis inhibitors with radiation therapy may control distant metastases.
The introduction of helical single-detector row computed tomography (CT) and, more recently, multi-detector row CT has greatly increased the clinical indications for CT. Correspondingly, CT examinations now account for greater than one-half of the radiation dose due to medical procedures in the population of North America. The level of CT radiation dose, especially in the pediatric population, is of concern to radiologists, medical physicists, government regulators, and the media. This review addresses this problem with particular reference to radiation dose in chest CT. Specifically it outlines the topics of measurement units used to quantify radiation exposure, factors affecting CT scanner dose efficiency, scanner settings that determine the administered radiation dose, and radiation dose reduction in chest CT. A table of reference dose values is provided. Given the wide variation documented in chest CT radiation exposure, the authors suggest that reference standards be promoted to minimize excessive CT radiation exposure. In addition, further research into the complex relationship between radiation exposure, image quality, and diagnostic accuracy should be encouraged in order to establish the minimum radiation dose necessary to provide adequate diagnostic information for standard clinical questions.
A system for cone-beam computed tomography (CBCT) has been developed based upon the technology of active matrix flat-panel imagers (FPIs), and the system has demonstrated the potential for fully three-dimensional volumetric imaging with high spatial and contrast resolution. This paper investigates the effects of image lag (arising from charge trapping and release in the FPI pixels) upon CBCT reconstructions. Hypotheses were derived based upon a simple, geometrical/physical model, suggesting that image lag in the projection data results primarily in two artifacts: a spatial blurring artifact in the direction opposite to the direction of rotation (called a "comet") and a line artifact along the direction of the first few projections (called a "streak"). The hypotheses were tested by means of computer simulations and experimental measurements that yielded CBCT images of a simple cylindrical water phantom containing an attenuating rod of varying size and composition. The computer simulations generated projection images based upon analysis of the system geometry and a simple model of the FPI that allowed free adjustment of the image lag. Experimental measurements involved CBCT scans of the phantom under various conditions and modes of acquisition followed by examination of the resulting CBCT axial slices for lag artifacts. Measurements were performed as a function of exposure level, position and contrast of the rod, and for three modes of acquisition designed to isolate and/or minimize the two hypothesized artifacts. The results clearly illustrate the comet and streak artifacts, particularly in relation to high-contrast objects imaged at high exposure levels. The significance of such artifacts under clinical conditions is expected to be small, considering the magnitude of the effect relative to the morphology and composition of typical anatomy. The artifacts may become appreciable, however, in the presence of high-contrast objects, such as marker BBs, dental fillings, and metal prosthetics. A procedural method of reducing lag artifacts is demonstrated.
Small-animal imaging has become increasingly more important as transgenic and knockout mice are produced to model human diseases. One imaging technique that has emerged is microcomputed tomography (micro-CT). For live-animal imaging, the precision in the images will be determined by the x-ray dose given to the animal. As a result, we propose a simple method to predict the noise performance of an x-ray micro-CT system as a function of dose and image resolution. An ideal, quantum-noise limited micro-CT scanner, assumed to have perfect resolution and ideal efficiency, was modeled. Using a simplified model, the coefficient of variation (COV) of the linear attenuation coefficient was calculated for a range of entrance doses and isotropic voxel sizes. COV calculations were performed for the ideal case and with simulated imperfections in efficiency and resolution. Our model was validated in phantom studies and mouse images were acquired with a specimen scanner to illustrate the results. A simplified model of noise propagation in the case of isotropic resolution indicates that the COV in the linear attenuation coefficient is proportional to (dose)(-1/2) and to the (isotropic voxel size)(-2) in the reconstructed volume. Therefore an improvement in the precision can be achieved only by increasing the isotropic voxel size (thereby decreasing the resolution of the image) or by increasing the x-ray dose. For the ideal scanner, a COV of 1% in the linear attenuation coefficient for an image of a mouse exposed to 0.25 Gy is obtained with a minimum isotropic voxel size of 135 microm. However, the same COV is achieved at a dose of 5.0 Gy with a 65 microm isotropic voxel size. Conversely, for a 68 mm diameter rat, a COV of 1% obtained from an image at 5.0 Gy would require an isotropic voxel size of 100 microm. These results indicate that short-term, potentially lethal, effects of ionizing radiation will limit high-resolution live animal imaging. As improvements in detector technology allow the resolution to improve, by decreasing the detector element size to tens of microns or less, high quality images will be limited by the x-ray dose administered. For the highest quality images, these doses will approach the lethal dose or LD50 for the animals. Approaching the lethal dose will affect the way experiments are planned, and may reduce opportunities for experiments involving imaging the same animal over time. Dose considerations will become much more important for live small-animal imaging as the limits of resolution are tested.
The goal is to perform geometric studies on cone-beam CT scanning along a three-dimensional (3D) spiral of variable radius. First, the background for variable radius spiral cone-beam scanning is given in the context of electron-beam CT/micro-CT. Then, necessary and sufficient conditions are proved for existence and uniqueness of PI lines inside the variable radius 3D spiral. These results are necessary steps toward exact cone-beam reconstruction from a 3D spiral scan of variable radius, adapting Katsevich's formula for the standard helical cone-beam scanning. It is shown in the paper that when the longitudinally projected planar spiral is not always convex toward the origin, the PI line may not be unique in the envelope defined by the tangents of the spiral. This situation can be avoided by using planar spirals whose curvatures are always positive. Using such a spiral, a longitudinally homogeneous region inside the corresponding 3D spiral is constructed in which any point is passed by one and only one PI line, provided the angle omega between planar spiral's tangent and radius is bounded by [omega - 90 degrees] < or = < epsilon for some positive epsilon < or = 32.48 degrees. If the radius varies monotonically, this region is larger and one may allow epsilon < or = 51.85 degrees. Examples for 3D spirals based on logarithmic and Archimedean spirals are given. The corresponding generalized Tam-Danielsson detection windows are also formulated.
Micro computed tomography is an important tool for small animal imaging. On many occasions, it is desirable to image lungs in a live instead of postmortem small animal to perform a pulmonary physiology study. Because the lungs are moving, gating with respect to the ventilatory phase has to be performed to reduce motion artifacts. Precapture ventilation gating may be difficult to achieve in some situations, which motivates us to propose and implement a simple postacquisition gating method.
Rats were used as the subjects in this study. A sequence of low-dose projection images were acquired at 30 frames per second for each view angle. During each capture sequence the rat undergoes multiple ventilation cycles. Using the sequence of projection images, an automated region of interest algorithm, based on integrated grayscale intensity, tracts the ventilatory phase of the lungs. In the processing of an image sequence, multiple projection images are identified at a particular phase and averaged to improve the signal-to-noise ratio. The resulting averaged projection images from different view angles are input to a Feldkamp cone-beam algorithm reconstruction algorithm to obtain isotropic image volumes.
Reconstructions with reduced movement artifacts are obtained. In the gated reconstruction, registration of the bone is much better, the edge of the lung is clearly defined, and structures within the lung parenchyma are better resolved. Also, different phases of a breathing cycle can be reconstructed from one single tomographic scan by the proposed gating method.
A postacquisition gating method using the phase information encoded in the 2-dimensional cone beam projections is proposed. This method is simple to implement and does not require additional experimental set-up to monitor the respiration. It may find applications in lung tumor detection, dynamic pulmonary physiology studies, and the respiratory systems modeling. Minimal motion artifact data sets improve qualitative and quantitative analysis techniques that are useful in physiologic studies of pulmonary structure and function.
Volumetric computed tomography (VCT) is a technology in which area detectors are used for imaging large volumes of a subject with isotropic imaging resolution. We are experimenting with a prototype VCT scanner that uses flat-panel X-ray detectors and is designed for high-resolution three-dimensional (3D) imaging. Using this technique, we have demonstrated microangiography of xeno-transplanted skin squamous cell carcinomas in nude mice. VCT shows the vessel architecture of tumors and animals with greater detail and plasticity than has previously been achieved, and is superior to contrast-enhanced magnetic resonance (MR) angiography. VCT and MR images correlate well for larger tumor vessels, which are tracked from their origin on 3D reconstructions of VCT images. When compared with histology, small tumor vessels with a diameter as small as 50 microm were clearly visualized. Furthermore, imaging small vessel networks inside the tumor tissue improved discrimination of vital and necrotic regions. Thus, VCT substantially improves imaging of vascularization in tumors and offers a promising tool for preclinical studies of tumor angiogenesis and antiangiogenic therapies.
Insufficient techniques exist for rapid and reliable phenotype characterization of genetically manipulated mouse models of cardiac dysfunction. We developed a new, robust, 3-dimensional echocardiography (3D-echo) technique and hypothesized that this 3D-echo technique is as accurate as magnetic resonance imaging (MRI) and histology for assessment of left ventricular (LV) volume, ejection fraction, mass, and infarct size in normal and chronically infarcted mice.
Using a high-frequency, 7/15-MHz, linear-array ultrasound transducer, we acquired ECG and respiratory-gated, 500-microm consecutive short-axis slices of the murine heart within 4 minutes. The short-axis movies were reassembled off-line in a 3D matrix by using the measured platform locations to position each slice in 3D. Epicardial and endocardial heart contours were manually traced, and a B-spline surface was fitted to the delineated image curves to reconstruct the heart volumes. Excellent correlations were obtained between 3D-echo and MRI for LV end-systolic volumes (r=0.99, P<0.0001), LV end-diastolic volumes (r=0.99, P<0.0001), ejection fraction (r=0.99, P<0.0001), LV mass (r=0.94, P<0.0019), and infarct size (r=0.98, P<0.0001). Also, excellent correlations were found between the 3D-echo-derived LV mass and necropsy LV mass in normal mice (r=0.99, P<0.0001), as well as for 3D-echo-derived infarct size and histologically determined infarct size (r=0.99, P<0.0001) in mice with chronic heart failure. Bland-Altman analysis showed excellent limits of agreement between techniques for all measured parameters.
This new, fast, and highly reproducible 3D-echo technique should be of widespread applicability for high-throughput murine cardiac phenotyping studies.
The presence of motion artifacts is a typical problem in thoracic imaging. However, synchronizing the respiratory cycle with computed tomography (CT) image acquisition can reduce these artifacts. We currently employ a method of in vivo respiratory-gated micro-CT imaging for small laboratory animals (mice). This procedure involves the use of a ventilator that controls the respiratory cycle of the animal and provides a digital output signal that is used to trigger data acquisition. After inspection of the default respiratory trigger timing, we hypothesized that image quality could be improved by moving the data-acquisition window to a portion of the cycle with less respiratory motion. For this reason, we developed a simple delay circuit to adjust the timing of the ventilator signal that initiates micro-CT data acquisition. This delay circuit decreases motion artifacts and substantially improves image quality.
The use of micro-CT in small animals has increased in recent years. Although the radiation levels used for micro-CT are generally not lethal to the animal, they are high enough where changes in the immune response and other biological pathways may alter the experimental outcomes. Therefore, it is important to understand what the doses are for a specific imaging procedure. Monte Carlo simulation was used to evaluate the radiation dose to small animals (5-40 mm in diameter) as a result of X-ray exposure. Both monoenergetic (6-100 keV) and polyenergetic (15-100 kVp) X-ray sources were simulated under typical mouse imaging geometries. X-ray spectral measurements were performed on a mouse imaging X-ray system using a commercially available X-ray spectrometer, and spectra from high-energy systems were used as well. For a typical X-ray system with 1.0 mm of added Al at 40 kVp, the dose coefficients (dose to mouse per air kerma at isocenter) were 0.80, 0.63, 0.52, and 0.44 mGy/mGy for mouse diameters of 10, 20, 30, and 40 mm, respectively. A number of tables and figures are provided for dose estimation over a range of mouse imaging geometries.
The authors explored micro-computed tomography (micro-CT) to quantify lung tumor number and volume in a specific genetic mouse model for lung cancer.
The authors used K-ras mice, which develop lung adenomas and adenocarcinomas through somatic activation of the K-ras oncogene. Tumor number measured using micro-CT and were compared at necropsy (n = 38 mice). Tumor volume measurement precision (n = 39 mice) and accuracy (multiple tumors from a single mouse) were evaluated. Serial lung tumor volume was assessed in a pilot group (n = 8) of mice in vivo.
Tumor number assessed at necropsy and using micro-CT were significantly correlated. Lung tumor volume measurements were both reproducible (2% operator variability) and accurate (6% average error). Strikingly, we observed both tumor growth and shrinkage within individual mice.
Serial measurements provided evidence of tumor heterogeneity, an unexpected finding given the uniformity of the initiating genetic event. Micro-CT may become a powerful tool for murine lung cancer research in vivo.
Transthoracic echocardiography is the predominant diagnostic tool to evaluate systolic and diastolic cardiac function noninvasively in mice. It is known that systolic function is substantially influenced by anesthetic agents used for sedation during echocardiography. However, the effect on diastolic function has not been investigated yet. The following study was conducted to evaluate the influence of different agents on diastolic left ventricular function in mice.
The effect of ketamine/xylazine (K/X), ketamine/midazolam (K/M), and tribromoethanol (TBE, Avertin) on diastolic function was measured 5, 15, and 25 minutes after the onset of anesthesia. Ratio of peak early-to-late myocardial diastolic velocities (Ea/Aa; determined by tissue Doppler imaging; TDI), ratio of peak transmitral early (E)- and late-diastolic velocity (E/A), deceleration time (DT), and isovolumic relaxation time (IVRT) correlated significantly with heart rate (HR). Overall, increasing HR contributed to a decrease of E/A-, Ea/Aa ratio, IVRT, and DT, whereas agents characterized by the strongest variation of HR (K/M and TBE) were associated with the greatest effect on diastolic function.
Left ventricular diastolic function in mice, determined by echocardiography, is dependent on anesthetic agent and timing of measurements after onset of anesthesia.
Our objective was to determine additional radiation dose associated with scanning beyond the anatomic limits of the thorax in chest CT protocol and to assess the effect of z-axis modulation on the additional radiation dose associated with the scanning protocol.
"Extra" images for routine chest CT were defined as those above lung apices (supraapical) and those caudal to the lowermost portion of lung parenchyma (infrapulmonary), including images obtained beyond the adrenal glands (infraadrenal). One hundred and forty-eight consecutive chest CT examinations (70 men, 78 women; age range, 15-90 years) performed September 13-25, 2003, were reviewed to determine the number of supraapical, infrapulmonary, and infraadrenal extra images. All examinations were performed using z-axis modulation (n = 70) or fixed tube current (n = 78). The CT dose index volume and dose-length product (DLP) values for the extra images were calculated. Two radiologists reviewed these extra images for pathologic findings. Student's t test was used to perform the statistical analysis.
One hundred forty-four (97%) examinations had supraapical extra images and 145 (98%) had infrapulmonary extra images. A total of 31 additional findings were observed in extra images. Most clinically important findings were identified in patients with a history of malignancy. With z-axis modulation, the mean DLP for supraapical and infrapulmonary extra images was 39.98 mGy x cm and 132.59 mGy x cm, respectively. With fixed tube current, the mean DLP for supraapical and infrapulmonary extra images was 30.31 mGy x cm and 95.91 mGy x cm, respectively.
A substantial number of extra images are acquired during chest CT that do not add clinically important information in patients with nonmalignant indications. The use of z-axis modulation increased radiation dose for the extra images.
The purpose of this study was to estimate the optimal volume of an iodine-based contrast agent to administer to mice via intraperitoneal injection and the optimal time after injection to perform micro-computed tomography for maximal enhancement of abdominal organs. Eight mice were paired randomly; three pairs underwent imaging after receiving intraperitoneal injections of 125, 250, or 500 microl of contrast agent, and the fourth pair underwent imaging without receiving an injection. Each mouse was scanned three consecutive times, and each scan lasted 25 min so that we could observe the clearance of the contrast agent from the abdomen. We determined that introducing 250 microl of contrast agent into the abdominal cavity of the mice and then having the mice undergo micro-computed tomography 15 min after injection provided the optimal degree of contrast enhancement needed to distinguish the abdominal organs. These results may lead to expanded use of this imaging modality to assess abdominal organ margins in small-animal studies in vivo.
Cardiac MR imaging in small animals presents some difficulties due to shorter cardiac cycles and smaller dimensions than in human beings, but prospectively gated techniques have been successfully applied. As with human imaging, there may be certain applications in animal imaging for which retrospective gating is preferable to prospective gating. For example, cardiac imaging in multiple mice simultaneously is one such application. In this work we investigate the use of retrospective gating for cardiac imaging in a mouse. Using a three-dimensional imaging protocol, we show that image quality with retrospective gating is comparable to prospectively gated imaging. We conclude that retrospective gating is applicable for small animal cardiac MRI and show how it can be applied to the problem of cardiac MRI in multiple mice.
ECG-gated cardiac MRI in the mouse is hindered by many technical difficulties in ECG signal recording inside static and variable high magnetic scanner fields. The present study proposes an alternative robust method of acquiring auto-gated cardiac and respiratory cine images in mouse heart. In our approach, a motion synchronization signal is extracted from the echo peak MR signal of a non-triggered radial acquisition. This signal is then used for both cardiac and respiratory retrospective gating before cine image reconstruction. Highly asymmetric echoes were acquired to achieve the radial k-space sampling in order to avoid radial acquisition related artifacts and to increase auto-gating robustness. In vivo experiments demonstrated the feasibility and robustness of self-gated cine-MRI in the mouse heart at 7T. The signal-to-noise and contrast-to-noise ratios of the self-gated and ECG-gated images were comparable, all parameters being equal. Magn Reson Med, 2006. (c) 2006 Wiley-Liss, Inc.
The objective of this study was to examine the feasibility of a multimodal system to effectively induce and maintain contrast enhancement in both computed tomography (CT) and magnetic resonance (MR) for radiation therapy applications.
The physicochemical characteristics of a liposome-encapsulated iohexol and gadoteridol formulation were assessed in terms of agent loading efficiencies, size and morphology, in vitro stability, and release kinetics. The imaging properties of the liposome formulation were assessed based on T1 and T2 relaxivity measurements and in vitro CT and MR imaging in a phantom. A preliminary imaging-based evaluation of the in vivo stability of this multimodal contrast agent was also performed in a lupine model.
The average agent loading levels achieved were 26.5+/-3.8 mg/mL for iodine and 6.6+/- 1.5 mg/mL for gadolinium. These concentrations correspond to approximately 10% of that found in the commercially available preparations of each of these agents. However, this liposome-based formulation is expected to have a smaller volume of distribution and prolonged circulation lifetime in vivo. This multimodal system was found to have high agent retention in vitro, which translated into maintained contrast enhancement (up to 3 days) and stability in vivo.
This study demonstrated the feasibility of engineering a multimodal contrast agent with prolonged contrast enhancement in vivo for use in CT and MR. This contrast agent may serve as a valuable tool for cardiovascular imaging as well as image registration and guidance applications in radiation therapy.
The objective of this study was to determine the time-course of computed tomography (CT) contrast enhancement of an iodinated blood-pool contrast agent.
Five C57BL/6 mice were anesthetized, imaged at baseline, and given an iodinated blood-pool contrast agent. Micro-CT scans were acquired at 0, 0.25, 0.5, 1, 2, 4, 8, and 24 hours after injection. The mean CT number was determined in a region of interest in 7 organs.
The CT contrast enhancement was plotted as a function of time for each organ. We identified an imaging window immediately after injection suitable for visualizing the vascular system and a second imaging window at 24 hours for visualizing liver and spleen.
A single injection of the blood-pool contrast agent can be used for dual-phase investigations of the vasculature (t = 0 hours) and liver (t = 24 hours), which can be applied to studies of liver tumors or disease.
The role of three-dimensional (3D) image guidance for interventional procedures and minimally invasive surgeries is increasing for the treatment of vascular disease. Currently, most interventional procedures are guided by two-dimensional x-ray angiography, but computed rotational angiography has the potential to provide 3D geometric information about the coronary arteries. The creation of 3D angiographic images of the coronary arteries requires synchronization of data acquisition with respect to the cardiac cycle, in order to minimize motion artifacts. This can be achieved by inferring the extent of motion from a patient's electrocardiogram (ECG) signal. However, a direct measurement of motion (from the 2D angiograms) has the potential to improve the 3D angiographic images by ensuring that only projections acquired during periods of minimal motion are included in the reconstruction. This paper presents an image-based metric for measuring the extent of motion in 2D x-ray angiographic images. Adaptive histogram equalization was applied to projection images to increase the sharpness of coronary arteries and the superior-inferior component of the weighted centroid (SIC) was measured. The SIC constitutes an image-based metric that can be used to track vessel motion, independent of apparent motion induced by the rotational acquisition. To evaluate the technique, six consecutive patients scheduled for routine coronary angiography procedures were studied. We compared the end of the SIC rest period (rho) to R-waves (R) detected in the patient's ECG and found a mean difference of 14 +/- 80 ms. Two simultaneous angular positions were acquired and rho was detected for each position. There was no statistically significant difference (P = 0.79) between rho in the two simultaneously acquired angular positions. Thus we have shown the SIC to be independent of view angle, which is critical for rotational angiography. A preliminary image-based gating strategy that employed the SIC was compared to an ECG-based gating strategy in a porcine model. The image-based gating strategy selected 61 projection images, compared to 45 selected by the ECG-gating strategy. Qualitative comparison revealed that although both the SIC-based and ECG-gated reconstructions decreased motion artifact compared to reconstruction with no gating, the SIC-based gating technique increased the conspicuity of smaller vessels when compared to ECG gating in maximum intensity projections of the reconstructions and increased the sharpness of a vessel cross section in multi-planar reformats of the reconstruction.
Computed tomography (CT) applications continue to expand, and they require faster data acquisition speeds and improved spatial resolution. Achieving isotropic resolution, by means of cubic voxels, in combination with longitudinal coverage beyond 20 mm would represent a substantial advance in clinical CT because few commercially available scanners are capable of this at present. To achieve this goal, a prototype CT system incorporating a movable array of 20 cm X 20 cm, 200-microm-pitch amorphous silicon flat-panel x-ray detectors and a conventional CT x-ray source was constructed at the General Electric Global Research Center and performance tested at The University of Texas M. D. Anderson Cancer Center. The device was designed for preclinical imaging applications and has a scan field of 13 to 33 cm, with a magnification of 1.5. Image quality performance measurements, such as spatial and contrast resolutions, were obtained using both industry standard and custom phantoms. Spatial resolution, quantified by the system's modulation transfer function, indicated improvement by a factor of 2.5 to 5 in isotropic spatial resolution over current commercially available systems, with 10% modulation transfer function modulations at frequencies from 19 to 31 lp/cm. Low-contrast detectability results were obtained from industry-standard phantoms and were comprised of embedded contrast regions of 0.3%, 0.5%, and 1.0% over areas of several mm2. Performance was sufficient to easily distinguish 1.0% contrast regions down to 2 mm in diameter relative to the background. On the basis of scans of specialized hydroxyapatite phantoms, the system response is extremely linear (R2=0.990) in bone-equivalent density regimens. Standard CT dose index CTDI100 and CTDIw measurements were also conducted to assess dose delivery using a 16-cm-CTDI phantom and a 120 kV 120 mAs scan technique. The CTDIw ranged from 30 mGy (one-panel mode) to 113 mGy (two-panel mode) for this system. Lastly, several in vivo canine and murine samples were examined, and preliminary results from these scans are presented. On the basis of our results, it is clear that flat-panel-based CT scanners are useful for high-contrast high-resolution clinical applications, providing up to a 20-fold increase in volumetric resolution over most commercially available scanners.