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Extended Coverage First-Pass Perfusion Imaging Using Slice-Interleaved TSENSE
Magnetic Resonance in Medicine
Abstract
Parallel imaging applied to first-pass, contrast-enhanced cardiac MR can yield greater spatial coverage for a fixed temporal resolution. The method combines rate R = 2 acceleration using TSENSE with shot-to-shot interleaving of two slices. The √R SNR loss is largely compensated for by a longer effective repetition time (TR) and increased flip angle associated with slice interleaving. In this manner, increased spatial coverage is achieved while comparable or better image quality is maintained. Single-heartbeat temporal resolution was accomplished with spatial coverage of eight slices at heart rates up to 71 bpm, six slices up to 95 bpm, and four slices up to 143 bpm. Experiments in normal subjects (N = 6) were performed to assess signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) values. Magn Reson Med 51:200–204, 2004. Published 2003 Wiley-Liss, Inc.
... Extended coverage of the LV can also be obtained by acquisition of a greater number of slices per cardiac cycle with multi-slice methods [Köstler et al., 2003] [Kellman et al., 2004a] [Stäb et al., 2014]. Such multi-slice acquisitions have shorter individual shot-times comparative to 3D, greatly improving robustness to cardiac motion while offering some of the same advantages from the increased coverage. ...
... Despite achieving full coverage, an undersampling acceleration factor of only 2 was applied and therefore spatial resolution was coarser than the ideal values considered earlier. Kellman et al. [Kellman et al., 2004a] extended the use of h-EPI with TSENSE [Kellman et al., 2001] to produce improved quality in extended coverage FPP, potentially whole-heart, but again limited by the parallel imaging performance to an acceleration factor of 2. ...
Myocardial perfusion imaging is of huge importance for the detection of coronary artery disease (CAD), one of the leading causes of morbidity and mortality worldwide, as it can provide non-invasive detection at the early stages of the disease. Magnetic resonance imaging (MRI) can assess myocardial perfusion by capturing the rst-pass perfusion (FPP) of a gadolinium-based contrast agent (GBCA), which is now a well-established technique and compares well with other modalities. However, current MRI methods are restricted by their limited coverage of the left ventricle. Interest has therefore grown in 3D volumetric \whole-heart" FPP by MRI, although many challenges currently limit this. For this thesis, myocardial perfusion assessment in general, and 3D whole-heart FPP in particular, were reviewed in depth, alongside MRI techniques important for achieving 3D FPP. From this, a 3D `stack-of-stars' (SOS) FPP sequence was developed with the aim of addressing some current limitations. These included the breath-hold requirement during GBCA rst-pass, long 3D shot durations corrupted by cardiac motion, and a propensity for artefacts in FPP. Parallel imaging and compressed sensing were investigated for accelerating whole-heart FPP, with modi cations presented to potentially improve robustness to free-breathing. Novel sequences were developed that were capable of individually improving some current sequence limits, including spatial resolution and signal-to-noise ratio, although with some sacri ces. A nal 3D SOS FPP technique was developed and tested at stress during free-breathing examinations of CAD patients and healthy volunteers. This enabled the rst known detection of an inducible perfusion defect with a free-breathing, compressed sensing, 3D FPP sequence; however, further investigation into the diagnostic performance is required. Simulations were performed to analyse potential artefacts in 3D FPP, as well as to examine ways towards further optimisation of 3D SOS FPP. The nal chapter discusses some limitations of the work and proposes opportunities for further investigation.
... Despite this, h-EPI in 2D FPP typically uses an ETL of around 4 at 1.5 T (no current examples at 3 T), corresponding to acceleration factors of approximately 2 compared to the FLASH timings calculated earlier (see Appendix). Early examples of 2D FPP with extended LV coverage used h-EPI [26,30,31] but so far 3D EPI imaging has largely been limited to non-cardiac work. ...
... Despite achieving full coverage, only an undersampling acceleration factor of 2 was applied and therefore spatial resolution was coarser than the ideal values considered earlier. Kellman et al. [31] extended the use of h-EPI with TSENSE [67] to produce improved quality in extended coverage FPP, potentially whole-heart, but again limited by the parallel imaging performance to an acceleration factor of 2. ...
A comprehensive review is undertaken of the methods available for 3D whole-heart first-pass perfusion (FPP) and their application to date, with particular focus on possible acceleration techniques. Following a summary of the parameters typically desired of 3D FPP methods, the review explains the mechanisms of key acceleration techniques and their potential use in FPP for attaining 3D acquisitions. The mechanisms include rapid sequences, non-Cartesian k-space trajectories, reduced k-space acquisitions, parallel imaging reconstructions and compressed sensing. An attempt is made to explain, rather than simply state, the varying methods with the hope that it will give an appreciation of the different components making up a 3D FPP protocol. Basic estimates demonstrating the required total acceleration factors in typical 3D FPP cases are included, providing context for the extent that each acceleration method can contribute to the required imaging speed, as well as potential limitations in present 3D FPP literature. Although many 3D FPP methods are too early in development for the type of clinical trials required to show any clear benefit over current 2D FPP methods, the review includes the small but growing quantity of clinical research work already using 3D FPP, alongside the more technical work. Broader challenges concerning FPP such as quantitative analysis are not covered, but challenges with particular impact on 3D FPP methods, particularly with regards to motion effects, are discussed along with anticipated future work in the field.
... Selfcalibration is conveniently incorporated in the time-domain approaches. Self-calibrated SENSE has been demonstrated for accelerated spatio-temporal hybrid techniques which rely on dynamic data [25][26][27][28][29][30][31]; a requirement which is not met by standardized CMR protocols used for single cardiac phase black blood FSE imaging of the heart [32,33]. ...
... SCSE-FSE helps to address some of the limitations of previous self-calibrated approaches using SENSE reconstruction tech-niques. These approaches are commonly based upon spatiotemporal hybrid algorithms -a concept behind techniques such as UNFOLD-SENSE, TSENSE, auto-SENSE, and k-t SENSEwhich are applied on a frame-by-frame basis [25][26][27][28][29][30][31]. These techniques share the need of a time series of data (for example CINE imaging or first pass bolus perfusion imaging) and hence do not support single cardiac phase black blood FSE imaging of the heart per se. ...
Design, validation and application of an accelerated fast spin-echo (FSE) variant that uses a split-echo approach for self-calibrated parallel imaging.
For self-calibrated, split-echo FSE (SCSE-FSE), extra displacement gradients were incorporated into FSE to decompose odd and even echo groups which were independently phase encoded to derive coil sensitivity maps, and to generate undersampled data (reduction factor up to R = 3). Reference and undersampled data were acquired simultaneously. SENSE reconstruction was employed.
The feasibility of SCSE-FSE was demonstrated in phantom studies. Point spread function performance of SCSE-FSE was found to be competitive with traditional FSE variants. The immunity of SCSE-FSE for motion induced mis-registration between reference and undersampled data was shown using a dynamic left ventricular model and cardiac imaging. The applicability of black blood prepared SCSE-FSE for cardiac imaging was demonstrated in healthy volunteers including accelerated multi-slice per breath-hold imaging and accelerated high spatial resolution imaging.
SCSE-FSE obviates the need of external reference scans for SENSE reconstructed parallel imaging with FSE. SCSE-FSE reduces the risk for mis-registration between reference scans and accelerated acquisitions. SCSE-FSE is feasible for imaging of the heart and of large cardiac vessels but also meets the needs of brain, abdominal and liver imaging.
... Two interleaved multi-shot EPI k-space trajectories are commonly used: top-down (1-3) and center-out (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12). In a top-down trajectory, the first echo in the echo train is used to acquire an outer line at the positive (or negative) edge of k-space. ...
... This holds true for flow and motion in both the frequency and phase encode directions (2). The center-out trajectory has therefore been recommended for applications such as functional imaging of the brain (5)(6)(7)(8), myocardial perfusion (3,9), coronary artery imaging (10), and velocity mapping and imaging in the presence of physiological motion (6,10,11). It has been suggested that the center-out trajectory be used instead of gradient moment nulling (GMN) to reduce motion and flow artifacts (11). ...
Segmented interleaved echo planar imaging (EPI) is a highly efficient data acquisition technique; however, EPI is sensitive to artifacts from off-resonance spins. The choice of k-space trajectories is important in determining how off-resonance spins contribute to image artifacts. Top-down and center-out trajectories are theoretically analyzed, simulated, implemented, and tested in phantom and volunteer experiments. Theoretical results show off-resonance artifact manifests as a simple positional shift for the top-down trajectory, while for the center-out trajectory off-resonance artifact manifests as a splitting of the object, which entails both shift and blurring. These results were validated using simulation and phantom scan data where a frequency-offset was introduced ranging from -300 Hz to +300 Hz. As predicted by the theoretical results, inferior image quality was observed for the center-out trajectory in a single volunteer. Off-resonance produces more severe and complex artifacts with the center-out trajectory than the top-down trajectory.
... With the advent of parallel imaging techniques (11,12) 2-fold scan acceleration has become feasible. In application to CMR perfusion imaging, this has enabled improved myocardial coverage without scan time penalty (13,14). Further improvements in scan efficiency have, however, been difficult to achieve as noise enhancement increases with increasing acceleration factors, severely compromising image quality. ...
... The boundaries of CMR myocardial perfusion imaging continue to be pushed, driven by technological advances that permit faster data acquisition. This increase in speed can be invested flexibly into better spatial or temporal resolution, and both are important to maximize diagnostic performance (13,14). The latest acceleration methods, such as k-t SENSE, continue to evolve, and several improvements over the initial implementations have been proposed (14,18,20), each addressing particular potential limitations of the method. ...
The aim of this study was to assess the clinical feasibility and diagnostic performance of an acceleration technique based on k-space and time (k-t) sensitivity encoding (SENSE) for rapid, high-spatial resolution cardiac magnetic resonance (CMR) myocardial perfusion imaging.
The assessment of myocardial perfusion is of crucial importance in the evaluation of patients with known or suspected coronary artery disease. CMR myocardial perfusion imaging performs favorably compared to single photon-emission computed tomography and offers higher spatial resolution, particularly when combined with scan acceleration techniques such as k-t SENSE. A previous study showed that k-t SENSE accelerated myocardial perfusion CMR with 5-fold acceleration is feasible and delivers high diagnostic accuracy for the detection of coronary artery disease. Higher acceleration factors have not been attempted clinically because of concerns over temporal blurring effects of the time-varying signal during contrast bolus passage.
Twenty patients underwent myocardial perfusion CMR imaging using a 3.0-T whole-body CMR imager before diagnostic X-ray coronary angiography. Perfusion images were obtained using an extension of the k-t SENSE method using parallel imaging to double the spatial resolution of the k-t SENSE training images. This extension, termed k-t SENSE+, permitted 8-fold nominal scan acceleration and an in-plane spatial resolution of up to 1.1 x 1.1 mm(2). Perfusion scores were derived by 2 blinded observers for 16 myocardial segments and compared to quantitative analysis of X-ray coronary angiography.
CMR data were successfully obtained in all 20 patients. High diagnostic accuracy was achieved using CMR, as reflected by areas under the receiver-operator characteristic curve of 0.94 and 0.82 for detecting stenoses >50% and >75%, respectively. Observer agreement between 2 readers had a kappa value of 0.92. The areas under the receiver-operator characteristic curves for the left anterior descending, left circumflex, and right coronary artery territories with stenoses >50% were 0.75, 0.92, and 0.79, respectively.
Accelerated CMR perfusion imaging is clinically feasible and offers excellent diagnostic performance in detecting coronary stenosis.
... Other examples include methods that use an estimate of the spatial-spectral support [15][16][17]; methods that incorporate parallel imaging to reduce data acquisition time [18][19][20][21][22]; methods that temporally model the dynamic sequence; and methods that attempt reconstruction of the dynamic sequence using severely reduced or sparse sampling [23][24][25][26]. Recent applications of these techniques include a parallel imaging technique called TSENSE introduced by Kellman et al. [27], a parallel technique called k-t SENSE that uses an estimated spatial-spectral support [28], and temporally constrained version of k-t SENSE called k-t PCA [29]. ...
... More recent work on cardiac perfusion dynamic imaging methods has shown promise in improving the spatial and temporal resolution, imaging time, or spatial coverage features. In the work by Kellman et al. [27], a dynamic parallel imaging method called TSENSE was used to perform a slice-interleaved acquisition where multiple slices are acquired per saturation preparation as opposed to the original one slice acquired per saturation preparation shown in Figure 2.1: Imaging sequences used for first-pass cardiac perfusion imaging. Top sequence shown is fast low-flip angle gradient echo sequence (also know as turboFLASH or spoiled GRE). ...
Cardiac perfusion MRI aims to analyze perfusion characteristics of the heart through the injection of a contrast agent. In clinical practice, cardiac perfusion imaging is used to diagnose ischemic myocardial tissue, coronary artery disease, and, to a lesser extent, myocardial infarctions. To diagnose these tissues and diseases, real time ECG triggered cardiac imaging techniques (fast T1-weighted gradient echo, echo planar, or steady-state free precession sequences) are typically used to capture the quick wash-in and wash out of the contrast agent. An alternative approach for real-time MRI based on the partially separable functions (PSF) model has been shown to provide reconstructions of dynamic sequences with good spatial and temporal resolutions without the need for ECG triggering. Although previous studies have demonstrated good results using the PSF model, a detailed analysis on the e®ect of the choice of sampling pattern has yet to be performed. Consequently, this thesis aims to analyze the ability of various Cartesian and projection sampling trajectories to characterize perfusion characteristics of the heart when the perfusion is spatially inhomogeneous using the PSF dynamic imaging method. Ten total sampling patterns (five Cartesian and five projection) were analyzed. Overall, the Cartesian sampling patterns provided better reconstructions in terms of image quality, image sequence NRMSE, and relative error of the quantitative perfusion parameters. Also, the Cartesian sampling patterns provided a more accurate reconstruction of the smallest simulated perfusion defect in the myocardium. However, the best sampling pattern performer for the other size defects varied. Also, projection sampling patterns showed a superior robustness to noise compared to Cartesian sampling patterns.
... Parallel imaging has long been used in perfusion CMR but is limited to two-to three-fold acceleration. 12 Spatio-temporal reconstruction (k-t) methods [14][15][16] have been proposed, but their acceleration rates remained limited. 17 Subsequently, compressed sensing, low-rank methods, and their combinations have been adopted to perfusion CMR reconstruction to enable higher acceleration rates. ...
Purpose
To develop a physics‐guided deep learning (PG‐DL) reconstruction strategy based on a signal intensity informed multi‐coil (SIIM) encoding operator for highly‐accelerated simultaneous multislice (SMS) myocardial perfusion cardiac MRI (CMR).
Methods
First‐pass perfusion CMR acquires highly‐accelerated images with dynamically varying signal intensity/SNR following the administration of a gadolinium‐based contrast agent. Thus, using PG‐DL reconstruction with a conventional multi‐coil encoding operator leads to analogous signal intensity variations across different time‐frames at the network output, creating difficulties in generalization for varying SNR levels. We propose to use a SIIM encoding operator to capture the signal intensity/SNR variations across time‐frames in a reformulated encoding operator. This leads to a more uniform/flat contrast at the output of the PG‐DL network, facilitating generalizability across time‐frames. PG‐DL reconstruction with the proposed SIIM encoding operator is compared to PG‐DL with conventional encoding operator, split slice‐GRAPPA, locally low‐rank (LLR) regularized reconstruction, low‐rank plus sparse (L + S) reconstruction, and regularized ROCK‐SPIRiT.
Results
Results on highly accelerated free‐breathing first pass myocardial perfusion CMR at three‐fold SMS and four‐fold in‐plane acceleration show that the proposed method improves upon the reconstruction methods use for comparison. Substantial noise reduction is achieved compared to split slice‐GRAPPA, and aliasing artifacts reduction compared to LLR regularized reconstruction, L + S reconstruction and PG‐DL with conventional encoding. Furthermore, a qualitative reader study indicated that proposed method outperformed all methods.
Conclusion
PG‐DL reconstruction with the proposed SIIM encoding operator improves generalization across different time‐frames /SNRs in highly accelerated perfusion CMR.
... These forms of data-redundancy in a "conventional" perfusion scan (ie, without use of undersampling) imply that undersampling in both the spatial and time domains (k-t) can be performed to reduce the image acquisition time within bounds deemed acceptable for signal-to-noise, image quality, artifact absence, and so forth. Various k-t undersampling/acceleration techniques have been used for myocardial perfusion CMR, such as TSENSE, 45,46 k-t BLAST, 47 and more recently k-t principal component analysis (PCA). 48,49 Other, arguably more esoteric methods relate to the highly constrained back-projection reconstruction (HYPR), 50 in which k-space data are obtained by undersampled radial projections, possibly combined with an overall rotation of the sampling pattern during the course of dynamic imaging. ...
Ischemic heart disease remains the foremost determinant of death and disability across the world. Quantification of the ischemia burden is currently the preferred approach to predict event risk and to trigger adequate treatment. Cardiac magnetic resonance (CMR) can be a prime protagonist in this scenario due to its synergistic features. It allows assessment of wall motility, myocardial perfusion, and tissue scar by means of late gadolinium enhancement imaging. We discuss the clinical and preclinical aspects of gadolinium-based, perfusion CMR imaging, including the relevance of high spatial resolution and 3-dimensional whole-heart coverage, among important features of this auspicious method.
... This can be achieved by acquiring multiple slices in an interleaved fashion following each saturation pulse, a concept which has previously been applied for EPI perfusion imaging. 24,25 The total imaging time collecting multiple slices per SR preparation is given by (2) where n Sat is the total number of SR pulses and determines the number of slices acquired during each SR pulse. The total imaging time can be dramatically reduced by using this strategy. ...
Purpose:
To design and evaluate two-dimensional (2D) L1-SPIRiT accelerated spiral pulse sequences for first-pass myocardial perfusion imaging with whole heart coverage capable of measuring eight slices at 2 mm in-plane resolution at heart rates up to 125 beats per minute (BPM).
Methods:
Combinations of five different spiral trajectories and four k-t sampling patterns were retrospectively simulated in 25 fully sampled datasets and reconstructed with L1-SPIRiT to determine the best combination of parameters. Two candidate sequences were prospectively evaluated in 34 human subjects to assess in vivo performance.
Results:
A dual density broad transition spiral trajectory with either angularly uniform or golden angle in time k-t sampling pattern had the largest structural similarity and smallest root mean square error from the retrospective simulation, and the L1-SPIRiT reconstruction had well-preserved temporal dynamics. In vivo data demonstrated that both of the sampling patterns could produce high quality perfusion images with whole-heart coverage.
Conclusion:
First-pass myocardial perfusion imaging using accelerated spirals with optimized trajectory and k-t sampling pattern can produce high quality 2D perfusion images with whole-heart coverage at the heart rates up to 125 BPM. Magn Reson Med, 2015. © 2015 Wiley Periodicals, Inc.
... Due to the intrinsic physical limitation of MRI, the speed of data acquisition is always the problem comparing to CT. Hence, a number of acceleration techniques have been developed over the past four decades and the shortened acquisition time has greatly expanded clinical applications of MRI, especially for dynamic or timeresolved MRI, such as perfusion imaging [103][104][105][106], contrastenhanced MR angiography [107][108][109][110][111], functional MRI [112,113], and cardiac function examinations [114][115][116][117]. Those acceleration techniques could be divided into two categories: parallel imaging and dynamic acceleration. ...
During the past decade, medical imaging has made the transition from anatomical imaging to functional and even molecular imaging. Such transition provides a great opportunity to begin the integration of imaging data and various levels of biological data. In particular, the integration of imaging data and multiomics data such as genomics, metabolomics, proteomics, and pharmacogenomics may open new avenues for predictive, preventive, and personalized medicine. However, to promote imaging-omics integration, the practical challenge of imaging techniques should be addressed. In this paper, we describe key challenges in two imaging techniques: computed tomography (CT) and magnetic resonance imaging (MRI) and then review existing technological advancements. Despite the fact that CT and MRI have different principles of image formation, both imaging techniques can provide high-resolution anatomical images while playing a more and more important role in providing molecular information. Such imaging techniques that enable single modality to image both the detailed anatomy and function of tissues and organs of the body will be beneficial in the imaging-omics field.
... In cardiac magnetic resonance, perfusion information is commonly imaged by detecting dynamic changes in myocardial signal intensity during the first passage of a contrast agent. Demands for high spatial and temporal resolution as well as sufficient cardiac coverage make scan acceleration techniques necessary (4)(5)(6)(7). Besides optimized and fast pulse sequences (8), k-space acquisition can be undersampled to reduce acquisition time and increase coverage in image space. ...
In this study, an iterative k-t principal component analysis (PCA) algorithm with nonrigid frame-to-frame motion correction is proposed for dynamic contrast-enhanced three-dimensional perfusion imaging.
An iterative k-t PCA algorithm was implemented with regularization using training data corrected for frame-to-frame motion in the x-pc domain. Motion information was extracted using shape-constrained nonrigid image registration of the composite of training and k-t undersampled data. The approach was tested for 10-fold k-t undersampling using computer simulations and in vivo data sets corrupted by respiratory motion artifacts owing to free-breathing or interrupted breath-holds. Results were compared to breath-held reference data.
Motion-corrected k-t PCA image reconstruction resolved residual aliasing. Signal intensity curves extracted from the myocardium were close to those obtained from the breath-held reference. Upslopes were found to be more homogeneous in space when using the k-t PCA approach with motion correction.
Iterative k-t PCA with nonrigid motion correction permits correction of respiratory motion artifacts in three-dimensional first-pass myocardial perfusion imaging. Magn Reson Med, 2013. © 2013 Wiley Periodicals, Inc.
... Multiple pulse sequences have been used for perfusion imaging and have their advantages and disadvantages (14). Parallel imaging with acceleration factors of 2 to 3 are routinely used (16) and multiple highly accelerated techniques have been evaluated in clinical studies (17). More recent techniques which exploit the spatial-temporal correlations in the series of first-pass perfusion imaging using techniques such as SENSitivity Encoding are enabling true 3-dimensional coverage of the ventricle with high spatial resolution (18). ...
Cardiac magnetic resonance imaging (CMR) is well established and considered the gold standard for assessing myocardial volumes and function, and for quantifying myocardial fibrosis in both ischemic and nonischemic heart disease. Recent developments in CMR imaging techniques are enabling clinically-feasible rapid parametric mapping of myocardial perfusion and magnetic relaxation properties (T1, T2, and T2* relaxation times) that are further expanding the range of unique tissue parameters that can be assessed using CMR. To generate a parametric map of perfusion or relaxation times, multiple images of the same region of the myocardium are acquired with different sensitivity to the parameter of interest, and the signal intensities of these images are fit to a model which describes the underlying physiology or relaxation parameters. The parametric map is an image of the fitted perfusion parameters or relaxation times. Parametric mapping requires acquisition of multiple images typically within a breath-hold and thus requires specialized rapid acquisition techniques. Quantitative perfusion imaging techniques can more accurately determine the extent of myocardial ischemia in coronary artery disease and provide the opportunity to evaluate microvascular disease with CMR. T1 mapping techniques performed both with and without contrast are enabling quantification of diffuse myocardial fibrosis and myocardial infiltration. Myocardial edema and inflammation can be evaluated using T2 mapping techniques. T2* mapping provides an assessment of myocardial iron-overload and myocardial hemorrhage. There is a growing body of evidence for the clinical utility of quantitative assessment of perfusion and relaxation times, although current techniques still have some important limitations. This article will review the current imaging technologies for parametric mapping, emerging applications, current limitations, and potential of CMR parametric mapping of the myocardium. The specific focus will be the assessment and quantification of myocardial perfusion and magnetic relaxation times.
... New acquisition strategies such as k-space and time sensitivity encoding (k–t SENSE), that simultaneously exploit coil encoding and spatiotemporal correlations, allow substantial acceleration of CMR data acquisition.12,13 This speed-up can be utilized to either improve spatial resolution, shorten acquisition time per slice or increase signal-to-noise ratio. ...
To evaluate the feasibility and diagnostic performance of high spatial resolution myocardial perfusion cardiac magnetic resonance (perfusion-CMR).
Fifty-four patients underwent adenosine stress perfusion-CMR. An in-plane spatial resolution of 1.4 × 1.4 mm(2) was achieved by using 5× k-space and time sensitivity encoding (k-t SENSE). Perfusion was visually graded for 16 left ventricular and two right ventricular (RV) segments on a scale from 0 = normal to 3 = abnormal, yielding a perfusion score of 0-54. Diagnostic accuracy of the perfusion score to detect coronary artery stenosis of >50% on quantitative coronary angiography was determined. Sources and extent of image artefacts were documented. Two studies (4%) were non-diagnostic because of k-t SENSE-related and breathing artefacts. Endocardial dark rim artefacts if present were small (average width 1.6 mm). Analysis by receiver-operating characteristics yielded an area under the curve for detection of coronary stenosis of 0.85 [95% confidence interval (CI) 0.75-0.95] for all patients and 0.82 (95% CI 0.65-0.94) and 0.87 (95% CI 0.75-0.99) for patients with single and multi-vessel disease, respectively. Seventy-four of 102 (72%) RV segments could be analysed.
High spatial resolution perfusion-CMR is feasible in a clinical population, yields high accuracy to detect single and multi-vessel coronary artery disease, minimizes artefacts and may permit the assessment of RV perfusion.
... To further improve temporal resolution parallel imaging techniques are generally used. 22 Multiple studies have compared various pulse sequences, but there has been no clear consensus on the optimal technique. 23,24 While the majority of published studies have been at 1.5T, multiple investigators have performed perfusion studies at 3T and have demonstrated improved SNR and contrast-to-noise ratio (CNR). ...
BACKGROUND: Cardiac magnetic resonance (CMR) imaging has emerged as an important cardiac imaging technique for the evaluation of multiple cardiac pathologies. OBJECTIVE/METHOD: The goal of this review is to describe recent advances in techniques which have extended the potential applications of CMR. The focus will be on the clinical applications of CMR for the evaluation of coronary artery disease and heart failure/cardiomyopathies which are major causes of morbidity and mortality worldwide. CONCLUSION: CMR provides unique tissue characterization which is not available from other imaging modalities and has demonstrated important diagnostic and prognostic information in many forms of heart disease.
... Also, for the imaging to be performed while the subject is breathing without gating, high temporal resolution is required (20 ms or better). A number of dynamic imaging methods have been applied to the first-pass perfusion cardiac imaging problem including TSENSE [10], k-t SENSE [11], and k-t PCA [12]; however, these applications used gating, and it is not clear that any of these methods are usable in the context of the rat heart without gating. The rat's heart rate is 5 times higher than that of a human, so temporal undersampling on the order of 100 times must be tolerated while these methods are known to function in the undersampling range from 2 to 8. In comparison, NIH Public Access ...
Dynamic imaging methods based on the Partially Separable Functions (PSF) model have been used to perform ungated cardiac MRI, and the critical parameter determining the quality of the reconstructed images is the order, L, of the PSF model. This work extends previous methods by increasing L in the cardiac region to improve the ability of the PSF model to represent complex spatiotemporal signals. The resulting higher order PSF model is fit to sparse (k, t)-space data using spatial-spectral support, spatial-eigenbasis support, and spectral sparsity constraints. This new method is demonstrated in the context of 2D first-pass perfusion MRI in a healthy rat heart.
... The phase-contrast CINE MR images confirmed good systolic flow with excellent valve leaflet opening and no evidence of turbulence, diastolic regurgitant flow, or paravalvular leak. First-pass perfusion studies [24] demonstrated adequacy of myocardial blood flow after valve placement in all animals following successful deployment. The perfusion results confirmed adequacy of blood flow at the tissue level, indicating proper valve positioning with respect to the coronary ostia. ...
Aortic valves have been implanted on self-expanding (SE) and balloon-expandable (BE) stents minimally invasively. We have demonstrated the advantages of transapical aortic valve implantation (tAVI) under real-time magnetic resonance imaging (rtMRI) guidance. Whether there are different advantages to SE or BE stents is unknown. We report rtMRI-guided tAVI in a porcine model using both SE and BE stents, and compare the differences between the stents.
A total of 22 Yucatan pigs (45-57 kg) underwent tAVI. Commercially available stentless bioprostheses (21-25 mm) were mounted on either BE platinum-iridium stents or SE-nitinol stents. rtMRI guidance was employed as the intraoperative imaging. Markers on both types of stents were used to enhance visualization in rtMRI. Pigs were allowed to survive and had follow-up MRI scans and echocardiography at 1, 3, and 6 months postoperatively.
rtMRI provided excellent visualization of the aortic valve implantation mounted on both stent types. The implantation times were shorter with the SE stents (60 ± 14s) than with the BE stents (74 ± 18s), (p=0.027). The total procedure time was 31 and 37 min, respectively (p=0.12). It was considerably easier to manipulate the SE stent during deployment, without hemodynamic compromise. This was not always the case with the BE stent, and its placement occasionally resulted in coronary obstruction and death. Long-term results demonstrated stability of the implants with preservation of myocardial perfusion and function over time for both stents.
SE stents were easier to position and deploy, thus leading to fewer complications during tAVI. Future optimization of SE stent design should improve clinical results.
... Parallel imaging methods (6)(7)(8) have enhanced the clinical applicability of CMR perfusion imaging (9)(10)(11). Using acceleration techniques, the trade-off between spatial and temporal resolution can be relaxed, leading to single heartbeat acquisitions and spatial resolutions of up to 1 Â 1 mm 2 in plane. ...
Three-dimensional myocardial perfusion imaging requires significant acceleration of data acquisition to achieve whole-heart coverage with adequate spatial and temporal resolution. The present article introduces a compartment-based k-t principal component analysis reconstruction approach, which permits three-dimensional perfusion imaging at 10-fold nominal acceleration. Using numerical simulations, it is shown that the compartment-based method results in accurate representations of dynamic signal intensity changes with significant improvements of temporal fidelity in comparison to conventional k-t principal component analysis reconstructions. Comparison of the two methods based on rest and stress three-dimensional perfusion data acquired with 2.3 × 2.3 × 10 mm(3) during a 225 msec acquisition window in patients confirms the findings and demonstrates the potential of compartment-based k-t principal component analysis for highly accelerated three-dimensional perfusion imaging.
... In this work, a GRAPPA PI strategy was employed. It is important to note that other PI techniques such as k-tSENSE (26), TSENSE (27), or TGRAPPA (28) can be used for studies involving temporal imaging series, such as cardiac cine imaging (29)(30)(31)(32)(33) or DCE-MRI (34)(35)(36). The performance trends that were observed during this work are not limited to temporal imaging series, however, or to the particular cine imaging protocol that was used to explore multiple-animal parallel imaging in the presence of motion. ...
Compared to traditional single-animal imaging methods, multiple-mouse MRI has been shown to dramatically improve imaging throughput and reduce the potentially prohibitive cost for instrument access. To date, up to a single radiofrequency coil has been dedicated to each animal being simultaneously scanned, thus limiting the sensitivity, flexibility, and ultimate throughput. The purpose of this study was to investigate the feasibility of multiple-mouse MRI with a phased-array coil dedicated to each animal. A dual-mouse imaging system, consisting of a pair of two-element phased-array coils, was developed and used to achieve acceleration factors greater than the number of animals scanned at once. By simultaneously scanning two mice with a retrospectively gated cardiac cine MRI sequence, a 3-fold acceleration was achieved with signal-to-noise ratio in the heart that is equivalent to that achieved with an unaccelerated scan using a commercial mouse birdcage coil.
... In all cases, including k-t PCA/ SENSE and k-t SENSE, the coil sensitivities were calculated using the sum-of-squares coil combination of the temporal average images as the reference. This procedure is similar to the one described in Ref. (12). ...
The k-t broad-use linear acquisition speed-up technique (BLAST) has become widespread for reducing image acquisition time in dynamic MRI. In its basic form k-t BLAST speeds up the data acquisition by undersampling k-space over time (referred to as k-t space). The resulting aliasing is resolved in the Fourier reciprocal x-f space (x = spatial position, f = temporal frequency) using an adaptive filter derived from a low-resolution estimate of the signal covariance. However, this filtering process tends to increase the reconstruction error or lower the achievable acceleration factor. This is problematic in applications exhibiting a broad range of temporal frequencies such as free-breathing myocardial perfusion imaging. We show that temporal basis functions calculated by subjecting the training data to principal component analysis (PCA) can be used to constrain the reconstruction such that the temporal resolution is improved. The presented method is called k-t PCA.
The rationale for perfusion imaging is based on the fact that patient outcome is mainly driven by myocardial ischemic events. With repetitive episodes of plaque ruptures the luminal narrowing increases and ischemia in situations of increased oxygen demand can develop, which can result in malignant arrhythmias. Also, acute vessel occlusions can occur by plaque ruptures with thrombus formation. The risk of total occlusion in case of plaque rupture increases with increasing stenosis degree or increased plaque volume. Therefore ischemia detected by cardiac magnetic resonance (CMR) identifies patients at risk for arrhythmias and cardiac death, and it also identifies high-degree stenoses prone to result in vessel occlusions (in case of rupture) potentially causing acute myocardial infarctions. In this chapter the different types of vasodilators used for perfusion CMR studies are discussed as well as the various contrast media and their dosing. Then, the various techniques of data analysis (i.e., visual vs. quantitative) and their diagnostic performances are illustrated. In particular, the diagnostic performance is given for single- and multi-center trials and for 1.5 T versus 3 T systems, with coronary anatomy or invasive FFR-based ischemia testing used as reference, and CMR performance is also compared to other modalities like scintigraphy. Finally, extensive evidence demonstrates the strong prognostic power of perfusion CMR (i.e., to exclude relevant coronary artery disease or to guide revascularization in case of ischemia-positive CMR studies). The prognostic data represent approximately 9800 patients of whom 3647 were collected in the European CMR registry, thus reflecting excellent daily clinical routine performance in 59 centers of 18 countries.
Purpose:
Simultaneous acquisition of myocardial first-pass perfusion MRI and 18F-FDG PET viability imaging on integrated whole-body PET/MR hybrid systems synergistically delivers both functional and metabolic information on the tissue state. While PET viability scans are inherently three-dimensional, conventional MR myocardial perfusion imaging is typically performed using only three short-axis slices with a temporal resolution of one RR-interval. To improve the integrated diagnostics, an acquisition and image reconstruction method based on "Multi-Slice Controlled Aliasing In Parallel Imaging Results IN Higher Acceleration (MS-CAIPIRINHA)" was developed extending anatomical coverage for MR perfusion imaging to six short-axis slices per RR-interval.
Methods:
An ECG-gated radial TurboFLASH MR pulse sequence with dual band excitation was implemented on an integrated whole-body PET/MR system and a model-based reconstruction technique was developed to fully reconstruct the undersampled CAIPIRINHA acquisitions. An 18F-FDG viability PET scan was performed simultaneously to the MR protocol, additionally complemented by a late enhancement MRI acquisition (LGE).
Results and conclusion:
The developed imaging technique was tested in five patients with known collateralized coronary total occlusions, resulting in improved characterization of perfusion across areas of decreased tissue viability as indicated by the simultaneously determined 18F-FDG uptake. While conventional MR perfusion with only three slice positions was occasionally missing substantial parts of the viable area, the new approach achieved LV coverage only slightly inferior to LGE imaging and therefore better comparable to PET results. The quality of first-pass enhancement curves was comparable between conventional and radial MS-CAIPIRINHA-based acquisitions.
Cardiovascular magnetic resonance (CMR) is a well established imaging modality for assessing patients with coronary artery disease. CMR images can provide high quality information about myocardial anatomy, function, perfusion, and viability. These are all critical elements in the diagnosis and management of coronary artery disease. Perfusion and viability imaging by CMR most commonly depend on gadolinium contrast agents. This chapter aims to provide background in the methods, validations, and clinical utility of these methods.
Coronary plaque formation impeding myocardial perfusion is the most frequent cause of myocardial ischemia. Myocardial perfusion imaging by MRI is rapidly gaining acceptance as a useful clinical tool to assess the hemodynamic significance of coronary artery stenoses, and to depict myocardial ischemia. MRI techniques have significantly advanced over the last years, enabling to accurately study first-pass myocardial perfusion providing visual, semiquantitative or quantitative data on myocardial blood flow patterns. This chapter focuses on how imaging techniques, in particular MRI, can be used to study myocardial perfusion. The main emphasis of this chapter is on the challenges MRI faces to appropriately study the first-pass of a contrast agent through the heart, and on the solutions and approaches that have been developed to tackle the issues. In a second part, the approaches for image interpretation and to deduct (semi-)quantitative measures of myocardial perfusion are highlighted, and the clinical use and relevance of MRI in daily practice is discussed.
Real-time magnetic resonance imaging (rtMRI) techniques have been described for intravascular guidance and may be immediately applicable to provide vision in a beating heart with circulating blood to guide the surgeon in a minimally invasive cardiac surgery environment. Operative rtMRI guidance has stringent requirements for surgical instruments and devices. Typically, devices that are used during interventions, such as catheters and wires, are not designed to be magnetic resonance visible or compatible as they often contain ferromagnetic materials or long electrical conductors. This chapter illustrates the interactive rtMRI, MRI compatibility of the instruments and devices, and device tracking techniques. It also presents a promising example of MRI-guided cardiac surgery, specifically, transapical aortic valve replacement under rtMRI.
One of the main goals in magnetic resonance angiography (MRA) is to acquire data quickly, both to reduce longer scan times to avoid artifacts due to patient motion (for time-of-flight and phase-contrast imaging) and to capture relevant dynamic information (contrast-enhanced MRA). However, the amount of time it takes to generate an MRI image depends directly on the desired spatial resolution. Images with higher spatial resolution require more k-space data, and thus, a longer time is needed to acquire these data. In order to meet both goals of high spatial resolution and high temporal resolution simultaneously, one must look to advanced image acquisition and reconstruction techniques, such as parallel imaging.
IntroductionThe source of the MRI signalRelaxationContrast agentsImage encodingBasic pulse sequencesCardiac and respiratory synchronizationMRI hardwareReferences
IntroductionDetection of ischemia-induced myocardial dysfunction by cardiac magnetic resonancePerfusion cardiac magnetic resonanceAssessment of myocardial viability by cardiac magnetic resonanceThe role of cardiac magnetic resonance in the workup and management of patients with suspected or known coronary artery diseaseReferences
Coronary artery disease (CAD) is the leading cause of death in the industrialized world [1]. As shown in Figure 15.1, CAD can manifest in different ways, including both chest pain during exercise (stable angina pectoris) and as acute ST-segment (on the ECG) elevation myocardial infarction (STEMI), which requires immediate revascularization. The diagnosis of STEMI is relatively simple and straightforward, with typical chest pain, ST-segment elevation in the resting ECG, and positive serum troponins. However, the non-ST-segment elevation MI (NSTEMI) or its precursor, the non-ST-segment elevation acute coronary syndrome (NSTE-ACS) without relevant necrosis, are more difficult to diagnose. In these patients ischemia should be detected by non-invasive imaging tests or by visualization of coronary stenoses by invasive coronary angiography. Despite substantial progress in the treatment of STEMI by percutaneous coronary interventions (PCI), these patients still suffer from a higher in-hospital mortality rate than those with NSTE-ACS (7% vs. 5%, respectively), but the mortality rates become similar at 6 months of approximately 12% [2, 3].
Objectives:
To demonstrate the feasibility of Real-time magnetic resonance imaging (rtMRI) guided transcatheter aortic valve replacement (TAVR) with an active guidewire and an MRI compatible valve delivery catheter system in a swine model.
Methods:
The CoreValve system was minimally modified to be MRI-compatible by replacing the stainless steel components with fluoroplastic resin and high-density polyethylene components. Eight swine weighing 60-90 kg underwent rtMRI-guided TAVR with an active guidewire through a left subclavian approach.
Results:
Two imaging planes (long-axis view and short-axis view) were used simultaneously for real-time imaging during implantation. Successful deployment was performed without rapid ventricular pacing or cardiopulmonary bypass. Postdeployment images were acquired to evaluate the final valve position in addition to valvular and cardiac function.
Conclusions:
Our results show that the CoreValve can be easily and effectively deployed through a left subclavian approach using rtMRI guidance, a minimally modified valve delivery catheter system, and an active guidewire. This method allows superior visualization before deployment, thereby allowing placement of the valve with pinpoint accuracy. rtMRI has the added benefit of the ability to perform immediate postprocedural functional assessment, while eliminating the morbidity associated with radiation exposure, rapid ventricular pacing, contrast media renal toxicity, and a more invasive procedure. Use of a commercially available device brings this rtMRI-guided approach closer to clinical reality.
PurposeAccurate quantification of myocardial perfusion is dependent on reliable electrocardiogram (ECG) triggering. Measuring myocardial blood flow (MBF) in patients with arrhythmias or poor ECGs is currently infeasible with MR. The purpose of this study was to demonstrate the feasibility of a non–ECG-triggered method with clinically useful three-slice ventricular coverage for measurement of MBF in healthy volunteers.MethodsA saturation recovery magnetization–prepared gradient recalled echo acquisition was continuously repeated during first-pass imaging. A slice-interleaved radial trajectory was employed to enable image-based retrospective triggering. The arterial input function was generated using a beat-by-beat T1 estimation method. The proposed technique was validated against a conventional ECG-triggered dual-bolus technique in 10 healthy volunteers. The technique was further demonstrated under adenosine stress in 12 healthy volunteers.ResultsThe proposed method produced MBF with no significant difference compared with the ECG-triggered technique. The proposed method yielded mean myocardial perfusion reserve comparable to published literature.Conclusion
We have developed a non–ECG-triggered quantitative perfusion imaging method. In this preliminary study, our results demonstrate that our method yields comparable MBF compared with the conventional ECG-triggered method and that it is feasible for stress imaging. Magn Reson Med, 2015. © 2015 Wiley Periodicals, Inc.
Time-adaptive sensitivity encoding (TSENSE) and generalized autocalibrating partially parallel acquisition (GRAPPA) were applied to a gradient-echo sequence used for first-pass myocardial perfusion imaging of 12 patients with coronary artery disease. The two parallel imaging methods were compared in terms of signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR) and image artefacts. Image acquisition was started during the administration of a Gd-contrast bolus (0.1 mmoL/kg) followed by a 20-mL saline flush (3 mL/s), and the next perfusion was started at least 15 min thereafter using an identical bolus. The order of perfusion sequences was inverted in every other patient. Both acquisitions had an acceleration rate of 2, and were performed during breath-holding. The SNR, CNR and image quality of the GRAPPA images were significantly better than were those of the TSENSE images. An exception was the lower CNR of GRAPPA when applied after the second bolus. Differences between subjects were larger with GRAPPA perfusion imaging than with TSENSE. The SNR and CNR also varied relatively much between the GRAPPA images, indicating that the diagnostic value of TSENSE may be superior after all.
To evaluate and to compare Parallel Imaging and Compressed Sensing acquisition and reconstruction frameworks based on simultaneous multislice excitation for high resolution contrast-enhanced myocardial first-pass perfusion imaging with extended anatomic coverage.
The simultaneous multislice imaging technique MS-CAIPIRINHA facilitates imaging with significantly extended anatomic coverage. For additional resolution improvement, equidistant or random undersampling schemes, associated with corresponding reconstruction frameworks, namely Parallel Imaging and Compressed Sensing can be used. By means of simulations and in vivo measurements, the two approaches were compared in terms of reconstruction accuracy. Comprehensive quality metrics were used, identifying statistical and systematic reconstruction errors.
The quality measures applied allow for an objective comparison of the frameworks. Both approaches provide good reconstruction accuracy. While low to moderate noise enhancement is observed for the Parallel Imaging approach, the Compressed Sensing framework is subject to systematic errors and reconstruction induced spatiotemporal blurring.
Both techniques allow for perfusion measurements with a resolution of 2.0 × 2.0 mm(2) and coverage of six slices every heartbeat. Being not affected by systematic deviations, the Parallel Imaging approach is considered to be superior for clinical studies.J. Magn. Reson. Imaging 2013;00:000-000. © 2013 Wiley Periodicals, Inc.
To develop a free-breathing cardiac MR perfusion sequence with slice tracking for use after physical exercise.
We propose to use a leading navigator, placed immediately before each 2D slice acquisition, for tracking the respiratory motion and updating the slice location in real-time. The proposed sequence was used to acquire CMR perfusion datasets in 12 healthy adult subjects and 8 patients. Images were compared with the conventional perfusion (i.e., without slice tracking) results from the same subjects. The location and geometry of the myocardium were quantitatively analyzed, and the perfusion signal curves were calculated from both sequences to show the efficacy of the proposed sequence.
The proposed sequence was significantly better compared with the conventional perfusion sequence in terms of qualitative image scores. Changes in the myocardial location and geometry decreased by 50% in the slice tracking sequence. Furthermore, the proposed sequence had signal curves that are smoother and less noisy.
The proposed sequence significantly reduces the effect of the respiratory motion on the image acquisition in both rest and stress perfusion scans. Magn Reson Med, 2013. © 2013 Wiley Periodicals, Inc.
To develop and evaluate an image reconstruction technique for cardiac MRI (CMR) perfusion that uses localized spatio-temporal constraints.
CMR perfusion plays an important role in detecting myocardial ischemia in patients with coronary artery disease. Breath-hold k-t-based image acceleration techniques are typically used in CMR perfusion for superior spatial/temporal resolution and improved coverage. In this study, we propose a novel compressed sensing-based image reconstruction technique for CMR perfusion, with applicability to free-breathing examinations. This technique uses local spatio-temporal constraints by regularizing image patches across a small number of dynamics. The technique was compared with conventional dynamic-by-dynamic reconstruction, and sparsity regularization using a temporal principal-component (pc) basis, as well as zero-filled data in multislice two-dimensional (2D) and three-dimensional (3D) CMR perfusion. Qualitative image scores were used (1 = poor, 4 = excellent) to evaluate the technique in 3D perfusion in 10 patients and five healthy subjects. On four healthy subjects, the proposed technique was also compared with a breath-hold multislice 2D acquisition with parallel imaging in terms of signal intensity curves.
The proposed technique produced images that were superior in terms of spatial and temporal blurring compared with the other techniques, even in free-breathing datasets. The image scores indicated a significant improvement compared with other techniques in 3D perfusion (x-pc regularization, 2.8 ± 0.5 versus 2.3 ± 0.5; dynamic-by-dynamic, 1.7 ± 0.5; zero-filled, 1.1 ± 0.2). Signal intensity curves indicate similar dynamics of uptake between the proposed method with 3D acquisition and the breath-hold multislice 2D acquisition with parallel imaging.
The proposed reconstruction uses sparsity regularization based on localized information in both spatial and temporal domains for highly accelerated CMR perfusion with potential use in free-breathing 3D acquisitions. Magn Reson Med, 2013. © 2013 Wiley Periodicals, Inc.
The clinical utility of myocardial perfusion MR imaging (MPI) is often restricted by the inability of current acquisition schemes to simultaneously achieve high spatio-temporal resolution, good volume coverage, and high signal to noise ratio. Moreover, many subjects often find it difficult to hold their breath for sufficiently long durations making it difficult to obtain reliable MPI data. Accelerated acquisition of free breathing MPI data can overcome some of these challenges. Recently, an algorithm termed as k - t SLR has been proposed to accelerate dynamic MRI by exploiting sparsity and low rank properties of dynamic MRI data. The main focus of this paper is to further improve k - t SLR and demonstrate its utility in considerably accelerating free breathing MPI. We extend its previous implementation to account for multi-coil radial MPI acquisitions. We perform k - t sampling experiments to compare different radial trajectories and determine the best sampling pattern. We also introduce a novel augmented Lagrangian framework to considerably improve the algorithm's convergence rate. The proposed algorithm is validated using free breathing rest and stress radial perfusion data sets from two normal subjects and one patient with ischemia. k - t SLR was observed to provide faithful reconstructions at high acceleration levels with minimal artifacts compared to existing MPI acceleration schemes such as spatio-temporal constrained reconstruction and k - t SPARSE/SENSE.
Dobutamine-cardiovascular magnetic resonance (dobutamine-CMR) and perfusion-CMR are readily available to assess patients with
suspected coronary artery disease or to determine the haemodynamic relevance of patients with intermediate coronary artery
stenoses. Both tests have good diagnostic accuracy (with dobutamine-CMR being more specific and perfusion-CMR being more sensitive)
and provide prognostically relevant information. Patients with normal MR stress studies show an excellent prognosis (0.7%
event rate per year for the first 2 years) and in most patients with negative studies, no further examinations need to be
performed. In combination with scar imaging and the assessment of LV and RV function and mass, a rapid and complete work-up
of a large group of patients can be offered.
In recent years, there has been an explosive growth of magnetic resonance imaging (MRI) techniques that allow faster scan speed by exploiting temporal or spatiotemporal redundancy of the images. These techniques improve the performance of dynamic imaging significantly across multiple clinical applications, including cardiac functional examinations, perfusion imaging, blood flow assessment, contrast-enhanced angiography, functional MRI, and interventional imaging, among others. The scan acceleration permits higher spatial resolution, increased temporal resolution, shorter scan duration, or a combination of these benefits. Along with the exciting developments is a dizzying proliferation of acronyms and variations of the techniques. The present review attempts to summarize this rapidly growing topic and presents conceptual frameworks to understand these techniques in terms of their underlying mechanics and connections. Techniques from view sharing, keyhole, k-t, to compressed sensing are covered.
Dynamic three-dimensional-cardiac magnetic resonance (3D-CMR) perfusion proved highly diagnostic for the detection of angiographically defined coronary artery disease (CAD) and has been used to assess the efficacy of coronary stenting procedures. The present study aimed to relate significant coronary lesions as assessed by fractional flow reserve (FFR) to the volume of myocardial hypoenhancement on 3D-CMR adenosine stress perfusion imaging and to define the inter-study reproducibility of stress inducible 3D-CMR hypoperfusion.
A total of 120 patients with known or suspected CAD were examined in two CMR centres using 1.5 T systems. The protocol included cine imaging, 3D-CMR perfusion during adenosine infusion, and at rest followed by delayed enhancement (DE) imaging. Fractional flow reserve was recorded in epicardial coronary arteries and side branches with ≥2 mm luminal diameter and >40% severity stenosis (pathologic FFR < 0.75). Twenty-five patients underwent an identical repeat CMR examination for the determination of inter-study reproducibility of 3D-CMR perfusion deficits induced by adenosine. Three-dimensional CMR perfusion scans were visually classified as pathologic if one or more segments showed an inducible perfusion deficit in the absence of DE. Myocardial ischaemic burden (MIB) was measured by segmentation of the area of inducible hypoenhancement and normalized to left ventricular myocardial volume (MIB, %). Three-dimensional CMR perfusion resulted in a sensitivity, specificity, and diagnostic accuracy of 90, 82, and 87%, respectively. Substantial concordance was found for inter-study reproducibility [Lin's correlation coefficient: 0.98 (95% confidence interval: 0.96-0.99)].
Three-dimensional CMR stress perfusion provided high diagnostic accuracy for the detection of functionally significant CAD. Myocardial ischaemic burden measurements were highly reproducible and allowed the assessment of CAD severity.
: The principal limitations of percutaneous techniques to replace the aortic valve are detailed visualization and durable prostheses. We report the feasibility of using real-time magnetic resonance imaging (MRI) to provide precise anatomic detail and visual feedback to implant a proven bioprosthesis.
: Twelve domestic pigs were anesthetized, and, through a minimally invasive approach using real-time MRI guidance, underwent aortic valve replacement. This was accomplished on the beating heart by using a commercially available bioprosthesis. MRI was used to precisely identify the anatomic landmarks of the aortic annulus, coronary artery ostia, and the mitral valve leaflets. Additional intraoperative perfusion, flow velocity, and functional imaging were used to confirm adequacy of placement and function of the valve.
: Under real-time MRI, multiple oblique planes were prescribed to delineate the anatomy of the native aortic valve and left ventricular outflow track. Enhanced by the use of an active marker wire, this imaging allowed correct placement and orientation of the valve. Through a transapical approach, a series of bioprosthetic aortic valves (21 to 25 mm) were inserted. The time to implantation after the placement of the trocar to deployment of the valve was less than 90 seconds. The average procedure duration was less than 40 minutes
: Real-time MRI provides excellent anatomic detail and intraoperative assessment that permits placement of durable valve prostheses on the beating heart without the limitations of percutaneous approaches.
Although spiral trajectories have multiple attractive features such as their isotropic resolution, acquisition efficiency, and robustness to motion, there has been limited application of these techniques to first-pass perfusion imaging because of potential off-resonance and inconsistent data artifacts. Spiral trajectories may also be less sensitive to dark-rim artifacts that are caused, at least in part, by cardiac motion. By careful consideration of the spiral trajectory readout duration, flip angle strategy, and image reconstruction strategy, spiral artifacts can be abated to create high-quality first-pass myocardial perfusion images with high signal-to-noise ratio. The goal of this article was to design interleaved spiral pulse sequences for first-pass myocardial perfusion imaging and to evaluate them clinically for image quality and the presence of dark-rim, blurring, and dropout artifacts.
Exciting multiple slices at the same time, "controlled aliasing in parallel imaging results in higher acceleration" (CAIPIRINHA) and "phase-offset multiplanar" have shown to be very effective techniques in 2D multislice imaging. Being provided with individual rf phase cycles, the simultaneously excited slices are shifted with respect to each other in the FOV and, thus, can be easily separated. For SSFP sequences, however, similar rf phase cycles are required to maintain the steady state, impeding a straightforward application of phase-offset multiplanar or controlled aliasing in parallel imaging results in higher acceleration. In this work, a new flexible concept for applying the two multislice imaging techniques to SSFP sequences is presented. Linear rf phase cycles are introduced providing both in one, the required shift between the slices and steady state in each slice throughout the whole measurement. Consequently, the concept is also appropriate for real-time and magnetization prepared imaging. Steady state properties and shifted banding behavior of the new phase cycles were investigated using simulations and phantom experiments. Moreover, the concept was applied to perform whole heart myocardial perfusion SSFP imaging as well as real-time and cine SSFP imaging with increased coverage. Showing no significant penalties in SNR or image quality, the results successfully demonstrate the general applicability of the concept.
Stress perfusion CMR can provide both excellent diagnostic and important prognostic information in the context of a comprehensive assessment of cardiac anatomy and function. This coupled with the high spatial resolution, and the lack of both attenuation artefacts and ionising radiation, make CMR stress perfusion imaging a highly attractive stress imaging modality. It is now in routine use in many centres, and shows promise in evaluating patients with clinical problems beyond those of epicardial coronary disease.
The purpose of this study is to develop and evaluate a displacement-encoded pulse sequence for simultaneous perfusion and strain imaging. Displacement-encoded images in two to three myocardial slices were repeatedly acquired using a single-shot pulse sequence for 3 to 4 min, which covers a bolus infusion of Gadolinium contrast. The magnitudes of the images were T(1) weighted and provided quantitative measures of perfusion, while the phase maps yielded strain measurements. In an acute coronary occlusion swine protocol (n = 9), segmental perfusion measurements were validated against microsphere reference standard with a linear regression (slope 0.986, R(2) = 0.765, Bland-Altman standard deviation = 0.15 mL/min/g). In a group of ST-elevation myocardial infarction patients (n = 11), the scan success rate was 76%. Short-term contrast washout rate and perfusion are highly correlated (R(2) = 0.72), and the pixelwise relationship between circumferential strain and perfusion was better described with a sigmoidal Hill curve than linear functions. This study demonstrates the feasibility of measuring strain and perfusion from a single set of images.
Percutaneous valve replacements are presently being evaluated in clinical trials. As delivery of the valve is catheter based, the safety and efficacy of these procedures may be influenced by the imaging used. To assist the surgeon and improve the success of the operation, we have performed transapical aortic valve replacements using real-time magnetic resonance imaging guidance.
Twenty-eight swine underwent aortic valve replacement by real-time magnetic resonance imaging on the beating heart. Stentless bioprostheses mounted on balloon-expandable stents were used. Magnetic resonance imaging (1.5 T) was used to identify the critical anatomic landmarks. In addition to anatomic confirmation of adequate placement of the prosthesis, functional assessment of the valve and left ventricle and perfusion were also obtained with magnetic resonance imaging. A series of short-term feasibility experiments were conducted (n = 18) in which the animals were humanely killed after valve placement and assessment by magnetic resonance imaging. Ten additional animals were allowed to survive and had follow-up magnetic resonance imaging scans and confirmatory echocardiography at 1, 3, and 6 months postoperatively.
Real-time magnetic resonance imaging provided superior visualization of the landmarks needed. The time to implantation after apical access was 74 +/- 18 seconds. Perfusion scanning demonstrated adequate coronary flow and functional imaging documented preservation of ventricular contractility in all animals after successful deployment. Phase contrast imaging revealed minimal intravalvular or paravalvular leaks. Longer term results demonstrated stability of the implants with preservation of myocardial perfusion and function over time.
Real-time magnetic resonance imaging provides excellent visualization for intraoperative guidance of aortic valve replacement on the beating heart. Additionally, it allows assessment of tissue perfusion and organ function that is not obtainable by conventional imaging alone.
Noninvasive assessment of myocardial perfusion is important in the diagnosis and risk stratification of patients with known or suspected coronary artery disease (CAD). Although single-photon emission computed tomography (SPECT) is most commonly used, multiple modalities including myocardial contrast echocardiography (MCE), positron emission tomography (PET), cardiac MRI (CMR), andcardiac computed tomography (CT) have emerged as promising techniques. This article will critically evaluate the strengths and weakness of these modalities for evaluating myocardial perfusion.
In k-t sensitivity encoding (SENSE), MR data acquisition performed in parallel by multiple coils is accelerated by sparsely sampling the k-space over time. The resulting aliasing is resolved by exploiting spatiotemporal correlations inherent in dynamic images of natural objects. In this article, a modified k-t SENSE reconstruction approach is presented, which aims at improving the temporal fidelity of first-pass, contrast-enhanced myocardial perfusion images at high accelerations. The proposed technique is based on applying parallel imaging on the training data in order to increase their spatial resolution. At a net acceleration of 5.8 (k-t factor = 8, training profiles = 11) accurate representations of dynamic signal-intensities were achieved. The efficacy of this approach as well as limitations due to noise amplification were investigated in computer simulations and in vivo experiments.
To improve myocardial perfusion magnetic resonance imaging (MRI) by reconstructing undersampled radial data with a spatiotemporal constrained reconstruction method (STCR).
The STCR method jointly reconstructs all of the time-frames for each slice. In 7 subjects at rest, on a 3-T scanner, the method was compared with a conventional (GRAPPA) Cartesian approach.
Increased slice coverage was obtained, as compared with Cartesian acquisitions. On average, 10 slices were obtained per heartbeat for radial acquisitions (8 of which are suitable for visual analysis with the remaining 2 slices, in theory, usable for quantitative purposes), whereas 4 slices were obtained for the conventional Cartesian acquisitions. The new method was robust to interframe motion, unlike using Cartesian undersampling and STCR. STCR produced images with an image quality rating (1 for best and 5 for worst) of 1.7 +/- 0.5; the Cartesian images were rated 2.6 +/- 0.4 (P = 0.0006). A mean improvement of 44 (+/-17) in signal-to-noise (SNR) ratio and 46 (+22) in contrast-to-noise ratio (CNR) was observed for STCR.
The new radial data acquisition and reconstruction scheme for dynamic myocardial perfusion imaging is a promising approach for obtaining significantly higher coverage and improved SNR ratios. Further testing of this approach is warranted during vasodilation in patients with coronary artery disease.
The purpose of this study was to compare fast single-shot gradient-echo (FLASH) and hybrid echo-planar imaging (EPI) magnetic resonance (MR) technologies regarding the relative contrast-to-noise ratio (CNR), spatiotemporal resolution, size of inducible perfusion defects, and presence of artifacts in patients with coronary artery disease (CAD). Fifteen patients with CAD underwent rest and adenosine stress gadolinium first-pass perfusion cardiovascular MR examinations with EPI and FLASH. The study was approved by the local ethics committee, and each subject gave written informed consent. The spatial resolution of the two sequences was made similar in nine patients, and the temporal resolution was made similar in six. The images were assessed for CNR, artifact, and size of inducible perfusion defects. The CNR was significantly higher with the EPI sequence, whether matched for spatial (32 vs 22 [46%], P < .001) or temporal (35 vs 23 [51%], P < .001) resolution. There was no significant difference in scoring for artifact or area and transmural extent of inducible perfusion defects with EPI and FLASH, whether matched for temporal or spatial resolution. Further work is warranted to determine the relative diagnostic accuracy of the two techniques.
Introduction: Cardiac cine imaging is typically performed by acquiring a single slice within a 15-20s breath-hold [1]. Using recent developments that apply short echo-planar readouts, high quality cine imaging can now be performed with shorter scan times, i.e. 3-5 heartbeats [2,3], enabling the acquisition of multiple slices during a single breath-hold. Currently, these techniques acquire data sequentially, completing the acquisition of data for one slice before proceeding to the next. We investigated the use of slice interleaving during a breath-hold acquisition to improve SNR by using longer TRs and higher flip angles without increasing the overall multi-slice scan time. Methods: In slice interleaving, data acquisition for N slices is time multiplexed (Fig.1) such that all slices are excited within each TR. This is distinguished from sequential data acquisition, where only one slice is excited per TR. Therefore, in slice interleaving, the TR increases by a factor of N, leading to increased SNR. Using steady state gradient echo theory [4], we developed the expected SNR improvement, defined as the ratio of the interleaved acquisition SNR to the counterpart sequential acquisition SNR, to be √N. This slice interleaving method was implemented in a fast gradient echo sequence with an echo train readout (FGRE-ET) [2,3]. Both phantom and normal volunteer validation studies were performed for 2,3 and 4 slice acquisitions. Phantom validations were done on a rotating gel cylinder phantom with a T 1 of 550ms. The phantom was rotated at 40 cycles per minute. Imaging took place with a standard head coil using the following parameters: TR 9.6ms, TE 1.7ms, matrix size 256X160, FOV 36cm, phase FOV 0.75 when applicable, RBW ±125kHz, 8mm slices spaced 4 mm apart, echo train length (ETL) of 4. The flip angles used were 11° for the sequential acquisitions and 15°, 18°, and 21° for the 2,3,and 4-slice interleaves respectively, as calculated using the Ernst Angle criterion. Multi-slice imaging of normal volunteers (4 male, 1 female, average age 27±8) was performed with a cardiac coil using the following parameters: TR 14.8ms, TE 2.3ms, matrix size 256X160 or 256X128, FOV 36cm, phase FOV 0.75 when applicable, RBW ±62.5kHz, ETL 4, 8mm slices spaced 4mm apart. The flip angles used were 11°, 15°, 18°, and 21° for the sequential, 2,3,and 4-slice interleaves respectively, as determined using a T 1 of 850ms [5] and the Ernst Angle criterion. All imaging, took place on a 1.5T GE Cardiovascular MRI Scanner (GE Medical Systems, Milwaukee WI). To measure SNR, the signal was taken as the average signal in a myocardial region of interest (ROI) outside any possible flow artifact from the blood pool. The mean background noise was calculated from an ROI and corrected to obtain the noise standard deviation [6]. The average SNR for all phases of a slice was obtained. The SNR improvement for each slice was averaged. Results: Theory predicts a √N improvement in SNR. Figure 2 shows the SNR improvement of the 2,3,and 4-slice interleaved acquisitions for both phantom and volunteer studies. The phantom trials achieved an average of 97±2% of the expected improvement and volunteer trials achieved an average of 92±5%. Figure 3 shows a 4 slice interleaved acquisition compared to a 4 slice sequential acquisition, both acquired with a 23s breath-hold. Interleaving allowed an increase in TR from 14.8ms to 59.2ms and flip angle from 11° to 21°, leading to an 86% increase in SNR. One further advantage of slice interleaving is that while the number of phases is the same as for a sequential acquisition, the sampling window per phase (dt in Fig.1) is reduced, leading shaper images. Discussion: Slice interleaving has shown to significantly increase the SNR compared to the standard sequential acquisition without any increases in total scan time. While the current studies were applied in a segmented gated acquisition, these finding will also hold in multi-slice real time imaging. Furthermore, the data acquisition window is reduced with slice interleaving, leading to a reduction in the temporal motion blurring. Note that interleaved acquisitions will yield cine loops with time offsets one TR between different slices. The implementation of slice interleaving represents an attempt to reinstate a mechanism for SNR improvement, previously used in spin echo imaging, lost due to time constraints imposed by cardiac imaging.
The authors evaluated a magnetization preparation scheme with a "notched" section profile for T1-weighted first-pass myocardial perfusion magnetic resonance (MR) imaging at 1.5 T. The pulse sequence consisted of a preparation sequence followed by an interleaved gradient-echo echo-planar sequence. Image contrast was evaluated in a feasibility study in 12 adult patients. The notched saturation pulse allowed long magnetization recovery times without sacrificing section coverage. Image contrast between normal and ischemic myocardium was excellent.
New theoretical and practical concepts are presented for considerably enhancing the performance of magnetic resonance imaging (MRI) by means of arrays of multiple receiver coils. Sensitivity encoding (SENSE) is based on the fact that receiver sensitivity generally has an encoding effect complementary to Fourier preparation by linear field gradients. Thus, by using multiple receiver coils in parallel scan time in Fourier imaging can be considerably reduced. The problem of image reconstruction from sensitivity encoded data is formulated in a general fashion and solved for arbitrary coil configurations and k-space sampling patterns. Special attention is given to the currently most practical case, namely, sampling a common Cartesian grid with reduced density. For this case the feasibility of the proposed methods was verified both in vitro and in vivo. Scan time was reduced to one-half using a two-coil array in brain imaging. With an array of five coils double-oblique heart images were obtained in one-third of conventional scan time. Magn Reson Med 42:952-962, 1999.
An interleaved gradient-echo echo-planar imaging (IGEPI) sequence was modified for and applied to dynamic contrast-enhanced imaging of the heart. Using IGEPI, images with 3.0 × 3.9 mm nominal in-plane resolution are acquired in 100 ms, enabling eight slices per heartbeat for a heart rate of 60 beats/min. The acquisition speed and use of saturation pre-pulses allows acquisition of short- and long-axis images during the same contrast bolus. IGEPI maintains the acquisition characteristics required for performing a quantitative first-pass perfusion analysis as well as providing improved coverage compared with conventional fast gradient echo.
Fast imaging techniques allow monitoring of contrast medium (CM) first-pass kinetics in a multislice mode. Employing shorter recovery times improves cardiac coverage during first-pass conditions, but potentially flattens signal response in the myocardium. The aim of this study was therefore to compare in patients with suspected coronary artery disease (CAD) two echo-planar imaging strategies yielding either extended cardiac coverage or optimized myocardial signal response (protocol A/B, six/four slices; preparation pulse, 60°/90°; delay time, 10/120 msec; readout flip angle, 10°/50°; respectively). In phantoms and myocardium of normal volunteers (N= 10) the CM-induced signal increase was 2.5–3 times higher with protocol B (P < 0.005) than with protocol A. For the detection of individually diseased coronary arteries (≥1 stenosis with ≥50% diameter reduction on quantitative coronary angiography (QCA)), receiver-operator characteristics of protocol B (signal upslope in 32 sectors/heart) yielded a sensitivity/specificity of 82%/73%, which was superior to protocol A (P < 0.05, N= 14). For the overall detection of CAD, the sensitivity/specificity of protocol B was 85%/81%. An adequate signal response in the myocardium is crucial for a reliable detection of perfusion deficits during first-pass conditions. The presented protocol B detects CAD with a sensitivity and specificity similar to scintigraphic techniques. J. Magn. Reson. Imaging 2001;14:556–562. © 2001 Wiley-Liss, Inc.
A number of different methods have been demonstrated which increase the speed of MR acquisition by decreasing the number of sequential phase encodes. The UNFOLD technique is based on time interleaving of k-space lines in sequential images and exploits the property that the outer portion of the field-of-view is relatively static. The differences in spatial sensitivity of multiple receiver coils may be exploited using SENSE or SMASH techniques to eliminate the aliased component that results from undersampling k-space. In this article, an adaptive method of sensitivity encoding is presented which incorporates both spatial and temporal filtering. Temporal filtering and spatial encoding may be combined by acquiring phase encodes in an interleaved manner. In this way the aliased components are alternating phase. The SENSE formulation is not altered by the phase of the alias artifact; however, for imperfect estimates of coil sensitivities the residual artifact will have alternating phase using this approach. This is the essence of combining temporal filtering (UNFOLD) with spatial sensitivity encoding (SENSE). Any residual artifact will be temporally frequency-shifted to the band edge and thus may be further suppressed by temporal low-pass filtering. By combining both temporal and spatial filtering a high degree of alias artifact rejection may be achieved with less stringent requirements on accuracy of coil sensitivity estimates and temporal low-pass filter selectivity than would be required using each method individually. Experimental results that demonstrate the adaptive spatiotemporal filtering method (adaptive TSENSE) with acceleration factor R = 2, for real-time nonbreath-held cardiac MR imaging during exercise induced stress are presented. Magn Reson Med 45:846–852, 2001. Published 2001 Wiley-Liss, Inc.
The effect of gradient system performance on segmented k-space gradient echo imaging is presented. Three cases were investigated. First, an ideal system that has infinite slew rates and unlimited maximum gradient strengths was considered. Second, a "high speed" imaging system (2.3 (G/cm), 23 (G/cm)/ms) was considered. These two cases were compared with a "conventional" imaging system (1(G/cm), 1.67 (G/cm)/ms). It was found that substantial increases in SNR can be achieved (approximately 45%) by using high speed versus a conventional gradient system, for a TR of 6 ms. For trapezoidal gradient waveforms, there exists an optimum maximum gradient strength for a given slew rate, and any increase in gradient strength above this optimum will not be utilized by an optimized sequence. These studies have shown that increasing TR without decreasing the bandwidth is not a good way to increase SNR for constant scan time.
New theoretical and practical concepts are presented for considerably enhancing the performance of magnetic resonance imaging (MRI) by means of arrays of multiple receiver coils. Sensitivity encoding (SENSE) is based on the fact that receiver sensitivity generally has an encoding effect complementary to Fourier preparation by linear field gradients. Thus, by using multiple receiver coils in parallel scan time in Fourier imaging can be considerably reduced. The problem of image reconstruction from sensitivity encoded data is formulated in a general fashion and solved for arbitrary coil configurations and k-space sampling patterns. Special attention is given to the currently most practical case, namely, sampling a common Cartesian grid with reduced density. For this case the feasibility of the proposed methods was verified both in vitro and in vivo. Scan time was reduced to one-half using a two-coil array in brain imaging. With an array of five coils double-oblique heart images were obtained in one-third of conventional scan time. Magn Reson Med 42:952-962, 1999.
In several applications, MRI is used to monitor the time behavior of the signal in an organ of interest; e.g., signal evolution because of physiological motion, activation, or contrast-agent accumulation. Dynamic applications involve acquiring data in a k-t space, which contains both temporal and spatial information. It is shown here that in some dynamic applications, the t axis of k-t space is not densely filled with information. A method is introduced that can transfer information from the k axes to the t axis, allowing a denser, smaller k-t space to be acquired, and leading to significant reductions in the acquisition time of the temporal frames. Results are presented for cardiac-triggered imaging and functional MRI (fMRI), and are compared with data obtained in a conventional way. The temporal resolution was increased by nearly a factor of two in the cardiac-triggered study, and by as much as a factor of eight in the fMRI study. This increase allowed the acquisition of fMRI activation maps, even when the acquisition time for a single full time frame was actually longer than the paradigm cycle period itself. The new method can be used to significantly reduce the acquisition time of the individual temporal frames in certain dynamic studies. This can be used, for example, to increase the temporal or spatial resolution, increase the spatial coverage, decrease the total imaging time, or alter sequence parameters e.g., repetition time (TR) and echo time (TE) and thereby alter contrast. Magn Reson Med 42:813-828, 1999.
To improve real-time control of interventional procedures such as guidance of catheters, monitoring of ablation therapy, or control of dosage during drug delivery, the image acquisition and reconstruction must be high speed and have low latency (small time delay) in processing. A number of different methods have been demonstrated which increase the speed of MR acquisition by decreasing the number of sequential phase-encodes. A design and implementation of the UNFOLD method which achieves the desired low latency with a recursive temporal filter is presented. The recursive filter design is characterized for this application and compared with more commonly used moving average filters. Experimental results demonstrate low-latency UNFOLD for two applications: 1) high-speed, real-time imaging of the heart to be used in conjunction with cardiac interventional procedures; and 2) the injection of drugs into muscle tissue with contrast enhancement, i.e., monitoring needle insertion and injection of a drug with contrast enhancement properties. Proof-of-concept was demonstrated by injecting a contrast agent. In both applications the UNFOLD technique was used to double the frame rate.
Dynamic contrast myocardial perfusion studies may benefit from methods that speed up the acquisition. Unaliasing by Fourier encoding the overlaps using the temporal dimension (UNFOLD), and a similar linear interpolation method have been shown to be effective at reducing the number of phase encodes needed for cardiac wall motion studies by using interleaved sampling and temporal filtering. Here such methods are evaluated in cardiac dynamic contrast studies, with particular regard to the effects of the choice of filter and the interframe motion. Four different filters were evaluated using a motion-free canine study. Full k-space was acquired and then downsampled to allow for a measure of truth. The different filters gave nearly equivalent images and quantitative flow estimates compared to full k-space. The effect of respiratory motion on these schemes was graphically depicted, and the performance of the four temporal filters was evaluated in seven human subjects with respiratory motion present. The four filters provided images of similar quality. However, none of the filters were effective at eliminating motion artifacts. Motion registration methods or motion-free acquisitions may be necessary to make these reduced FOV approaches clinically useful.
First‐pass cardiac MRI using UNFOLD
- Epstein FH
Epstein FH, Kellman P, McVeigh ER. First-pass cardiac MRI using UNFOLD. Radiology 2000;217
(Suppl):379.
- Madore