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Effects of breathing and cardiac motion on spatial resolution in the microscopic imaging of rodents
Magnetic Resonance in Medicine
Abstract
One can acquire high-resolution pulmonary and cardiac images in live rodents with MR microscopy by synchronizing the image acquisition to the breathing cycle across multiple breaths, and gating to the cardiac cycle. The precision with which one can synchronize image acquisition to the motion defines the ultimate resolution limit that can be attained in such studies. The present work was performed to evaluate how reliably the pulmonary and cardiac structures return to the same position from breath to breath and beat to beat across the prolonged period required for MR microscopy. Radiopaque beads were surgically glued to the abdominal surface of the diaphragm and on the cardiac ventricles of anesthetized, mechanically ventilated rats. We evaluated the range of motion for the beads (relative to a reference vertebral bead) using digital microradiography with two specific biological gating methods: 1) ventilation synchronous acquisition, and 2) both ventilation synchronous and cardiac-gated acquisitions. The standard deviation (SD) of the displacement was < or =100 microm, which is comparable to the resolution limit for in vivo MRI imposed by signal-to-noise ratio (SNR) constraints. With careful control of motion, its impact on resolution can be limited. This work provides the first quantitative measure of the motion-imposed resolution limits for in vivo imaging.
... The volume-rendered image of the cardiac and pulmonary vascular trees acquired after injection of the liposomal contrast agent shows the right and left ventricles, aorta, pulmonary trunk, and inferior vena cava (Fig. 5). The long residence time at stable, high opacity enables simultaneous respiratory-and cardiacgated image acquisition, thus limiting motion artifacts to less than 100 µm [8] and enabling the extremely high resolution of these images (100-µm isotropic voxels) and, in turn, the visualization of submillimeter features in the vasculature. The ability to perform gated 9-msec acquisitions of data based on both cardiac and respiratory telemetry allowed for the acquisition of 4D (3D spatial + time) cardiac images of the beating heart and the display of the images as a cine series (Fig. 6). ...
... Therefore, in order for a feature to be clearly visible, its opacity needs to be greater than both the contrast resolution limit and the noise limit. Also, it is important to note that physiologic motion such as cardiac and respiratory motion usually reduces the effective spatial resolution [8]. Noise limits are primarily dependent on the scanner used. ...
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.
... Rodent respiratory and cardiac rates are up to an order of magnitude faster than in human subjects and animals cannot be instructed to hold their breath during imaging. Constant motion can degrade image quality with blurring and artifacts, so animals are placed on an MRcompatible ventilator Mai et al., 2005) that controls the breathing cycle. Positioning of the lung is precisely reproduced from breath to breath (Mai et al., 2005) as the MR-compatible ventilator triggers the MR scanner to acquire data only during a specified phase of the breathing cycle, such as at end-expiration or at full inspiratory volume. ...
... Constant motion can degrade image quality with blurring and artifacts, so animals are placed on an MRcompatible ventilator Mai et al., 2005) that controls the breathing cycle. Positioning of the lung is precisely reproduced from breath to breath (Mai et al., 2005) as the MR-compatible ventilator triggers the MR scanner to acquire data only during a specified phase of the breathing cycle, such as at end-expiration or at full inspiratory volume. Heart motion can also be frozen by providing a second image gating signal synchronized with the cardiac cycle (Brau et al., 2004). ...
In vivo magnetic resonance microscopy (MRM) of the small animal lung has become a valuable research tool, especially for preclinical studies. MRM offers a noninvasive and nondestructive tool for imaging small animals longitudinally and at high spatial resolution. We summarize some of the technical and biologic problems and solutions associated with imaging the small animal lung and describe several important pulmonary disease applications. A major advantage of MR is direct imaging of the gas spaces of the lung using breathable gases such as helium and xenon. When polarized, these gases become rich MR signal sources. In animals breathing hyperpolarized helium, the dynamics of gas distribution can be followed and airway constrictions and obstructions can be detected. Diffusion coefficients of helium can be calculated from diffusion-sensitive images, which can reveal micro-structural changes in the lungs associated with pathologies such as emphysema and fibrosis. Unlike helium, xenon in the lung is absorbed by blood and exhibits different frequencies in gas, tissue, or erythrocytes. Thus, with MR imaging, the movement of xenon gas can be tracked through pulmonary compartments to detect defects of gas transfer. MRM has become a valuable tool for studying morphologic and functional changes in small animal models of lung diseases.
... Previously, we have used x-ray radiopaque beads surgically glued to the abdominal diaphragmatic surface and cardiac ventricles of anesthetized, mechanically ventilated rats to describe the range of motion using digital microradiography with respiratory gating methods [22]. For this study, a mouse phantom similar to the CT human "pocket phantom" [5] was fabricated using a number of polyethylene spheres (http://www.cospheric.com/) with diameters of~0.7 mm. ...
Small animal imaging has become essential in evaluating new cancer therapies as they are translated from the preclinical to clinical domain. However, preclinical imaging faces unique challenges that emphasize the gap between mouse and man. One example is the difference in breathing patterns and breath-holding ability, which can dramatically affect tumor burden assessment in lung tissue. As part of a co-clinical trial studying immunotherapy and radiotherapy in sarcomas, we are using micro-CT of the lungs to detect and measure metastases as a metric of disease progression. To effectively utilize metastatic disease detection as a metric of progression, we have addressed the impact of respiratory gating during micro-CT acquisition on improving lung tumor detection and volume quantitation. Accuracy and precision of lung tumor measurements with and without respiratory gating were studied by performing experiments with in vivo images, simulations, and a pocket phantom. When performing test-retest studies in vivo, the variance in volume calculations was 5.9% in gated images and 15.8% in non-gated images, compared to 2.9% in post-mortem images. Sensitivity of detection was examined in images with simulated tumors, demonstrating that reliable sensitivity (true positive rate (TPR) ≥ 90%) was achievable down to 1.0 mm³ lesions with respiratory gating, but was limited to ≥ 8.0 mm³ in non-gated images. Finally, a clinically-inspired “pocket phantom” was used during in vivo mouse scanning to aid in refining and assessing the gating protocols. Application of respiratory gating techniques reduced variance of repeated volume measurements and significantly improved the accuracy of tumor volume quantitation in vivo.
... Previously, we have used radiopaque beads surgically glued to the abdominal diaphragmatic surface and cardiac ventricles of anesthetized, mechanically ventilated rats to describe the range of motion using digital microradiography with respiratory gating methods 17 . For this study, a mouse phantom similar to the CT human "pocket phantom" 5 was fabricated using a number of polyethylene spheres (http://www.cospheric.com/) with diameters of ~0.7 mm. ...
... Even if respiratory gating is sufficiently effective at reducing chest and diaphragm blurring, our last experiment results indicate that respiratory gating is not sufficient for taking into account all movements (Fig. 6, experiment 6). 21 This confirms that intrinsic movements could be considered a limiting factor where coregistration accuracy is concerned. ...
... Even if respiratory gating is sufficiently effective at reducing chest and diaphragm blurring, our last experiment results indicate that respiratory gating is not sufficient for taking into account all movements (Fig. 6, experiment 6). 21 This confirms that intrinsic movements could be considered a limiting factor where coregistration accuracy is concerned. ...
We use high-resolution [Formula: see text] data in multiple experiments to estimate the sources of error during coregistration of images acquired on separate preclinical instruments. In combination with experiments with phantoms, we completed in vivo imaging on mice, aimed at identifying the possible sources of registration errors, caused either by transport of the animal, movement of the animal itself, or methods of coregistration. The same imaging cell was used as a holder for phantoms and animals. For all procedures, rigid coregistration was carried out using a common landmark coregistration system, placed inside the imaging cell. We used the fiducial registration error and the target registration error to analyze the coregistration accuracy. We found that moving an imaging cell between two preclinical devices during a multimodal procedure gives an error of about [Formula: see text] at most. Therefore, it could not be considered a source of coregistration errors. Errors linked to spontaneous movements of the animal increased with time, to nearly 1 mm at most, excepted for body parts that were properly restrained. This work highlights the importance of animal intrinsic movements during a multiacquisition procedure and demonstrates a simple method to identify and quantify the sources of error during coregistration.
... To counteract the effects of motion associated with respiration and to control lung volume, animals can be supported by a ventilator. This way, the position of the lungs is precisely reproduced from one breath to the next [47,77], and the image acquisition can be triggered as specific phases in the respiratory cycle, where motion is minimal (end expiration or full inspiration). To achieve ventilatorsynchronous acquisition, animals have to be intubated with an endotracheal tube, and closely monitored. ...
This article has no abstract.
... It is noted that in both Figs. 4(e) and 4(f), the speed values in the area off the visible vessels are not zero, which might be caused by blood flow in deep capillaries and other biological motions [21,29]. ...
Laser speckle contrast imaging (LSCI) is a simple yet powerful tool to image blood flow. However, traditional LSCI has limited quantitative analysis capabilities due to various factors affecting flow speed evaluation, including illumination intensity, scattering from static tissues, and mathematical complexity of blood flow estimation. Here, we present a frequency-domain laser speckle imaging (FDLSI) method that can directly measure absolute flow speed. In phantom experiments, the measured flow speed agreed well with the preset actual values (10% deviation). Furthermore, in vivo experiments demonstrated that FDLSI was minimally affected by illumination condition changes.
... W ith the technical advances in optics and the development of novel fluorescent reporters and probes, intravital microscopy is entering a new era of in vivo high-resolution real-time imaging, helping to answer biological questions under physiological conditions [1][2][3][4] . Unfortunately, naturally occurring periodic and random motion artifacts continue to pose one of the biggest challenges for intravital imaging 5 . Image degradation by motion artifacts is directly proportional to the spatial resolution: at lower resolutions there are fewer effects whereas motion can become the limiting factor at higher spatial resolutions. ...
Intravital fluorescence microscopy, through extended penetration depth and imaging resolution, provides the ability to image at cellular and subcellular resolution in live animals, presenting an opportunity for new insights into in vivo biology. Unfortunately, physiological induced motion components due to respiration and cardiac activity are major sources of image artifacts and impose severe limitations on the effective imaging resolution that can be ultimately achieved in vivo. Here we present a novel imaging methodology capable of automatically removing motion artifacts during intravital microscopy imaging of organs and orthotopic tumors. The method is universally applicable to different laser scanning modalities including confocal and multiphoton microscopy, and offers artifact free reconstructions independent of the physiological motion source and imaged organ. The methodology, which is based on raw data acquisition followed by image processing, is here demonstrated for both cardiac and respiratory motion compensation in mice heart, kidney, liver, pancreas and dorsal window chamber.
... Tissue movement imposes a practical limitation on the imaging resolution regardless of the physical limitation of the instrument [5]. This is particularly true for high-resolution imaging modalities such as confocal and multiphoton microscopy, optical coherence tomography, and super-resolution microscopy. ...
Intravital microscopy has emerged in the recent decade as an indispensible imaging modality for the study of the micro-dynamics of biological processes in live animals. Technical advancements in imaging techniques and hardware components, combined with the development of novel targeted probes and new mice models, have enabled us to address long-standing questions in several biology areas such as oncology, cell biology, immunology and neuroscience. As the instrument resolution has increased, physiological motion activities have become a major obstacle that prevents imaging live animals at resolutions analogue to the ones obtained in vitro. Motion compensation techniques aim at reducing this gap and can effectively increase the in vivo resolution. This paper provides a technical review of some of the latest developments in motion compensation methods, providing organ specific solutions.
... This is especially true for imaging of the neck, thorax and upper abdomen. Although image comparisons between gated and nongated images are not shown, the clarity and spatial Furthermore, in a recent publication (31), it was shown that without gating, the errors associated with motion of the lungs (and thus presumably in tumors placed over the thorax) could be on the order of a few millimeters. Imaging regions distant from the lungs and heart, such as the lower abdomen and legs, is far less problematic in relation to motion control and does not strictly require gated imaging. ...
X-ray based micro-computed tomography (CT) and micro-digital subtraction angiography (DSA) are important non-invasive imaging modalities for following tumorogenesis in small animals. To exploit these imaging capabilities further, the two modalities were combined into a single system to provide both morphological and functional data from the same tumor in a single imaging session. The system is described and examples are given of imaging implanted fibrosarcoma tumors in rats using two types of contrast media: (a) a new generation of blood pool contrast agent containing iodine with a concentration of 130 mg/mL (Fenestra TM VC, Alerion Biomedical, San Diego, CA, USA) for micro-CT and (b) a conventional iodinated contrast agent (Isovue 1-370 mg/mL iodine, trademark of Bracco Diagnostics, Princeton, NJ, USA) for micro-DSA. With the blood pool contrast agent, the 3D vascular architecture is revealed in exquisite detail at 100 mm resolution. Micro-DSA images, in perfect registration with the 3D micro-CT datasets, provide complementary functional information such as mean transit times and relative blood flow through the tumor. This imaging approach could be used to understand tumor angiogenesis better and be the basis for evaluating anti-angiogenic therapies.
... Magnetic resonance imaging (MRI) with natural high soft tissue contrasts achieves a sub-millimetre three-dimensional (3D) resolution that makes it a proper method to study in vivo some models in rats. Nowadays, two kinds of MRI systems are used for rat imaging: dedicated high magnetic field magnets (hfMRI)234567 rather present in specialized imaging centres for small animal imaging and clinical MRI (cMRI) devices widely spread but with accessibility for small animal imaging limited in time8910111213. Low field MRI devices have been very little considered yet for in vivo MRI in rat [14]. ...
Objective: This work aims at demonstrating the interest of low field magnetic resonance imaging (MRI) for in vivo imaging in rat. Material and methods: MRI is performed using an open resistive 0.1 T magnet with a homogeneous zone measuring 6 cm × 10 cm × 10 cm. Rats under isoflurane gaseous anesthesia are placed in a technical cell for small animal imaging. Radio frequency solenoidal coils are developed for each application. Gradient echo T1 and T2-weighted sequences are programmed to acquire 3D data in vivo in rats. Results: Acquisition times are comprised between 1 min for scout sequences and 2 h 10 min to acquire volumes with in-plane pixel size ranging from 0.37 to 1 mm and a slice thickness between 0.37 mm and 1.88 mm. Information obtained from our acquisitions are comparable to those reported in the literature about five applications: 1: intracerebral glioma; 2: cerebral contrast enhancement using manganese; 3: description of knee joint anatomy; 4: cardiac dynamic acquisition synchronized on cardiac rhythm; 5: whole body (thorax and abdomen) acquisition. Conclusion: Rat MRI using an 0.1 T device allows to answer to the biological questions studied in this work at the expense of long acquisition times compared to high field MRI. Low field MRI offers economical and technical characteristics that could make it attractive to a non-specialized scientific community aiming at realizing daily MRI in vivo in rat.
... The spatial resolution of the mouse cardiac CT images collected using the present protocol is comparable to the published results obtained by prospective gating and ventilation 4 and slightly better than the results from retrospective gating. 5 In addition to the system spatial resolution, the quality of the mouse cardiac CT images also depends on the cycle-to-cycle repeatability of the respiratory and cardiac motions, which has been shown by a recent study to be on the order of 50-100 m. 26 Thus it is desirable for the micro-CT scanner to have a system spatial resolution in the comparable range. The 10% system MTF of the present scanner is 6.2 lp/mm, as compared to 1.9-3.1 lp/mm of the systems used for the prior cardiac micro-CT imaging studies. ...
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
... The beating was modelled by a 0.35-mm radial oscillation of the wall associated with a longitudinal twisting of 3° [16]. The breathing was modelled by a 2-mm transverse oscillation of the whole ventricle [17]. The breathing oscillation direction was modelled perpendicularly and tangentially to the defect as well. ...
Background
There is a growing interest in developing non-invasive imaging techniques permitting infarct size (IS) measurements in mice. The aim of this study was to validate the high-resolution rodent Linoview single photon emission computed tomography (SPECT) system for non-invasive measurements of IS in mice by using a novel algorithm independent of a normal database, in comparison with histology.
Methods
Eleven mice underwent a left coronary artery ligature. Seven days later, animals were imaged on the SPECT 2h30 after injection of 173 ± 27 MBq of Tc-99m-sestamibi. Mice were subsequently killed, and their hearts were excised for IS determination with triphenyltetrazolium chloride (TTC) staining. SPECT images were reconstructed using the expectation maximization maximum likelihood algorithm, and the IS was calculated using a novel algorithm applied on the 20-segment polar map provided by the commercially available QPS software (Cedars-Sinai Medical Center, CA, USA). This original method is attractive by the fact that it does not require the implementation of a normal perfusion database.
Results
Reconstructed images allowed a clear delineation of the left ventricles borders in all mice. No significant difference was found between mean IS determined by SPECT and by TTC staining [37.9 ± 17.5% vs 35.6 ± 17.2%, respectively (P = 0.10)]. Linear regression analysis showed an excellent correlation between IS measured on the SPECT images and IS obtained with TTC staining (y = 0.95x + 0.03 (r = 0.97; P < 0.0001)), without bias, as demonstrated by the Bland-Altman plot.
Conclusion
Our results demonstrate the accuracy of the method for the measurement of myocardial IS in mice with the Linoview SPECT system.
... Moreover, besides the heart motion itself, anatomic positioning of the mouse heart in space and time over the cardiac cycle is affected by motion of other organs or tissues in the chest, such as lungs, diaphragm, ribs, and to a lesser extent by blood motion in large vessels. All these physiological events and motion considerations limit spatial resolution to about 100 μm (Maï et al., 2005). Prospective or retrospective single (ECG) or dual (ECG and respiration) gating strategies have been therefore implemented to improve mouse cardiac image quality. ...
This overview first summarizes the last decade of continuous developments and improvements in pre-clinical imaging methods that are now essential tools for in vivo evaluation of cardiac morphology and function in living mice, involving nuclear emission of labeled molecules (micro-PET and micro-SPECT) and electromagnetic wave interactions with biological tissues (micro-CT and micro-MRI). In the following, and for better understanding, the basic physical principles and specific technical innovations of the aforementioned imaging methods are reviewed. Specificity, sensitivity, and spatial and temporal resolutions, together with the corresponding advantages and weaknesses of each method are then discussed, and cardiac image-acquisition protocols and illustrative examples are given for each modality. Emerging hybrid cardiac imaging is also presented and illustrated. Then, recent biological insights provided by mouse cardiac imaging are presented. Finally, imaging strategies in mouse cardiac phenotyping involving the aforementioned methods, adding metabolic and molecular information to morphological data, are emphasized and discussed. Curr. Protoc. Mouse Biol. 2:129-144 © 2012 by John Wiley & Sons, Inc.
Copyright © 2012 John Wiley & Sons, Inc.
... Furthermore, measurements are often profoundly affected by respiratory and cardiac motion. Thus, motion compensation and tissue stabilization are two major practical challenges currently faced by researchers using intravital microscopy in murine models 6 . To overcome these issues, a number of different approaches [7][8][9][10][11][12][13][14] have been proposed, including the use of restraining cover slips (Supplementary Fig. S1) or suctioning devices. ...
Real-time imaging of moving organs and tissues at microscopic resolutions represents a major challenge in studying the complex biology of live animals. Here we present a technique based on a novel stabilizer setup combined with a gating acquisition algorithm for the imaging of a beating murine heart at the single-cell level. The method allows serial in vivo fluorescence imaging of the beating heart in live mice in both confocal and nonlinear modes over the course of several hours. We demonstrate the utility of this technique for in vivo optical sectioning and dual-channel time-lapse fluorescence imaging of cardiac ischaemia. The generic method could be adapted to other moving organs and thus broadly facilitate in vivo microscopic investigations.
... On a longer term, however, the liquid-jet x-ray source can potentially be scaled to higher powers because of the regenerative nature of the anode (Hemberg et al 2004). Lung and heart gating can reduce motion blur to less than 100 μm (Maï et al 2005), but to obtain lower than 10 μm will be a challenge. Another technique is to use mechanical breathing and stop it for a short time while taking an image. ...
We demonstrate a laboratory method for imaging small blood vessels using x-ray propagation-based phase-contrast imaging and carbon dioxide (CO(2)) gas as a contrast agent. The limited radiation dose in combination with CO(2) being clinically acceptable makes the method promising for small-diameter vascular visualization. We investigate the possibilities and limitations of the method for small-animal angiography and compare it with conventional absorption-based x-ray angiography. Photon noise in absorption-contrast imaging prevents visualization of blood vessels narrower than 50 µm at the highest radiation doses compatible with living animals, whereas our simulations and experiments indicate the possibility of visualizing 20 µm vessels at radiation doses as low as 100 mGy. Experimental computed tomography of excised rat kidney shows blood vessels of diameters down to 60 µm with improved image quality compared to absorption-based methods. With our present prototype x-ray source, the acquisition time for a tomographic dataset is approximately 1 h, which is long compared to the 1-20 min common for absorption-contrast micro-CT systems. Further development of the liquid-metal-jet microfocus x-ray sources used here and high-resolution x-ray detectors shows promise to reduce exposure times and make this high-resolution method practical for imaging of living animals.
... To counteract the effects of motion associated with respiration and to control lung volume, animals can be supported by a ventilator. This way, the position of the lungs is precisely reproduced from one breath to the next (Hedlund, Cofer et al. 2000) (Mai, Badea et al. 2005), and the image acquisition can be triggered as specific phases in the respiratory cycle, where motion is minimal (end expiration or full inspiration). To achieve ventilator-synchronous acquisition, animals have to be intubated with an endotracheal tube, and closely monitored. ...
MRI, one of the major clinical imaging modalities, has gained an important role in studying small animal models, e.g., rats and mice. But imaging rodents comes with challenges, since the image resolution needs to be ~ 3000-times higher to resolve anatomical details at a level comparable to clinical imaging. A resolution on the order of 100 microns or less redefines MR imaging as MR microscopy. We discuss in this chapter the basic components of the MR imaging chain, with a particular emphasis on small animal imaging demands: from hardware design to basic physical principles of MR image formation, and contrast mechanisms. We discuss special considerations of animal preparation for imaging, and staining methods to enhance contrast. Attention is given to factors that increase sensitivity, including exogenous contrast agents, high performance radiofrequency detectors, and advanced MR encoding sequences. Among these, diffusion tensor imaging and tractography add novel information on white matter tracts, helping to better understand important aspects of development and neurodegeneration. These developments open avenues for efficient phenotyping of small animal models, in vivo - to include anatomical as well as functional estimates, or ex-vivo - with exquisite anatomical detail. The need for higher resolution results in larger image arrays that need to be processed efficiently. We discuss image-processing approaches for quantitative characterization of animal cohorts, and building population atlases. High throughput is essential for these methods to become practical. We discuss current trends for increasing detector performance, the use of cryoprobes, as well as strategies for imaging multiple animals at the same time. Ultimately, the development of highly specific probes, with the possibility to be used in multimodal imaging, will offer new insights into histology. MRM, alone or in combination with other imaging modalities, will increase the knowledge of fundamental biological processes, help understanding the genetic basis of human diseases, and test pharmacological interventions.
... An inherent problem with lung imaging is motion, since the lung and heart are dynamic organs. Others have shown that, even under the most ideal conditions, optimal resolution of live animal lung imaging is about 50-100 mm [18]. Although motional blurring cannot be avoided, it can be minimized. ...
Pulmonary computational fluid dynamics models require that three-dimensional images be acquired over multiple points in the dynamic breathing cycle without breath holds or changes in ventilatory mechanics. With small animals, these requirements can result in long imaging times (∼90 minutes), over which lung mechanics, such as compliance, may gradually change if not carefully monitored and controlled. These changes, caused by derecruitment of parenchymal tissue, are manifested as an upward drift in peak inspiratory pressure (PIP) or by changes in the pressure waveform and/or lung volume over the course of the experiment. We demonstrate highly repeatable mechanical ventilation in anesthetized rats over a long duration for dynamic lung x-ray computed tomography (CT) imaging. We describe significant updates to a basic commercial ventilator that was acquired for these experiments. Key to achieving consistent results was the implementation of periodic deep breaths, or sighs, of extended duration to maintain lung recruitment. In addition, continuous monitoring of breath-to-breath pressure and volume waveforms and long-term trends in PIP and flow provide diagnostics of changes in breathing mechanics.
... Rat proton images (Fig. 7) demonstrate a more enhanced sensitivity to the susceptibility differences (or T 2 * ) inside an animal brain at 21.1 T, which may help in distinguishing brain anatomy features. No motion compensation was used in the present imaging experiments; thus, some blur of MR images could be present because of the incidence of in vivo motion [31]. It was our intention to reach a maximum available resolution using a volume coil during the limited time available in vivo. ...
The first in vivo sodium and proton magnetic resonance (MR) images and localized spectra of rodents were attained using the wide bore (105 mm) high resolution 21.1-T magnet, built and operated at the National High Magnetic Field Laboratory (Tallahassee, FL, USA). Head images of normal mice (C57BL/6J) and Fisher rats (approximately 250 g) were acquired with custom designed radiofrequency probes at frequencies of 237/900 MHz for sodium and proton, respectively. Sodium MR imaging resolutions of approximately 0.125 microl for mouse and rat heads were achieved by using a 3D back-projection pulse sequence. A gain in SNR of approximately 3 for sodium and approximately 2 times for proton were found relative to corresponding MR images acquired at 9.4 T. 3D Fast Low Angle Shot (FLASH) proton mouse images (50x50x50 microm(3)) were acquired in 90 min and corresponding rat images (100x100x100 microm(3)) within a total time of 120 min. Both in vivo large rodent MR imaging and localized spectroscopy at the extremely high field of 21.1 T are feasible and demonstrate improved resolution and sensitivity valuable for structural and functional brain analysis.
... The radiographic system used for this work has been constructed for functional small animal imaging with both highspatial resolution ͑100ϫ 100 m 2 pixels͒ and high-temporal resolution ͑10 ms exposures at 10 frames/s͒. 2,[23][24][25][26][35][36][37] The system employs a unique LABVIEW ͑National Instruments, Austin, TX͒ sequencer that allows synchronization of breathing, contrast injection, radiographic exposure, and digital frame acquisition with cardiac cycle. Images were acquired with x-ray techniques ͑80 kVp, 160 mA, and 10 ms͒ optimized for small animal DSA and thus limited beam hardening effects. ...
The use of preclinical rodent models of disease continues to grow because these models help elucidate pathogenic mechanisms and provide robust test beds for drug development. Among the major anatomic and physiologic indicators of disease progression and genetic or drug modification of responses are measurements of blood vessel caliber and flow. Moreover, cardiopulmonary blood flow is a critical indicator of gas exchange. Current methods of measuring cardiopulmonary blood flow suffer from some or all of the following limitations--they produce relative values, are limited to global measurements, do not provide vasculature visualization, are not able to measure acute changes, are invasive, or require euthanasia.
In this study, high-spatial and high-temporal resolution x-ray digital subtraction angiography (DSA) was used to obtain vasculature visualization, quantitative blood flow in absolute metrics (ml/min instead of arbitrary units or velocity), and relative blood volume dynamics from discrete regions of interest on a pixel-by-pixel basis (100 x 100 microm2).
A series of calibrations linked the DSA flow measurements to standard physiological measurement using thermodilution and Fick's method for cardiac output (CO), which in eight anesthetized Fischer-344 rats was found to be 37.0 +/- 5.1 ml/min. Phantom experiments were conducted to calibrate the radiographic density to vessel thickness, allowing a link of DSA cardiac output measurements to cardiopulmonary blood flow measurements in discrete regions of interest. The scaling factor linking relative DSA cardiac output measurements to the Fick's absolute measurements was found to be 18.90 x CODSA = COFick.
This calibrated DSA approach allows repeated simultaneous visualization of vasculature and measurement of blood flow dynamics on a regional level in the living rat.
... Accurate physiological monitoring of the animal, as well as establishing homeostatic conditions inside the magnet, is mandatory during the whole imaging procedure to be sure that images were obtained in the best conditions for the animal and also to avoid blurring of the images. All these requirements remain difficult to achieve properly, some limits being due to physiological movements (Mai et al 2005). ...
The rationale of this work is to point out the relevance of in vivo MR images of mice obtained using a dedicated low-field system. For this purpose a small 0.1 T water-cooled electro-magnet and solenoidal radio frequency (RF) transmit-receive coils were used. All MR images were acquired in three-dimensional (3D) mode. An isolation cell was designed allowing easy placement of the RF coils and simple delivery of gaseous anesthesia as well as warming of the animal. Images with and without contrast agent were obtained in total acquisition times on the order of half an hour to four hours on normal mice as well as on animals bearing tumors. Typical in plane pixel dimensions range from 200 x 200 to 500 x 500 microm(2) with slice thicknesses ranging between 0.65 and 1.50 mm. This work shows that, besides light installation and low cost, dedicated low-field MR systems are suitable for small rodents imaging, opening this technique even to small research units.
... The precision with which one can control motion, using gating, defines the resolution limit that can be attained in such studies, and the magnitude of motion. Mai et al. (Mai et al., 2005) performed a study in rats to evaluate how reproducibly the heart and lung return to the same position through multiple exposures required for acquisition of a micro-CT study. Radio-opaque beads were surgically glued on the diaphragm of anesthetized, mechanically ventilated rats. ...
Small-animal imaging has a critical role in phenotyping, 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 in vivo x-ray based small-animal imaging, with a focus on in vivo micro-computed tomography (micro-CT) and digital subtraction angiography (DSA). We present the principles, technologies, image quality parameters and types of applications. We show that both methods can be used not only to provide morphological, but also functional information, such as cardiac function estimation or perfusion. Compared to other modalities, x-ray based imaging is usually regarded as being able to provide higher throughput at lower cost and adequate resolution. The limitations are usually associated with the relatively poor contrast mechanisms and potential radiation damage due to ionizing radiation, although the use of contrast agents and careful design of studies can address these limitations. We hope that the information will effectively address how x-ray based imaging can be exploited for successful in vivo preclinical imaging.
... The scanning time for a complete projections set was 8– 10 min depending on the synchronization between the ECG and ventilation, that is, sometimes, the QRS complex does not ''occur'' every bit in the window placed at end-expiration and the sampling time is increased. A recent investigation suggests that with the proposed gating procedure, the position of the heart and the diaphragm can be reliably reproduced through the course of a physiologically controlled study to $100 mm [19]. Scanning was conducted with the animal placed at a source-to-object distance (SOD = 480 mm), an objectto-detector distance (ODD = 40 mm), and a source-todetector distance (SDD = 520 mm), resulting in aFigure 1. ...
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.
... Although the quality of images produced by this method of synchronizing imaging to biological motion is a testimony to its effectiveness, we have quantitatively assessed the precision of this method. We measured how consistently the diaphragm of the rat returned to the same point at each breath (21). The excursion of metal beads cemented to the abdominal surface of the diaphragm with reference to a stationary bead on spinal column was measured using an X-ray system with in-plane resolution of 20 m. ...
Small-animal imaging with magnetic resonance microscopy (MRM) has become an important tool in biomedical research. When MRM is used to image perfusion-fixed and "stained" whole mouse specimens, cardiopulmonary morphology can be visualized, nondestructively, in exquisite detail in all three dimensions. This capability can be a valuable tool for morphologic phenotyping of different mouse strains commonly used in genomics research. When these imaging techniques are combined with specialized methods for biological motion control and animal support, the lungs of the live, small animal can be imaged. Although in vivo imaging may not achieve the high resolution possible with a fixed specimen, dynamic functional studies and survival studies that follow the progression of pulmonary change related to disease or environmental exposure are possible. By combining conventional proton imaging with gas imaging, using hyperpolarized 3He, it is possible to image the tissue and gas compartments of the lung. This capability is illustrated in studies on an emphysema model in rats and on radiation damage of the lung. With further improvements in imaging and animal handling technology, we will be able to image faster and at higher resolutions, making MRM an even more valuable research tool.
... In contrast to studies based on analog hardware devices (6,(11)(12)(13)(14)(15)(16)(17)(18)(19)(20), we adopted a real-time gating software approach. In general, the performance of a gating protocol depends on three criteria: ECG noise reduction, filtering delay, and gating efficiency. ...
Mouse cardiac MR gating using ECG is affected by the hostile MR environment. It requires appropriate signal processing and correct QRS detection, but gating software methods are currently limited. In this study we sought to demonstrate the feasibility of digital real-time automatically updated gating methods, based on optimizing a signal-processing technique for different mouse strains. High-resolution MR images of mouse hearts and aortic arches were acquired using a chain consisting of ECG signal detection, digital signal processing, and gating signal generation modeled using Simulink (The MathWorks, Inc., Natick, MA, USA). The signal-processing algorithms used were respectively low-pass filtering, nonlinear passband, and wavelet decomposition. Both updated and nonupdated gating signal generation methods were tested. Noise reduction was assessed by comparison of the ECG signal-to-noise ratio (SNR) before and after each processing step. Gating performance was assessed by measuring QRS detection accuracy before and after online trigger-level adjustments. Low-pass filtering with trigger-level adjustment gave the best performance for mouse cardiovascular imaging using gradient-echo (GE), spin-echo (SE), and fast SE (FSE) sequences with minimum induced delay and maximum gating efficiency (99% sensitivity and R-peak detection). This simple digital gating interface will allow various gating strategies to be optimized for cardiovascular MR explorations in mice.
... Throughout the respiratory cycle, the thorax and upper abdomen experience motion, which causes artifacts in the CT image, including blurring of the lungs (39). Clinically, these artifacts can be avoided by acquiring the CT projections during a breath hold; however, this breath-hold technique is not feasible for imaging rodents because the acquisition time for the majority of commercially available micro-CT equipment ranges from 10 to 30 min per scan. ...
Lung morphology and function in human subjects can be monitored with computed tomography (CT). Because many human respiratory diseases are routinely modeled in rodents, a means of monitoring the changes in the structure and function of the rodent lung is desired. High-resolution images of the rodent lung can be attained with specialized micro-CT equipment, which provides a means of monitoring rodent models of lung disease noninvasively with a clinically relevant method. Previous studies have shown respiratory-gated images of intubated and respirated mice. Although the image quality and resolution are sufficient in these studies to make quantitative measurements, these measurements of lung structure will depend on the settings of the ventilator and not on the respiratory mechanics of the individual animals. In addition, intubation and ventilation can have unnatural effects on the respiratory dynamics of the animal, because the airway pressure, tidal volume, and respiratory rate are selected by the operator. In these experiments, important information about the symptoms of the respiratory disease being studied may be missed because the respiration is forced to conform to the ventilator settings. In this study, we implement a method of respiratory-gated micro-CT for use with anesthetized free-breathing rodents. From the micro-CT images, quantitative analysis of the structure of the lungs of healthy unconscious mice was performed to obtain airway diameters, lung and airway volumes, and CT densities at end expiration and during inspiration. Because the animals were free breathing, we were able to calculate tidal volume (0.09 +/- 0.03 ml) and functional residual capacity (0.16 +/- 0.03 ml).
... 11 Therefore, efficient synchronization techniques must associate cardiac gating with respiratory blanking, where the heart beat governed acquisitions are only enabled during at-rest periods (between breaths) to overcome the respiratory motion artifacts. 4,13,15 Cardiac gating is usually done by detecting the R peaks on the simultaneously recorded electrocardiogram (ECG), which are then used to trigger the consecutive image acquisition sequences. However, accomplishing an accurate R peak detection, hence a correct synchronization is often obstructed by the high corruption levels of the ECG signal due to electromagnetic effects. ...
Cardiac Magnetic Resonance Imaging (MRI) requires synchronization to overcome motion related artifacts caused by the heart's contractions and the chest wall movements during respiration. Achieving good image quality necessitates combining cardiac and respiratory gating to produce, in real time, a trigger signal that sets off the consecutive image acquisitions. This guarantees that the data collection always starts at the same point of the cardiac cycle during the exhalation phase. In this paper, we present a real time algorithm for extracting a cardiac-respiratory trigger signal using only one, adequately placed, ECG sensor. First, an off-line calculation phase, based on wavelet decomposition, is run to compute an optimal QRS filter. This filter is used, afterwards, to accomplish R peak detection, while a low pass filtering process allows the retrieval of the respiration cycle. The algorithm's synchronization capabilities were assessed during mice cardiac MRI sessions employing three different imaging sequences, and three specific wavelet functions. The prominent image enhancement gave a good proof of correct triggering. QRS detection was almost flawless for all signals. As for the respiration cycle retrieval it was evaluated on contaminated simulated signals, which were artificially modulated to imitate respiration. The results were quite satisfactory.
... MR acquisition can then be performed during the expiratory phase, which corresponds to minimal breathing motion. However, the fundamental resolution limit in any study employing synchronous and/or gated acquisition is determined by the reliability of the gating in capturing the moving anatomy at the same phase of the physiological cycle (4). Mouse cardiovascular MRI is particularly affected by cardiac and respiratory motion artifacts, exacerbated by the high heart and respiratory rates and the small cardiac anatomy of mice and by the high magnetic field strengths (Ͼ4.7 T) of the research systems (12,15). ...
Atherosclerosis initially develops predominantly at the aortic root and carotid origin, where effective visualization in mice requires efficient cardiac and respiratory gating. The present study sought to first compare the high-resolution MRI gating performance of two digital gating strategies using: 1) separate cardiac and respiratory signals (double-sensor); and 2) a single-sensor cardiorespiratory signal (ECG demodulation), and second, to apply an optimized processing technique to dynamic contrast-enhanced (CE) carotid origin vessel-wall imaging in mice. High-resolution MR mouse heart and aortic arch images were acquired by ECG signal detection, digital signal processing, and gating signal generation modeled using Simulink (MathWorks, USA). Double-sensor gating used a respiratory sensor while single-sensor gating used breathing-modulated ECG to generate a demodulated respiratory signal. Pre- and postcontrast T(1)-weighted images were acquired to evaluate vessel-wall enhancement with a gadolinium blood-pool agent (P792; Guerbet, France) at the carotid origin in vivo in ApoE(-/-) and C57BL/6 mice, using the optimized cardiorespiratory gating processing technique. Both strategies provided images with improved spatial resolution, less artifacts, and 100% correct transistor-to-transistor logic (TTL) signals. Image quality allowed vessel-wall enhancement measurement in all the ApoE(-/-) mice, with maximal (32%) enhancement 27 min postinjection. The study demonstrated the efficiency of both cardiorespiratory gating strategies for dynamic contrast-enhanced vessel-wall imaging.
Small animal imaging has become essential in evaluating new cancer therapies as they are translated from the preclinical to clinical domain. However, preclinical imaging faces unique challenges that emphasize the gap between mouse and man. One example is the difference in breathing patterns and breath-holding ability, which can dramatically affect tumor burden assessment in lung tissue. As part of a co-clinical trial studying immunotherapy and radiotherapy in sarcomas, we are using micro-CT of the lungs to detect and measure metastases as a metric of disease progression. To effectively utilize metastatic disease detection as a metric of progression, we have addressed the impact of respiratory gating during micro-CT acquisition on improving lung tumor detection and volume quantitation. Accuracy and precision of lung tumor measurements with and without respiratory gating were studied by performing experiments with in vivo images, simulations, and a pocket phantom. When performing test-retest studies in vivo , the variance in volume calculations was 5.9% in gated images and 15.8% in non-gated images, compared to 2.9% in post-mortem images. Sensitivity of detection was examined in images with simulated tumors, demonstrating that reliable sensitivity (true positive rate (TPR) ≥ 90%) was achievable down to 1.0 mm 3 lesions with respiratory gating, but was limited to ≥ 8.0 mm 3 in non-gated images. Finally, a clinically-inspired “pocket phantom” was used during in vivo mouse scanning to aid in refining and assessing the gating protocols. Application of respiratory gating techniques reduced variance of repeated volume measurements and significantly improved the accuracy of tumor volume quantitation in vivo .
Cellular or molecular dynamics and function are continuously changing in vivo. Their properties also vary under the influence of tissue micro-environment. Dynamic information of a cell or a molecule obtained through intravital imaging allows spatio-temporal analysis of various phenomena in a living body. However, a common hurdle in achieving high resolution imaging is the motion artifact which is caused by motion of organs, such as breathing, cardiac contractions, pulsatile blood flow or peristalsis. In this section, we will introduce the techniques for the fluorescence imaging in a living mouse in real time at cellular level resolution.
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.
Noninvasive cardiacpulmonary gating is proposed to improve the imaging resolution. It produces signals based on the cardiacpulmonary motion of an animal in realtime. The system with the noninvasive gating consists of a digital signal processor (DSP), an electrocardiography (ECG) detection circuit and a thermocouple circuit. An enhanced R wave detection algorithm based on zero crossing counts is used to adjust the low sample frequency associated with the respiratory rate of an animal. The thermocouple recognizes the respiration phase by sensing the temperature changes of the nasal airflow of an animal. The proposed gating can accurately generate the gating signal for freely breathing mice (weight of around 0.03 kg), and its respiratory signal is too weak to be detected. Apart from noninvasiveness, compared with other existing gating techniques, it occupies minimal space at lower cost. Actually, it can be used in microcomputed tomography (CT) and other systems needed to detect the cardiacpulmonary motion. Several tests validate that the proposed cardiacpulmonary gating can generate the gating signal as required. By using the gating technique, the image resolution is improved.
Few publications have compared non-invasive in vivo imaging with either gross or histopathology in rodent models. Many researchers are using either in vivo imaging or histology to test their hypotheses. In vivo imaging methods are ideal for longitudinal surveys to follow disease progression or to probe different disease processes that histopathology does not address, but in most cases imaging has been used in investigational toxicity studies rather than routine safety assessment. Evaluation of drugs for toxicity or efficacy typically uses gross pathology as an initial morphologic screening method followed by the gold standard, histopathology. Each of these methods has its own merits and, in most cases, can complement or validate one another. This chapter will focus on examples that utilize both imaging (in vivo or ex vivo) and pathologic analysis to compare and contrast the strengths and weaknesses of each morphological method.
A prototype physiologically gated micro-computed tomography (micro-CT) system based on a field emission micro-focus x-ray source has been developed for in vivo imaging of small animal models. The novel x-ray source can generate radiation with a programmable waveform that can be readily synchronized and gated with non-periodic physiological signals. The system performance is evaluated using phantoms and sacrificed and anesthetized mouse models. Prospective respiratory-gated CT images of anesthetized free-breathing mice are collected using this scanner at 100 msec temporal resolution and 101p/mm of 10% system MTF.
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.
Micro Computed Tomography (micro-CT) was suggested in biomedical research to investigate tissues and small animals. Its use to characterize bone structures, vessels (e.g. tumor vascularization), tumors and soft tissues such as lung parenchyma has been shown. When co-registered, micro-CT can add structural information to other small animal imaging modalities. However, due to fundamental CT principles, high-resolution imaging with micro-CT demands for high x-ray doses and long scan times to generate a sufficiently high signal-to-noise ratio. Long scan times in turn make the use of extravascular contrast agents difficult. Recently introduced flat-panel based mini-CT systems offer a valuable trade-off between resolution (~200 µm), scan time (0.5 s), applied x-ray dose and scan field-of-view. This allows for angiography scans and follow-up examinations using iodinated contrast agents having a similar performance compared to patient scans. Furthermore, dynamic examinations such as perfusion studies as well as retrospective motion gating are currently implemented using flat-panel CT.
This review summarizes applications of experimental CT in basic research and provides an overview of current hardware developments making CT a powerful tool to study tissue morphology and function in small laboratory animals such as rodents.
Magnetic resonance imaging (MRI) can be used in pre-clinical studies as a non-invasive imaging tool for assessing the morphological and functional impact of lung diseases and for evaluating the efficacy of potential treatments for airways diseases. Hyperpolarized gases ((3)He or (129)Xe) MRI provides insight into the lung ventilation function. Lung proton MRI provides information on lung diseases associated with inflammatory activity or with changes in lung tissue density. These imaging techniques can be implemented with non-invasive protocols appropriate for longitudinal investigations in small animal models of lung diseases. This chapter will detail two (3)He and proton lung MR imaging protocols applied on two models of lung pathology in rodents.
The interest in small animal models of human diseases has generated a need to design a computed tomography (CT) system that operates at a microscopic level. It is particularly important to be able to visualize the dramatic rhythmical motion of organs such as the heart and lungs. In order to evaluate the motion of the heart and lungs of small animals (rats and mice), we developed in the present study a high-resolution 4D in vivo-CT system for small animals that uses synchrotron radiation. To reduce motion artifacts and the radiation dose, the projections were synchronized with airway pressure, the ECG, the x-ray shutter and the CCD shutter. For cardiovascular imaging, a blood pool contrast agent was injected and the data sets were acquired at several ECG points during the end-expiratory phase. For imaging of the lungs, the data sets were acquired at several airway pressures during diastole. The dynamic motion of the cardiovascular system (the ventricles and coronary arteries) and small airways (diameter > 250 microm of rats and 125 microm of mice) was visualized. This high-resolution imaging tool may be very useful for the development of novel drugs in murine models, in addition to its use in the study of cardiovascular and respiratory physiology.
Carbon nanotube (CNT) based field emission x-ray source technology has recently been investigated for diagnostic imaging applications because of its attractive characteristics including electronic programmability, fast switching, distributed source, and multiplexing. The purpose of this article is to demonstrate the potential of this technology for high-resolution prospective-gated cardiac micro-CT imaging.
A dynamic cone-beam micro-CT scanner was constructed using a rotating gantry, a stationary mouse bed, a flat-panel detector, and a sealed CNT based microfocus x-ray source. The compact single-beam CNT x-ray source was operated at 50 KVp and 2 mA anode current with 100 microm x 100 microm effective focal spot size. Using an intravenously administered iodinated blood-pool contrast agent, prospective cardiac and respiratory-gated micro-CT images of beating mouse hearts were obtained from ten anesthetized free-breathing mice in their natural position. Four-dimensional cardiac images were also obtained by gating the image acquisition to different phases in the cardiac cycle.
High-resolution CT images of beating mouse hearts were obtained at 15 ms temporal resolution and 6.2 lp/mm spatial resolution at 10% of system MTF. The images were reconstructed at 76 microm isotropic voxel size. The data acquisition time for two cardiac phases was 44 +/- 9 min. The CT values observed within the ventricles and the ventricle wall were 455 +/- 49 and 120 +/- 48 HU, respectively. The entrance dose for the acquisition of a single phase of the cardiac cycle was 0.10 Gy.
A high-resolution dynamic micro-CT scanner was developed from a compact CNT microfocus x-ray source and its feasibility for prospective-gated cardiac micro-CT imaging of free-breathing mice under their natural position was demonstrated.
We describe the technique for in vivo cardiac-gated magnetic resonance imaging (MRI) in normal dogs and its application in two dogs with a large right atrial tumor. The dogs with a cardiac tumor were also imaged using contrast-enhanced magnetic resonance angiography (CE-MRA). Cardiac-gated MRI and CE-MRA are both feasible in animals with short acquisition times compatible with breath-hold imaging under anesthesia, and provide detailed two- and three-dimensional (3D) depiction of the cardiac anatomy and great vessels with or without contrast medium. Although cardiac MRI will not replace echocardiography, it is a powerful alternative technique to use when knowledge of the 3D anatomy of the vasculature is required, when precise volume measurements are needed or when myocardial characterization is indicated. As opposed to contrast-enhanced computed tomography angiography, cardiac MRI does not use ionizing radiation or iodinated contrast medium.
One fundamental limitation of spatial resolution for in vivo MR lung imaging is related to motion in the thoracic cavity. To overcome this limitation, several methods have been proposed, including scan-synchronous ventilation and the cardiac gating approach. However, with cardiac and ventilation triggered techniques, the use of a predetermined and constant sequence repetition time is not possible, resulting in variable image contrast. In this study, the potential of two "constant repetition time" approaches based on retrospective self-gating and signal averaging were investigated for lung imaging. Image acquisitions were performed at a very short echo time for visualization of the lung structures and the parenchyma. Highly spatially resolved images acquired using retrospective self-gating, signal averaging technique and conventional cardiorespiratory gating are presented and compared.
With the development of numerous mouse models of cancer, there is a tremendous need for an appropriate imaging technique to study the disease evolution. High-field T(2)-weighted imaging using PROPELLER (Periodically Rotated Overlapping ParallEL Lines with Enhanced Reconstruction) MRI meets this need. The two-shot PROPELLER technique presented here provides (a) high spatial resolution, (b) high contrast resolution, and (c) rapid and noninvasive imaging, which enables high-throughput, longitudinal studies in free-breathing mice. Unique data collection and reconstruction makes this method robust against motion artifacts. The two-shot modification introduced here retains more high-frequency information and provides higher signal-to-noise ratio than conventional single-shot PROPELLER, making this sequence feasible at high fields, where signal loss is rapid. Results are shown in a liver metastases model to demonstrate the utility of this technique in one of the more challenging regions of the mouse, which is the abdomen.
The capability to use high-resolution (3)He MRI to depict regional ventilation changes and airway narrowing in mice challenged with methacholine (MCh) offers the opportunity to gain new insights into the study of asthma. However, to fully exploit the value of this novel technique, it is important to move beyond visual inspection of the images toward automated and quantitative analysis. To address this gap, we describe a postprocessing approach to create ventilation difference maps to better visualize and quantify regional ventilation changes before and after MCh challenge. We show that difference maps reveal subtle changes in airway caliber, and highlight both focal and diffuse regional alterations in ventilation. Ventilation changes include both hypoventilation and compensatory areas of hyperventilation. The difference maps can be quantified by a histogram plot of the ventilation changes, in which the standard deviation increases with MCh dose (R(2) = 0.89). This method of analysis is shown to be more sensitive than simple threshold-based detection of gross ventilation defects.
Gating is necessary in cardio-thoracic small-animal imaging because of the physiological motions that are present during scanning. In small-animal computed tomography (CT), gating is mainly performed on a projection base because full scans take much longer than the motion cycle. This paper presents and discusses various gating concepts of small-animal CT, and provides examples of concrete implementation. Since a wide variety of small-animal CT scanner systems exist, scanner systems are discussed with respect to the most suitable gating methods. Furthermore, an overview is given of cardio-thoracic imaging and gating applications. The necessary contrast media are discussed as well as gating limitations. Gating in small-animal imaging requires the acquisition of a gating signal during scanning. This can be done extrinsically (additional hardware, e.g. electrocardiogram) or intrinsically from the projection data itself. The gating signal is used retrospectively during CT reconstruction, or prospectively to trigger parts of the scan. Gating can be performed with respect to the phase or the amplitude of the gating signal, providing different advantages and challenges. Gating methods should be optimized with respect to the diagnostic question, scanner system, animal model, type of narcosis and actual setup. The software-based intrinsic gating approaches increasingly employed give the researcher independence from difficult and expensive hardware changes.
The increasing use of small animals in basic research has spurred interest in new imaging methodologies. Digital subtraction angiography (DSA) offers a particularly appealing approach to functional imaging in the small animal. This study examines the optimal x-ray, molybdenum (Mo) or tungsten (W) target sources, and technique to produce the highest quality small animal functional subtraction angiograms in terms of contrast and signal-difference-to-noise ratio squared (SdNR2). Two limiting conditions were considered-normalization with respect to dose and normalization against tube loading. Image contrast and SdNR2 were simulated using an established x-ray model. DSA images of live rats were taken at two representative tube potentials for the W and Mo sources. Results show that for small animal DSA, the Mo source provides better contrast. However, with digital detectors, SdNR2 is the more relevant figure of merit. The W source operated at kVps >60 achieved a higher SdNR2. The highest SdNR2 was obtained at voltages above 90 kVp. However, operation at the higher potential results in significantly greater dose and tube load and reduced contrast quantization. A reasonable tradeoff can be achieved at tube potentials at the beginning of the performance plateau, around 70 kVp, where the relative gain in SdNR2 is the greatest.
X-ray based micro-computed tomography (CT) and micro-digital subtraction angiography (DSA) are important non-invasive imaging modalities for following tumorogenesis in small animals. To exploit these imaging capabilities further, the two modalities were combined into a single system to provide both morphological and functional data from the same tumor in a single imaging session. The system is described and examples are given of imaging implanted fibrosarcoma tumors in rats using two types of contrast media: (a) a new generation of blood pool contrast agent containing iodine with a concentration of 130 mg/mL (Fenestratrade mark VC, Alerion Biomedical, San Diego, CA, USA) for micro-CT and (b) a conventional iodinated contrast agent (Isovue(R)-370 mg/mL iodine, trademark of Bracco Diagnostics, Princeton, NJ, USA) for micro-DSA. With the blood pool contrast agent, the 3D vascular architecture is revealed in exquisite detail at 100 microm resolution. Micro-DSA images, in perfect registration with the 3D micro-CT datasets, provide complementary functional information such as mean transit times and relative blood flow through the tumor. This imaging approach could be used to understand tumor angiogenesis better and be the basis for evaluating anti-angiogenic therapies.
Because of superior soft-tissue contrast compared to other imaging techniques, non-invasive abdominal magnetic resonance imaging (MRI) is ideal for monitoring organ regeneration, tissue repair, cancer stage, and treatment effects in a wide variety of experimental animal models. Currently, sophisticated MR protocols, including technically demanding procedures for motion artefact compensation, achieve an MRI resolution limit of < 100 microm under ideal conditions. However, such a high spatial resolution is not required for most experimental rodent studies. This article describes both a detailed imaging protocol for MR data acquisition in a ubiquitously and commercially available 1.5 T MR unit and 3-dimensional volumetry of organs, tissue components, or tumors. Future developments in MR technology will allow in vivo investigation of physiological and pathological processes at the cellular and even the molecular levels. Experimental MRI is crucial for non-invasive monitoring of a broad range of biological processes and will further our general understanding of physiology and disease.
The objective of this study was to develop a technique for dynamic respiratory imaging using retrospectively gated high-speed micro-CT imaging of free-breathing mice. Free-breathing C57Bl6 mice were scanned using a dynamic micro-CT scanner, comprising a flat-panel detector mounted on a slip-ring gantry. Projection images were acquired over ten complete gantry rotations in 50 s, while monitoring the respiratory motion in synchrony with projection-image acquisition. Projection images belonging to a selected respiratory phase were retrospectively identified and used for 3D reconstruction. The effect of using fewer gantry rotations--which influences both image quality and the ability to quantify respiratory function--was evaluated. Images reconstructed using unique projections from six or more gantry rotations produced acceptable images for quantitative analysis of lung volume, CT density, functional residual capacity and tidal volume. The functional residual capacity (0.15 +/- 0.03 mL) and tidal volumes (0.08 +/- 0.03 mL) measured in this study agree with previously reported measurements made using prospectively gated micro-CT and at higher resolution (150 microm versus 90 microm voxel spacing). Retrospectively gated micro-CT imaging of free-breathing mice enables quantitative dynamic measurement of morphological and functional parameters in the mouse models of respiratory disease, with scan times as short as 30 s, based on the acquisition of projection images over six gantry rotations.
Implementation and evaluation of retrospective respiratory and cardiac gating of mice and rats using a flat-panel volume-CT prototype (fpVCT).
Respiratory and cardiac gating was implemented by equipping a fpVCT with a small animal monitoring unit. ECG and breathing excursions were recorded and 2 binary gating signals derived. Mice and rats were scanned continuously over 80 seconds after administration of blood-pool contrast media. Projections were chosen to reconstruct volumes that fall within defined phases of the cardiac/respiratory cycle.
Multireader analysis indicated that in gated still images motion artifacts were strongly reduced and diaphragm, tracheobronchial tract, heart, and vessels sharply delineated. From 4D series, functional data such as respiratory tidal volume and cardiac ejection fraction were calculated and matched well with values known from literature.
Implementation of retrospective gating in fpVCT improves image quality and opens new perspectives for functional cardiac and lung imaging in small animals.
A temperature control system consisting of a thermistor, signal processor, and computer algorithm was developed for magnetic resonance (MR) microscopy of small live animals. With control of body temperature within +/- 0.2 degree C of the set point, heart rate is stabilized and, in turn, repetition time (TR) during cardiac-gated studies is less variable. Thus, image quality and resolution are improved.
This review emphasizes some of the challenges and benefits of in vivo imaging of the small animal lung. Because mechanical ventilation plays a key role in high-quality, high-resolution imaging of the small animal lung, the article focuses particularly on the problems of ventilation support, control of breathing motion and lung volume, and imaging during different phases of the breathing cycle. Solutions for these problems are discussed primarily in relation to magnetic resonance imaging, both conventional proton imaging and the newer, hyperpolarized helium imaging of pulmonary airways. Examples of applications of these imaging solutions to normal and diseased lung are illustrated in the rat and guinea pig. Although difficult to perform, pulmonary imaging in the small animal can be a valuable source of information not only for the normal lung, but also for the lung challenged by disease.
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.
Using in vivo magnetic resonance microscopy, registered 1H and hyperpolarized 3He images of the rat lung were obtained with a resolution of 0.098 × 0.098 × 0.469 mm (4.5 × 10–3 mm3). The requisite stability and SNR was achieved through an integration of scan-synchronous ventilation, dual-frequency RF coils, anisotropic projection encoding, and variable RF excitation. The total acquisition time was 21 min for the 3He images and 64 min for the 1H image. Airways down to the 6th and 7th orders are clearly visible. Magn Reson Med 45:365–370, 2001. © 2001 Wiley-Liss, Inc.
The limits to resolution in NMR imaging, arising from available signal-to-noise ratios, are examined. It is shown that NMR microscopy can achieve a transverse resolution comparable with the optical limit of order 1 μm but with an imaging time of around 1000 s. The specific imaging techniques of Fourier zeugmatography (FZ) and filtered back projection (FBP) are considered in detail and in particular it is shown that these techniques treat the signal and noise differently. Significant differences arise when smoothing filters are applied and exact expressions are given for image signal to noise and resolution for both FZ and FBP under conditions of optimal and asymptotic broadening.
Sensitivity considerations suggest that a voxel resolution of (10 μm)3 should be possible for NMR microscopy at 400 MHz. It is shown however that diamagnetic susceptibility inhomogeneity can degrade this resolution and examples of image reconstruction for simple geometries are given. This degradation increases with the size of the polarizing field and the optimal resolution cannot be restored by simply increasing the gradient strength unless appropriate echo summation methods are used. Alternatively, susceptibility effects can be avoided by employing pulse sequences which refocus magnetic inhomogeneity but retain the k-space evolution due to the linear gradient.
Self-diffusion of nuclear spins in the presence of an imaging magnetic field gradient imparts a broadening to the spectrum. This affects the spatial resolution in NMR imaging. The resulting limit is not simply the rms nuclear displacement over the experimental time scale but depends on the system sensitivity and the desired signal-to-noise ratio. The diffusive limit to spatial resolution in two-dimensional NMR imaging is derived and optimal conditions are defined. It is shown that a Carr-Purcell-Meiboom-Gill sequence can lead to a resolution limit close to optimal over a wide range of acquisition bandwidths.
Based on 39 cineangiographies in 23 patients performed during respiration with tracing of the cardiac chambers and the diaphragm, it has been found that the heart moves significantly with respiration, approximately half as much as the diaphragm during shallow or normal respiration. The cardiac respiratory motion indicates that gating of the respiratory cycle as well as the cardiac cycle is necessary in three-dimensional reconstruction of the heart using a large number of heart beats for recording.
Breathing motion severely degrades the quality of magnetic resonance images (MRI) of the thorax and upper abdomen and interferes with the acquisition of quantitative data. To minimize these motion effects, we built an MRI compatible ventilator for use in animal studies. Solid state circuitry is used for controlling ventilation parameters. The ventilator can be triggered internally at frequencies of 0.1 to 30 Hz or it can be triggered externally such as by the MRI pulse sequence. When triggered by the scanner, ventilation is synchronized to occur between image data acquisitions. Thus, image data are obtained when there is no breathing motion and at a minimum lung volume when hydrogen density is maximum. Since the ventilator can be adjusted to operate at virtually any frequency from conventional to high frequency, ventilation can be synchronized to all commonly used repetition times (100 ms to 2000 ms or more; 600 to 30 breaths/min). Scan synchronous ventilation eliminates breathing motion artifacts from most imaging sequences (single and multiple spin echo and inversion recovery). Best image quality is obtained when scan synchronous ventilation is combined with cardiac gating. These methods are also useful for quantitative research studies of thoracic and abdominal organs.
With magnetic resonance (MR) microscopy, high-resolution volumetric imaging (3DFT) of small animals is possible. Although these techniques are suitable for imaging the head and other small stationary objects, breathing and cardiac motion degrade the quality of body images. Scan synchronous ventilation and cardiac gating methods have been developed that permit acquisition of high-resolution images from anywhere in the body of small animals (150 to 400 g). Anesthetized rats were ventilated in synchrony with three-dimensional Fourier spin warp (3DFT) sequence (TR = 400 to 1000 ms, TE = 20 ms). Eight or 16 slices (1.2 or 2.5 mm thick) were acquired simultaneously. Effective pixel size was 200 X 200 mu. Imaging was performed in a 1.5 T, 1-m bore research system using a 28-cm diameter high field gradient coil and a 6-cm diameter radio frequency coil. For thoracic imaging, acquisitions were gated to the QRS of the ECG. Scan synchronous ventilation eliminated breathing motion artifacts and permitted visualization of peripheral vascular structures in the lung and liver. In images that were also cardiac gated, cardiac chambers and major thoracic vessels, including the coronary arteries, were well demonstrated. Thus, thoroughly characterized rodent models can now be studied with MR not only to explore noninvasively the intricacies of mammalian pathomorphology, but also to test the capabilities of MR and aid in interpreting MR data.
Projection reconstruction has been implemented with self-refocused selection pulses on a small bore, 2.0 T MR microscope, to allow imaging of lung parenchyma. Scan synchronous ventilation and cardiac gating have been integrated with the sequence to minimize motion artifacts. A systematic survey of the pulse sequence parameters has been undertaken in conjunction with the biological gating parameters to optimize resolution and signal-to-noise (SNR). The resulting projection images with effective echo time of < 300 microseconds allow definition of lung parenchyma with an SNR improvement of approximately 15 x over a more conventional 2DFT short echo gradient sequence.
The pathogenesis of atherosclerosis is currently being investigated in genetically engineered small animals. Methods to follow the time course of the developing pathology and/or the responses to therapy in vivo are limited. METHODS and
To address this problem, we developed a noninvasive MR microscopy technique to study in vivo atherosclerotic lesions (without a priori knowledge of the lesion location or lesion type) in live apolipoprotein E knockout (apoE-KO) mice. The spatial resolution was 0.0012 to 0.005 mm3. The lumen and wall of the abdominal aorta and iliac arteries were identified on all images in apoE-KO (n=8) and wild-type (n=5) mice on chow diet. Images obtained with MR were compared with corresponding cross-sectional histopathology (n=58). MR accurately determined wall area in comparison to histopathology (slope=1.0, r=0.86). In addition, atherosclerotic lesions were characterized in terms of lesion shape and type. Lesion type was graded by MR according to morphological appearance/severity and by histopathology according to the AHA classification. There was excellent agreement between MR and histopathology in grading of lesion shape and type (slope=0.97, r=0.91 for lesion shape; slope=0. 64, r=0.90 for lesion type).
The combination of high-resolution MR microscopy and genetically engineered animals is a powerful tool to investigate serially and noninvasively the progression and regression of atherosclerotic lesions in an intact animal model and should greatly enhance basic studies of atherosclerotic disease.
The use of a high-temperature superconducting probe for in vivo magnetic resonance microscopy at 2 T is described. To evaluate the performance of the probe, a series of SNR comparisons are carried out. The SNR increased by a factor of 3.7 compared with an equivalent copper coil. Quantitative measures of the SNR gain are in good agreement with theoretical predictions. A number of issues that are unique to the application of HTS coils are examined, including the difficulty in obtaining homogenous excitation without degrading the SNR of the probe. The use of the HTS probe in transmit-receive mode is simple to implement but results in nonuniform excitation. The effect of using the probe in this mode of operation on the T1 and T2 contrast is investigated. Methods for improving homogeneity are explored, such as employing a transmit volume coil. It is found that the cost of using an external transmit coil is an increased probe noise temperature and a reduced SNR by approximately 30%. Other important aspects of the probe are considered, including the effect of temperature on probe stability. Three-dimensional in vivo imaging sets are acquired to assess the stability of the probe for long scans. High-resolution images of the rat brain demonstrate the utility of the probe for microscopy applications.
Cardiovascular transgenic mouse models with an early phenotype or even premature death require noninvasive imaging methods that allow for accurate visualization of cardiac morphology and function. Thus the purpose of our study was to assess the feasibility of magnetic resonance imaging (MRI) to characterize cardiac function and mass in newborn, juvenile, and adult mice. Forty-five C57bl/6 mice from seven age groups (3 days to 4 mo after birth) were studied by MRI under isoflurane anesthesia. Electrocardiogram-gated cine MRI was performed with an in-plane resolution of (78-117 microm)(2). Temporal resolution per cine frame was 8.6 ms. MRI revealed cardiac anatomy in mice from all age groups with high temporal and spatial resolution. There was close correlation between MRI- and autopsy-determined left ventricular (LV) mass (r = 0.95, SE of estimate = 9.5 mg). The increase of LV mass (range 9.6-101.3 mg), cardiac output (range 1.1-14.3 ml/min), and stroke volume (range 3. 2-40.2 microl) with age could be quantified by MRI measurements. Ejection fraction and cardiac index did not change with aging. However, LV mass index decreased with increasing age (P < 0.01). High-resolution MRI allows for accurate in vivo assessment of cardiac function in neonatal, juvenile, and adult mice. This method should be useful when applied in transgenic mouse models.
We describe an MR-compatible ventilator that is computer controlled to generate a variety of breathing patterns, to minimize image degrading effects of breathing motion, and to support delivery of gas anesthesia and experimental inhalational gases. A key feature of this ventilator is the breathing valve that attaches directly to the endotracheal tube to reduce dead volume and allows independent control of inspiratory and expiratory phases of ventilation. This ventilator has been used in a wide variety of MR and x-ray microscopy studies of small animals, especially for MR imaging the lungs with hyperpolarized gases ((3)He & (129)Xe).
Signal of lung parenchymal tissue from the living rat and mouse lung was detected at 4.7 T with a good signal‐to‐noise ratio and motion‐suppressed artifacts using a short TE gradient‐echo sequence. Neither cardiac nor respiratory gating were applied, and animals respired freely during data collection. Mean T 2 * relaxation times of parenchyma in the anterior, middle and posterior regions of both lungs ranged between 403 and 657 µs and 397 and 751 µs, respectively for the rat and mouse. For the rat in the prone position, there was a gradient in T 2 * values, from the posterior to the anterior regions of both lungs. In the supine position, however, T 2 * values were larger in the posterior and in the anterior portions. For the mouse in both prone and supine positions, there was a tendential gradient in T 2 * from the anterior to the posterior portions. The robustness of the approach renders it well suited for routine applications, e.g. in pharmacological studies concerning asthma models in small rodents. The method was applied to lung inflammation models involving challenge with ovalbumin or lipopolysaccharide. Copyright © 2001 John Wiley & Sons, Ltd.
Abbreviations used
ECG electrocardiogram
HASTE half‐Fourier single‐shot turbo spin‐echo
i.t. intra‐tracheal
LPS lipopolysaccharide
OA ovalbumin
PET positron emission tomography
RF radio‐frequency
SPECT single‐photon emission computer tomography
SW sweep width
TE echo time
TR repetition time
TSE turbo spin‐echo
A fundamental problem associated with using the conventional electrocardiograph (ECG) to monitor a subject's cardiac activity during magnetic resonance imaging (MRI) is the distortion of the ECG due to electromagnetic interference. This problem is particularly pronounced in MR microscopy (MRI of small animals at microscopic resolutions (< 0.03 mm(3))) because the strong, rapidly-switching magnetic field gradients induce artifacts in the animal's ECG that often mimic electrophysiologic activity, impairing the use of the ECG for cardiac monitoring and gating purposes. The fiber-optic stethoscope system offers a novel approach to measuring cardiac activity that, unlike the ECG, is immune to electromagnetic effects. The fiber-optic stethoscope is perorally inserted into the esophagus of small animals to optically detect pulsatile compression of the esophageal wall. The optical system is shown to provide a robust cardiac monitoring and gating signal in rats and mice during routine cardiac MR microscopy.
Genetically engineered mouse models provide enormous potential for investigation of the underlying mechanisms of atherosclerotic disease, but noninvasive imaging methods for analysis of atherosclerosis in mice are currently limited. This study aimed to demonstrate the feasibility of MRI to noninvasively visualize atherosclerotic plaques in the thoracic aorta in mice deficient in apolipoprotein-E, who develop atherosclerotic lesions similar to those observed in humans. To freeze motion, MR data acquisition was both ECG- and respiratory-gated. T(1)-weighted MR images were acquired with TR/TE approximately 1000/10 ms. Spatial image resolution was 49 x 98 x 300 micro m(3). MRI revealed a detailed view of the lumen and the vessel wall of the entire thoracic aorta. Comparison of MRI with corresponding cross-sectional histopathology showed excellent agreement of aortic vessel wall area (r = 0.97). Hence, noninvasive MRI should allow new insights into the mechanisms involved in progression and regression of atherosclerotic disease.
The aim of this study was to test the feasibility of cine magnetic resonance imaging (MRI) for assessment of the infarcted rat and mouse heart and to compare the results with established methods. These models have been proven to predict genesis and prevention of heart failure in patients. The value of cine MRI was tested in studies investigating interventions to change the course of the remodeling process. MRI was performed for determination of left ventricular (LV) volumes and mass, myocardial infarct (MI) size and cardiac output. LV wet weight was determined after MRI. Rats underwent conventional hemodynamic measurements for determination of cardiac output and LV volumes by electromagnetic flowmeter and pressure-volume curves. Infarct size was determined by histology. MRI-acquired MI-size (18.5+/-2%) was smaller than that found by histology (22.8+/-2.5%, p<0.05) with close correlation (r=0.97). There was agreement in LV mass between MRI and wet weight (r=0.97, p<0.05) and in the MRI- and flowmeter measurements of cardiac output (r=0.80, p<0.05). Volume by MRI differed from pressure-volume curves with good correlation (r=0.96, p<0.05). In a serial study of mice after coronary ligation, LV hypertrophy at 8 weeks was detected (Sham 105.1+/-7.9 mg, MI 144.4+/-11.7 mg, p<0.05). Left ventricles were enlarged in infarcted mice (end-diastolic volume, week 8: Sham 63.5+/-4 microl, MI 94.2 microl, p<0.05). In conclusion, cine MRI is a valuable diagnostic tool applicable to the rat and mouse model of MI. Being non-invasive and exact it offers new insights into the remodeling process after MI because serial measurements are possible. The technique was applied to study several interventions and proved its usefulness.
To tailor a cardiac magnetic resonance (MR) microscopy technique for the rat that combines improvements in pulse sequence design and physiologic control to acquire high-resolution images of cardiac structure and function.
Projection reconstruction (PR) was compared to conventional Cartesian techniques in point-spread function simulations and experimental studies to evaluate its artifact sensitivity. Female Sprague-Dawley rats were imaged at 2.0 T using PR with direct encoding of the free induction decay. Specialized physiologic support and monitoring equipment ensured consistency of biological motion and permitted synchronization of imaging with the cardiac and respiratory cycles.
The reduced artifact sensitivity of PR offered improved delineation of cardiac and pulmonary structures. Ventilatory synchronization further increased the signal-to-noise ratio by reducing inter-view variability. High-quality short-axis and long-axis cine images of the rat heart were acquired with 10-msec temporal resolution and microscopic spatial resolution down to 175 microm x 175 microm x 1 mm.
Integrating careful biological control with an optimized pulse sequence significantly limits both the source and impact of image artifacts. This work represents a novel integration of techniques designed to support measurement of cardiac morphology and function in rodent models of cardiovascular disease.
Cardiopulmonary anatomy and physiology: essentials for respiratory care
- Des Jardins
Des Jardins TR. Cardiopulmonary anatomy and physiology: essentials for respiratory care. Albany, NY: Delmar Thomson Learning; 2002. 550 p.
Performance of a high-temperature superconducting probe for in vivo microscopy at 2.0 T
- Miller