Sodium MRI of the human heart at 7.0 T: preliminary results

The objective of this work was to examine the feasibility of three‐dimensional (3D) and whole heart coverage ²³Na cardiac MRI at 7.0 T including single‐cardiac‐phase and cinematic (cine) regimes. A four‐channel transceiver RF coil array tailored for ²³Na MRI of the heart at 7.0 T (f = 78.5 MHz) is proposed. An integrated bow‐tie antenna building block is used for ¹H MR to support shimming, localization and planning in a clinical workflow. Signal absorption rate simulations and assessment of RF power deposition were performed to meet the RF safety requirements. ²³Na cardiac MR was conducted in an in vivo feasibility study. 3D gradient echo (GRE) imaging in conjunction with Cartesian phase encoding (total acquisition time T_AQ = 6 min 16 s) and whole heart coverage imaging employing a density‐adapted 3D radial acquisition technique (T_AQ = 18 min 20 s) were used. For 3D GRE‐based ²³Na MRI, acquisition of standard views of the heart using a nominal in‐plane resolution of (5.0 × 5.0) mm² and a slice thickness of 15 mm were feasible. For whole heart coverage 3D density‐adapted radial ²³Na acquisitions a nominal isotropic spatial resolution of 6 mm was accomplished. This improvement versus 3D conventional GRE acquisitions reduced partial volume effects along the slice direction and enabled retrospective image reconstruction of standard or arbitrary views of the heart. Sodium cine imaging capabilities were achieved with the proposed RF coil configuration in conjunction with 3D radial acquisitions and cardiac gating. Cardiac‐gated reconstruction provided an enhancement in blood–myocardium contrast of 20% versus the same data reconstructed without cardiac gating. The proposed transceiver array enables ²³Na MR of the human heart at 7.0 T within clinical acceptable scan times. This capability is in positive alignment with the needs of explorations that are designed to examine the potential of ²³Na MRI for the assessment of cardiovascular and metabolic diseases. Copyright © 2015 John Wiley & Sons, Ltd.     

Accelerated three‐dimensional cine phase contrast imaging using randomly undersampled echo planar imaging with compressed sensing reconstruction

The aim of this study was to implement and evaluate an accelerated three‐dimensional (3D) cine phase contrast MRI sequence by combining a randomly sampled 3D k‐space acquisition sequence with an echo planar imaging (EPI) readout. An accelerated 3D cine phase contrast MRI sequence was implemented by combining EPI readout with randomly undersampled 3D k‐space data suitable for compressed sensing (CS) reconstruction. The undersampled data were then reconstructed using low‐dimensional structural self‐learning and thresholding (LOST). 3D phase contrast MRI was acquired in 11 healthy adults using an overall acceleration of 7 (EPI factor of 3 and CS rate of 3). For comparison, a single two‐dimensional (2D) cine phase contrast scan was also performed with sensitivity encoding (SENSE) rate 2 and approximately at the level of the pulmonary artery bifurcation. The stroke volume and mean velocity in both the ascending and descending aorta were measured and compared between two sequences using Bland–Altman plots. An average scan time of 3 min and 30 s, corresponding to an acceleration rate of 7, was achieved for 3D cine phase contrast scan with one direction flow encoding, voxel size of 2 × 2 × 3 mm³, foot–head coverage of 6 cm and temporal resolution of 30 ms. The mean velocity and stroke volume in both the ascending and descending aorta were statistically equivalent between the proposed 3D sequence and the standard 2D cine phase contrast sequence. The combination of EPI with a randomly undersampled 3D k‐space sampling sequence using LOST reconstruction allows a seven‐fold reduction in scan time of 3D cine phase contrast MRI without compromising blood flow quantification. Copyright © 2014 John Wiley & Sons, Ltd.     

Evaluation of highly accelerated real‐time cardiac cine MRI in tachycardia

Electrocardiogram (ECG)‐gated breath‐hold cine MRI is considered to be the gold standard test for the assessment of cardiac function. However, it may fail in patients with arrhythmia, impaired breath‐hold capacity and poor ECG gating. Although ungated real‐time cine MRI may mitigate these problems, commercially available real‐time cine MRI pulse sequences using parallel imaging typically yield relatively poor spatiotemporal resolution because of their low image acquisition efficiency. As an extension of our previous work, the purpose of this study was to evaluate the diagnostic quality and accuracy of eight‐fold‐accelerated real‐time cine MRI with compressed sensing (CS) for the quantification of cardiac function in tachycardia, where it is challenging for real‐time cine MRI to provide sufficient spatiotemporal resolution. We evaluated the performances of eight‐fold‐accelerated cine MRI with CS, three‐fold‐accelerated real‐time cine MRI with temporal generalized autocalibrating partially parallel acquisitions (TGRAPPA) and ECG‐gated breath‐hold cine MRI in 21 large animals with tachycardia (mean heart rate, 104 beats per minute) at 3T. For each cine MRI method, two expert readers evaluated the diagnostic quality in four categories (image quality, temporal fidelity of wall motion, artifacts and apparent noise) using a Likert scale (1–5, worst to best). One reader evaluated the left ventricular functional parameters. The diagnostic quality scores were significantly different between the three cine pulse sequences, except for the artifact level between CS and TGRAPPA real‐time cine MRI. Both ECG‐gated breath‐hold cine MRI and eight‐fold accelerated real‐time cine MRI yielded all four scores of ≥ 3.0 (acceptable), whereas three‐fold‐accelerated real‐time cine MRI yielded all scores below 3.0, except for artifact (3.0). The left ventricular ejection fraction (LVEF) measurements agreed better between ECG‐gated cine MRI and eight‐fold‐accelerated real‐time cine MRI (mean difference, –1.6%) than between ECG‐gated cine MRI and three‐fold‐accelerated real‐time cine MRI (mean difference, –5.7%). Eight‐fold‐accelerated real‐time cine MRI with CS yields acceptable diagnostic quality and relatively accurate LVEF measurements in the challenging setting of tachycardia. Copyright © 2013 John Wiley & Sons, Ltd.     

Comprehensive MRI for the detection of subtle alterations in diastolic cardiac function in apoE/LDLR–/– mice with advanced atherosclerosis

ApoE/LDLR^–/– mice represent a reliable model of atherosclerosis. However, it is not clear whether cardiac performance is impaired in this murine model of atherosclerosis. Here, we used MRI to characterize cardiac performance in vivo in apoE/LDLR^–/– mice with advanced atherosclerosis. Six‐month‐old apoE/LDLR^–/– mice and age‐matched C57BL/6J mice (control) were examined using highly time‐resolved cine‐MRI [whole‐chamber left ventricle (LV) imaging] and MR tagging (three slices: basal, mid‐cavity and apical). Global and regional measures of cardiac function included LV volumes, kinetics, time‐dependent parameters, strains and rotations. Histological analysis was performed using OMSB (orceine with Martius, Scarlet and Blue) and ORO (oil red‐O) staining to demonstrate the presence of advanced coronary atherosclerosis. MR‐tagging‐based strain analysis in apoE/LDLR^–/– mice revealed an increased frequency of radial and circumferential systolic stretch (25% and 50% of segments, respectively, p ≤ 0.012), increased radial post‐systolic strain index (45% of segments, p = 0.009) and decreased LV untwisting rate (?30.3° (11.6°)/cycle, p = 0.004) when compared with control mice. Maximal strains and LV twist were unchanged. Most of the cine‐MRI‐based LV functional and anatomical parameters also remained unchanged in apoE/LDLR^–/–mice, with only a lower filling rate, longer filling time, shorter isovolumetric contraction time and slower heart rate observed in comparison with control mice. The coronary arteries displayed severe atherosclerosis, as evidenced by histological analysis. Using comprehensive MRI methods, we have demonstrated that, despite severe coronary atherosclerosis in six‐month‐old apoE/LDLR^–/– mice, cardiac performance including global parameters, twist and strains, was well preserved. Only subtle diastolic alterations, possibly of ischemic background, were uncovered. Copyright © 2016 John Wiley & Sons, Ltd.     

Rapid reconstruction of highly undersampled,non‐Cartesian real‐time cine k‐space data using a perceptual complex neural network (PCNN)

Highly accelerated real‐time cine MRI using compressed sensing (CS) is a promising approach to achieve high spatio‐temporal resolution and clinically acceptable image quality in patients with arrhythmia and/or dyspnea. However, its lengthy image reconstruction time may hinder its clinical translation. The purpose of this study was to develop a neural network for reconstruction of non‐Cartesian real‐time cine MRI k‐space data faster (<1 min per slice with 80 frames) than graphics processing unit (GPU)‐accelerated CS reconstruction, without significant loss in image quality or accuracy in left ventricular (LV) functional parameters. We introduce a perceptual complex neural network (PCNN) that trains on complex‐valued MRI signal and incorporates a perceptual loss term to suppress incoherent image details. This PCNN was trained and tested with multi‐slice, multi‐phase, cine images from 40 patients (20 for training, 20 for testing), where the zero‐filled images were used as input and the corresponding CS reconstructed images were used as practical ground truth. The resulting images were compared using quantitative metrics (structural similarity index (SSIM) and normalized root mean square error (NRMSE)) and visual scores (conspicuity, temporal fidelity, artifacts, and noise scores), individually graded on a five‐point scale (1, worst; 3, acceptable; 5, best), and LV ejection fraction (LVEF). The mean processing time per slice with 80 frames for PCNN was 23.7 ± 1.9 s for pre‐processing (Step 1, same as CS) and 0.822 ± 0.004 s for dealiasing (Step 2, 166 times faster than CS). Our PCNN produced higher data fidelity metrics (SSIM = 0.88 ± 0.02, NRMSE = 0.014 ± 0.004) compared with CS. While all the visual scores were significantly different (P < 0.05), the median scores were all 4.0 or higher for both CS and PCNN. LVEFs measured from CS and PCNN were strongly correlated (R² = 0.92) and in good agreement (mean difference = ?1.4% [2.3% of mean]; limit of agreement = 10.6% [17.6% of mean]). The proposed PCNN is capable of rapid reconstruction (25 s per slice with 80 frames) of non‐Cartesian real‐time cine MRI k‐space data, without significant loss in image quality or accuracy in LV functional parameters.     

Self‐gated cardiac Cine MRI of the rat on a clinical 3 T MRI system

The ability to perform small animal functional cardiac imaging on clinical MRI scanners may be of particular value in cases in which the availability of a dedicated high field animal MRI scanner is limited. Here, we propose radial MR cardiac imaging in the rat on a whole‐body clinical 3 T scanner in combination with interspersed projection navigators for self‐gating without any additional external triggering requirements for electrocardiogram (ECG) and respiration. Single navigator readouts were interspersed using the same TR and a high navigator frequency of 54 Hz into a radial golden‐angle acquisition. The extracted navigator function was thresholded to exclude data for reconstruction from inhalation phases during the breathing cycle, enabling free breathing acquisition. To minimize flow artifacts in the dynamic cine images a center‐out half echo radial acquisition scheme with ramp sampling was used. Navigator functions were derived from the corresponding projection navigator data from which both respiration and cardiac cycles were extracted. Self‐gated cine acquisition resulted in high‐quality cardiac images which were free of major artifacts with spatial resolution of up to 0.21 × 0.21 × 1.00 mm³ and a contrast‐to‐noise ratio (CNR) of 21 ± 3 between the myocardium and left ventricle. Self‐gated golden ratio based radial acquisition successfully acquired cine images of the rat heart on a clinical MRI system without the need for dedicated animal ECG equipment. Copyright © 2014 John Wiley & Sons, Ltd.     

Feasibility of real‐time cardiac MRI in mice using tiny golden angle radial sparse

Cardiovascular magnetic resonance imaging has proven valuable for the assessment of structural and functional cardiac abnormalities. Even although it is an established imaging method in small animals, the long acquisition times of gated or self‐gated techniques still limit its widespread application. In this study, the application of tiny golden angle radial sparse MRI (tyGRASP) for real‐time cardiac imaging was tested in 12 constitutive nexilin (Nexn) knock‐out (KO) mice, both heterozygous (Het, N = 6) and wild‐type (WT, N = 6), and the resulting functional parameters were compared with a well‐established self‐gating approach. Real‐time images were reconstructed for different temporal resolutions of between 16.8 and 79.8 ms per image. The suggested approach was additionally tested for dobutamine stress and qualitative first‐pass perfusion imaging. Measurements were repeated twice within 2 weeks for reproducibility assessment. In direct comparison with the high‐quality, self‐gated technique, the real‐time approach did not show any significant differences in global function parameters for acquisition times below 50 ms (rest) and 31.5 ms (stress). Compared with WT, the end‐diastolic volume (EDV) and end‐systolic volume (ESV) were markedly higher (P < 0.05) and the ejection fraction (EF) was significantly lower in the Het Nexn‐KO mice at rest (P < 0.001). For the stress investigation, a clear decrease of EDV and ESV, and an increase in EF, but maintained stroke volume, could be observed in both groups. Combined with ECG‐triggering, tyGRASP provided first‐pass perfusion data with a temporal resolution of one image per heartbeat, allowing the quantitative assessment of upslope curves in the blood‐pool and myocardium. Excellent inter‐study reproducibility was achieved in all the functional parameters. The tyGRASP is a valuable real‐time MRI technique for mice, which significantly reduces the scan time in preclinical cardiac functional imaging, providing sufficient image quality for deriving accurate functional parameters, and has the potential to investigate real‐time and beat‐to‐beat changes.     

Accelerated ferumoxytol‐enhanced 4D multiphase,steady‐state imaging with contrast enhancement (MUSIC) cardiovascular MRI: validation in pediatric congenital heart disease

The purpose of this work was to validate a parallel imaging (PI) and compressed sensing (CS) combined reconstruction method for a recently proposed 4D non‐breath‐held, multiphase, steady‐state imaging technique (MUSIC) cardiovascular MRI in a cohort of pediatric congenital heart disease patients. We implemented a graphics processing unit accelerated CS‐PI combined reconstruction method and applied it in 13 pediatric patients who underwent cardiovascular MRI after ferumoxytol administration. Conventional breath‐held contrast‐enhanced magnetic resonance angiography (CE‐MRA) was first performed during the first pass of ferumoxytol injection, followed by the original MUSIC and the proposed CS‐PI MUSIC during the steady‐state distribution phase of ferumoxytol. Qualities of acquired images were then evaluated using a four‐point scale. Left ventricular volumes and ejection fractions calculated from the original MUSIC and the CS‐PI MUSIC were also compared with conventional multi‐slice 2D cardiac cine MRI. The proposed CS‐PI MUSIC reduced the imaging time of the MUSIC acquisition to 4.6 ± 0.4 min from 8.9 ± 1.2 min. Computationally intensive image reconstruction was completed within 5 min without interruption of sequential clinical scans. The proposed method (mean 3.3–4.0) provided image quality comparable to that of the original MUSIC (3.2–4.0) (all P ≥ 0.42), and better than conventional breath‐held first‐pass CE‐MRA (1.1–3.3) for 13 anatomical structures (all P ≤ 0.0014) with good inter‐observer agreement (κ > 0.46). The calculated ventricular volumes and ejection fractions from both original MUSIC (r > 0.90) and CS‐PI MUSIC (r > 0.85) correlated well with 2D cine imaging. In conclusion, PI and CS were successfully incorporated into the 4D MUSIC acquisition to further reduce scan time by approximately 50% while maintaining highly comparable image quality in a clinically practical reconstruction time.     

A radial sampling strategy for uniform k‐space coverage with retrospective respiratory gating in 3D ultrashort‐echo‐time lung imaging

The purpose of this work was to develop a 3D radial‐sampling strategy which maintains uniform k‐space sample density after retrospective respiratory gating, and demonstrate its feasibility in free‐breathing ultrashort‐echo‐time lung MRI. A multi‐shot, interleaved 3D radial sampling function was designed by segmenting a single‐shot trajectory of projection views such that each interleaf samples k‐space in an incoherent fashion. An optimal segmentation factor for the interleaved acquisition was derived based on an approximate model of respiratory patterns such that radial interleaves are evenly accepted during the retrospective gating. The optimality of the proposed sampling scheme was tested by numerical simulations and phantom experiments using human respiratory waveforms. Retrospectively, respiratory‐gated, free‐breathing lung MRI with the proposed sampling strategy was performed in healthy subjects. The simulation yielded the most uniform k‐space sample density with the optimal segmentation factor, as evidenced by the smallest standard deviation of the number of neighboring samples as well as minimal side‐lobe energy in the point spread function. The optimality of the proposed scheme was also confirmed by minimal image artifacts in phantom images. Human lung images showed that the proposed sampling scheme significantly reduced streak and ring artifacts compared with the conventional retrospective respiratory gating while suppressing motion‐related blurring compared with full sampling without respiratory gating. In conclusion, the proposed 3D radial‐sampling scheme can effectively suppress the image artifacts due to non‐uniform k‐space sample density in retrospectively respiratory‐gated lung MRI by uniformly distributing gated radial views across the k‐space. Copyright © 2016 John Wiley & Sons, Ltd.     

3D variable‐density SPARKLING trajectories for high‐resolution T2*‐weighted magnetic resonance imaging

We have recently proposed a new optimization algorithm called SPARKLING (Spreading Projection Algorithm for Rapid K‐space sampLING) to design efficient compressive sampling patterns for magnetic resonance imaging (MRI). This method has a few advantages over conventional non‐Cartesian trajectories such as radial lines or spirals: i) it allows to sample the k‐space along any arbitrary density while the other two are restricted to radial densities and ii) it optimizes the gradient waveforms for a given readout time. Here, we introduce an extension of the SPARKLING method for 3D imaging by considering both stacks‐of‐SPARKLING and fully 3D SPARKLING trajectories. Our method allowed to achieve an isotropic resolution of 600 μm in just 45 seconds for T2? ‐weighted ex vivo brain imaging at 7 Tesla over a field‐of‐view of 200 × 200 × 140 mm³ . Preliminary in vivo human brain data shows that a stack‐of‐SPARKLING is less subject to off‐resonance artifacts than a stack‐of‐spirals.     

Partitioning k‐space for cylindrical three‐dimensional rapid acquisition with relaxation enhancement imaging in the mouse brain

Three‐dimensional rapid acquisition with relaxation enhancement (RARE) scans require the assignment of each phase encode step in two dimensions to an echo in the echo train. Although this assignment is frequently made across the entire Cartesian grid, collection of only the central cylinder of k‐space by eliminating the corners in each phase encode dimension reduces the scan time by ~22% with negligible impact on image quality. The recipe for the assignment of echoes to grid points for such an acquisition is less straightforward than for the simple full Cartesian acquisition case, and has important implications for image quality. We explored several methods of partitioning k‐space—exploiting angular symmetry in one extreme or emulating a cropped Cartesian acquisition in the other—and acquired three‐dimensional RARE magnetic resonance imaging (MRI) scans of the ex vivo mouse brain. We evaluated each partitioning method for sensitivity to artifacts and then further considered strategies to minimize these through averaging or interleaving of echoes and by empirical phase correction. All scans were collected 16 at a time with multiple‐mouse MRI. Although all schemes considered could be used to generate images, the results indicate that the emulation of a standard Cartesian echo assignment, by partitioning preferentially along one dimension within the cylinder, is more robust to artifacts. Samples at the periphery of the bore showed larger phase deviations and higher sensitivity to artifacts, but images of good quality could still be obtained with an optimized acquisition protocol. A protocol for high‐resolution (40 μm) ex vivo images using this approach is presented, and has been used routinely with a success rate of 99% in over 1000 images.     

Highly accelerated,real‐time phase‐contrast MRI using radial k‐space sampling and GROG‐GRASP reconstruction: a feasibility study in pediatric patients with congenital heart disease

Retrospective electrocardiogram‐gated, 2D phase‐contrast (PC) flow MRI is routinely used in clinical evaluation of valvular/vascular disease in pediatric patients with congenital heart disease (CHD). In patients not requiring general anesthesia, clinical standard PC is conducted with free breathing for several minutes per slice with averaging. In younger patients under general anesthesia, clinical standard PC is conducted with breath‐holding. One approach to overcome this limitation is using either navigator gating or self‐navigation of respiratory motion, at the expense of lengthening scan times. An alternative approach is using highly accelerated, free‐breathing, real‐time PC (rt‐PC) MRI, which to date has not been evaluated in CHD patients. The purpose of this study was to develop a 38.4‐fold accelerated 2D rt‐PC pulse sequence using radial k‐space sampling and compressed sensing with 1.5 × 1.5 × 6.0 mm³ nominal spatial resolution and 40 ms nominal temporal resolution, and evaluate whether it is capable of accurately measuring flow in 17 pediatric patients (aortic valve, pulmonary valve, right and left pulmonary arteries) compared with clinical standard 2D PC (either breath‐hold or free breathing). For clinical translation, we implemented an integrated reconstruction pipeline capable of producing DICOMs of the order of 2 min per time series (46 frames). In terms of association, forward volume, backward volume, regurgitant fraction, and peak velocity at peak systole measured with standard PC and rt‐PC were strongly correlated (R² > 0.76; P < 0.001). Compared with clinical standard PC, in terms of agreement, forward volume (mean difference = 1.4% (3.0% of mean)) and regurgitant fraction (mean difference = ?2.5%) were in good agreement, whereas backward volume (mean difference = ?1.1 mL (28.2% of mean)) and peak‐velocity at peak systole (mean difference = ?21.3 cm/s (17.2% of mean)) were underestimated by rt‐PC. This study demonstrates that the proposed rt‐PC with the said spatial resolution and temporal resolution produces relatively accurate forward volumes and regurgitant fractions but underestimates backward volumes and peak velocities at peak systole in pediatric patients with CHD.     

Real-time phase-contrast MRI of cardiovascular blood flow using undersampled radial fast low-angle shot and nonlinear inverse reconstruction

Velocity‐encoded phase‐contrast MRI of cardiovascular blood flow commonly relies on electrocardiogram‐synchronized cine acquisitions of multiple heartbeats to quantitatively determine the flow of an averaged cardiac cycle. Here, we present a new method for real‐time phase‐contrast MRI that combines flow‐encoding gradients with highly undersampled radial fast low‐angle shot acquisitions and phase‐sensitive image reconstructions by regularized nonlinear inversion. Apart from calibration studies using steady and pulsatile flow, preliminary in vivo applications focused on through‐plane flow in the ascending aorta of healthy subjects. With bipolar velocity‐encoding gradients of alternating polarity that overlap the slice‐refocusing gradient, the method yields flow‐encoded images with an in‐plane resolution of 1.8 mm, section thickness of 6 mm and measuring time at 3 T of 24 ms (TR/TE = 3.44/2.76 ms; flip angle, 10º; seven radial spokes per image). Accordingly, phase‐contrast maps and corresponding velocity profiles achieve a temporal resolution of 48 ms. The evaluated peak velocities, stroke volumes, flow rates and respective variances over at least 20 consecutive heartbeats are in general agreement with literature data. Copyright © 2011 John Wiley & Sons, Ltd.     

Fast,reproducible measurement of the vascular input function in mice using constrained reconstruction and cardiac sampling

Dynamic contrast‐enhanced MRI is often used to assess the response to therapy in small animal models of cancer. Rigorous quantification of dynamic contrast‐enhanced MRI data using common pharmacokinetic models requires dynamic determination of the concentration of contrast in tumor tissue and in blood. Measurement of the blood concentration, or vascular input function (VIF), requires high temporal resolution and is prone to distortion as a result of flow and partial volume artifacts when measured in local blood vessels. We have developed a strategy for the robust measurement of VIF in mice that uses a constrained reconstruction algorithm to enable sampling from the left ventricle of the heart. The feasibility of the algorithm and its resistance to cardiac motion are demonstrated in vivo and through numerical simulations. VIF sampling is interleaved with slices dedicated to tumor coverage to yield a fast VIF sampling period (81 ms) that is decoupled from the temporal resolution of tumor data (3.9 s). The algorithm provides results that agree with fully encoded measurements in the slowly varying component of VIF to within a 4% root‐mean‐square signal difference. Analysis of a parametric representation of VIFs measured in a population of mice showed a significant reduction in variations observed within subjects (5%–58% over four parameters; p < 0.05) and a reduction in variations between subjects (19%–62%) when using this technique. Preliminary dynamic measurements in an orthotopic xenograft model of anaplastic thyroid cancer revealed a decrease in the variation of pharmacokinetic parameters between mice by a factor of two. Copyright © 2010 John Wiley & Sons, Ltd.     

Left ventricular function in the post-infarct failing mouse heart by magnetic resonance imaging and conductance catheter: a comparative analysis

Aim: Murine myocardial infarction (MI) models are increasingly used in heart failure studies. Magnetic resonance imaging (MRI) and pressure–volume loops by conductance catheter (CC) enable physiological phenotyping. We performed a comparative analysis of MRI vs. CC to assess left ventricular (LV) function in the failing mouse heart. Methods: MI was created by LAD ligation. MRI (day 14) and CC (day 15) were used to determine LV end‐diastolic volume (EDV), end‐systolic volume (ESV) and ejection fraction (EF). Results: Pooled data yielded moderate‐to‐strong linear correlations: EDV: R = 0.61; ESV: R = 0.72; EF: R = 0.81. We analysed three groups, no MI (sham, n = 10), small MI (<30% of LV, n = 14) and large MI (>30%, n = 20). Volumes and EF were consistently lower by CC than by MRI, but group differences were evident for both techniques. Receiver‐operating characteristic analysis indicated good sensitivity and specificity for both techniques, with superior results for MRI. Conclusions: CC and MRI are highly valuable for evaluation of LV volume and function. MRI is recommended for longitudinal studies, accurate absolute volumes and anatomical information. Unique features of CC are its online signal with high temporal resolution, and advanced analysis of LV function and energetics.     

Rapid and robust pulmonary proton ZTE imaging in the mouse

Pulmonary MRI is challenging because of the low proton density and rapid transverse relaxation in the lung associated with microscopic magnetic field inhomogeneities caused by tissue–air interfaces. Therefore, low signal is obtained in gradient and spin echo proton images. Alternatively, non‐proton MRI using hyperpolarized gases or radial techniques with ultrashort or zero TE have been proposed to image the lung. Also with the latter approach, the general challenge remains to provide full coverage of the lung at sufficient spatial resolution, signal‐to‐noise ratio (SNR) and image quality within a reasonable scan time. This task is further aggravated by physiological motion and is particularly demanding in small animals, such as mice. In this work, three‐dimensional (3D) zero echo time (ZTE) imaging is employed for efficient pulmonary MRI. Four protocols with different averaging and respiratory triggering schemes are developed and compared with respect to image quality and SNR. To address the critical issue of background signal in ZTE images, a subtraction approach is proposed, providing images virtually free of disturbing signal from nearby hardware parts. The protocols are tested for pulmonary MRI in six mice at 4.7 T, consistently providing images of high quality with a 3D isotropic resolution of 313 µm and SNR values in the lung between 8.0 and 18.5 within scan times between 1 min 21 s and 4 min 44 s. A generally high robustness of the ZTE approach against motion is observed, whilst respiratory triggering further improves the SNR and visibility of image details. The developed techniques are expected to enable efficient preclinical animal studies in the lung and will also be of importance for human applications. Further improvements are expected from radiofrequency (RF) coils with increased SNR and reduced background signal. Copyright © 2014 John Wiley & Sons, Ltd.     

Comparison of different compressed sensing algorithms for low SNR 19F MRI applications—Imaging of transplanted pancreatic islets and cells labeled with perfluorocarbons

Transplantation of pancreatic islets is a possible treatment option for patients suffering from Type I diabetes. In vivo imaging of transplanted islets is important for assessment of the transplantation site and islet distribution. Thanks to its high specificity, the absence of intrinsic background signal in tissue and its potential for quantification, ¹⁹F MRI is a promising technique for monitoring the fate of transplanted islets in vivo. In order to overcome the inherent low sensitivity of ¹⁹F MRI, leading to long acquisition times with low signal‐to‐noise ratio (SNR), compressed sensing (CS) techniques are a valuable option. We have validated and compared different CS algorithms for acceleration of ¹⁹F MRI acquisition in a low SNR regime using pancreatic islets labeled with perfluorocarbons both in vitro and in vivo. Using offline simulation on both in vitro and in vivo low SNR fully sampled ¹⁹F MRI datasets of labeled islets, we have shown that CS is effective in reducing the image acquisition time by a factor of three to four without seriously affecting SNR, regardless of the particular algorithms used in this study, with the exception of CoSaMP. Using CS, signals can be detected that might have been missed by conventional ¹⁹F MRI. Among different algorithms (SPARSEMRI, OMMP, IRWL1, Two‐level and CoSAMP), the two‐level l₁ method has shown the best performance if computational time is taken into account. We have demonstrated in this study that different existing CS algorithms can be used effectively for low SNR ¹⁹F MRI. An up to fourfold gain in SNR/scan time could be used either to reduce the scan time, which is beneficial for clinical and translational applications, or to increase the number of averages, to potentially detect otherwise undetected signal when compared with conventional ¹⁹F MRI acquisitions. Potential applications in the field of cell therapy have been demonstrated.     

Accelerated high‐frame‐rate mouse heart cine‐MRI using compressed sensing reconstruction

We introduce a new protocol to obtain very high‐frame‐rate cinematographic (Cine) MRI movies of the beating mouse heart within a reasonable measurement time. The method is based on a self‐gated accelerated fast low‐angle shot (FLASH) acquisition and compressed sensing reconstruction. Key to our approach is that we exploit the stochastic nature of the retrospective triggering acquisition scheme to produce an undersampled and random k–t space filling that allows for compressed sensing reconstruction and acceleration. As a standard, a self‐gated FLASH sequence with a total acquisition time of 10 min was used to produce single‐slice Cine movies of seven mouse hearts with 90 frames per cardiac cycle. Two times (2×) and three times (3×) k–t space undersampled Cine movies were produced from 2.5‐ and 1.5‐min data acquisitions, respectively. The accelerated 90‐frame Cine movies of mouse hearts were successfully reconstructed with a compressed sensing algorithm. The movies had high image quality and the undersampling artifacts were effectively removed. Left ventricular functional parameters, i.e. end‐systolic and end‐diastolic lumen surface areas and early‐to‐late filling rate ratio as a parameter to evaluate diastolic function, derived from the standard and accelerated Cine movies, were nearly identical. Copyright © 2012 John Wiley & Sons, Ltd.     

Rapid dealiasing of undersampled,non‐Cartesian cardiac perfusion images using U‐net

Compressed sensing (CS) is a promising method for accelerating cardiac perfusion MRI to achieve clinically acceptable image quality with high spatial resolution (1.6 × 1.6 × 8 mm³) and extensive myocardial coverage (6–8 slices per heartbeat). A major disadvantage of CS is its relatively lengthy processing time (~8 min per slice with 64 frames using a graphics processing unit), thereby making it impractical for clinical translation. The purpose of this study was to implement and test whether an image reconstruction pipeline including a neural network is capable of reconstructing 6.4‐fold accelerated, non‐Cartesian (radial) cardiac perfusion k‐space data at least 10 times faster than CS, without significant loss in image quality. We implemented a 3D (2D + time) U‐Net and trained it with 132 2D + time datasets (coil combined, zero filled as input; CS reconstruction as reference) with 64 time frames from 28 patients (8448 2D images in total). For testing, we used 56 2D + time coil‐combined, zero‐filled datasets (3584 2D images in total) from 12 different patients as input to our trained U‐Net, and compared the resulting images with CS reconstructed images using quantitative metrics of image quality and visual scores (conspicuity of wall enhancement, noise, artifacts; each score ranging from 1 (worst) to 5 (best), with 3 defined as clinically acceptable) evaluated by readers. Including pre‐ and post‐processing steps, compared with CS, U‐Net significantly reduced the reconstruction time by 14.4‐fold (32.1 ± 1.4 s for U‐Net versus 461.3 ± 16.9 s for CS, p < 0.001), while maintaining high data fidelity (structural similarity index = 0.914 ± 0.023, normalized root mean square error = 1.7 ± 0.3%, identical mean edge sharpness of 1.2 mm). The median visual summed score was not significantly different (p = 0.053) between CS (14; interquartile range (IQR) = 0.5) and U‐Net (12; IQR = 0.5). This study shows that the proposed pipeline with a U‐Net is capable of reconstructing 6.4‐fold accelerated, non‐Cartesian cardiac perfusion k‐space data 14.4 times faster than CS, without significant loss in data fidelity or image quality.     

Effects of sex,gonadectomy, and oestrogen substitution on ischaemic preconditioning and ischaemia–reperfusion injury in mice

Aim: Ischaemic preconditioning (IPC) has been demonstrated to protect heart function and viability, but has been predominantly studied in male animals. Methods: We studied a possible influence of sex and oestrogen for protection in IPC. Infarct size and heart function after 40 min global ischaemia and 60 min reperfusion with or without preceding classic IPC was investigated in Langendorff‐perfused hearts. Hearts were harvested from 10‐week‐old male and female C57BL6 mice with or without gonadectomy 6 weeks earlier, or gonadectomy and substitution with 17β‐oestradiol for 4 weeks (n = 104). Results: Classic IPC reduced depression of left ventricular developed pressure (P < 0.01), attenuated the increase of end‐diastolic pressure (P < 0.01), and reduced infarct size (P < 0.01) in hearts of untreated male mice, but failed to protect untreated females which had improved functional recovery and smaller infarctions than untreated males. After gonadectomy of female mice, developed pressure was reduced (P < 0.01) and infarct size increased (P < 0.01) compared with normal females, with no protection of preconditioning. The changes were not reversed by 17β‐oestradiol substitution. In hearts of gonadectomized males, the post‐ischaemic increase of end‐diastolic pressure was attenuated (P < 0.01), and enhanced after substitution with 17β‐oestradiol (P < 0.01). The preconditioning effect disappeared after gonadectomy and gonadectomy with substitution in male mice. Conclusion: There is a sex difference in evoking preconditioning in male and female mice which is only partially dependent on sex hormones.