Faculty Bibliography

PURPOSE: To allow fast and high-quality reconstruction of clinical accelerated multi-coil MR data by learning a variational network that combines the mathematical structure of variational models with deep learning. THEORY AND METHODS: Generalized compressed sensing reconstruction formulated as a variational model is embedded in an unrolled gradient descent scheme. All parameters of this formulation, including the prior model defined by filter kernels and activation functions as well as the data term weights, are learned during an offline training procedure. The learned model can then be applied online to previously unseen data. RESULTS: The variational network approach is evaluated on a clinical knee imaging protocol for different acceleration factors and sampling patterns using retrospectively and prospectively undersampled data. The variational network reconstructions outperform standard reconstruction algorithms, verified by quantitative error measures and a clinical reader study for regular sampling and acceleration factor 4. CONCLUSION: Variational network reconstructions preserve the natural appearance of MR images as well as pathologies that were not included in the training data set. Due to its high computational performance, that is, reconstruction time of 193 ms on a single graphics card, and the omission of parameter tuning once the network is trained, this new approach to image reconstruction can easily be integrated into clinical workflow. Magn Reson Med, 2017. (c) 2017 International Society for Magnetic Resonance in Medicine.

PURPOSE: To describe a new bench measurement based on quality (Q) factors to estimate the coil noise relative to the sample noise of dipole antennas at 7 T. METHODS: Placing a dipole antenna close to a highly conductive sample surrogate (HCSS) greatly reduces radiation loss, and using QHCSS gives a more accurate estimate of coil resistance than Qunloaded . Instead of using the ratio of unloaded and sample-loaded Q factors, the ratio of HCSS-loaded and sample-loaded Q factors should be used at ultra-high fields. A series of simulations were carried out to analyze the power budget of sample-loaded or HCSS-loaded dipole antennas. Two prototype dipole antennas were also constructed for bench measurements to validate the simulations. RESULTS: Simulations showed that radiation loss was suppressed when the dipole antenna was HCSS-loaded, and coil loss was largely the same as when the dipole was loaded by the sample. Bench measurements also showed good alignment with simulations. CONCLUSIONS: Using the ratio QHCSS /Qloaded gives a good estimate of the coil loss for dipole antennas at 7 T, and provides a convenient bench measurement to predict the body noise dominance of dipole antenna designs. The new approach also applies to conventional surface loop coils at ultra-high fields. Magn Reson Med 2017. (c) 2017 International Society for Magnetic Resonance in Medicine.

PURPOSE: Previous work with body-size objects suggested that loops are optimal MR detectors at low fields, whereas electric dipoles are required to maximize signal-to-noise ratio (SNR) at ultrahigh fields ( >/= 7 T). Here we investigated how many loops and/or dipoles are needed to approach the ultimate intrinsic SNR (UISNR) at various field strengths. METHODS: We calculated the UISNR inside dielectric cylinders mimicking different anatomical regions. We assessed the performance of various arrays with respect to the UISNR. We validated our results by comparing simulated and experimental coil performance maps. RESULTS: Arrays with an increasing number of loops can rapidly approach the UISNR at fields up to 3 T, but are suboptimal at ultrahigh fields for body-size objects. The opposite is true for dipole arrays. At 7 T and above, 16 dipoles provide considerably larger central SNR than any possible loop array, and minimal g factor penalty for parallel imaging. CONCLUSIONS: Electric dipoles can be advantageous at ultrahigh fields because they can produce both curl-free and divergence-free currents, whereas loops are limited to divergence-free contributions only. Combining loops and dipoles may be optimal for body imaging at 3 T, whereas arrays of loops or dipoles alone may perform better at lower or higher field strengths, respectively. Magn Reson Med, 2017. (c) 2017 International Society for Magnetic Resonance in Medicine.

PURPOSE: A 5D whole-heart sparse imaging framework is proposed for simultaneous assessment of myocardial function and high-resolution cardiac and respiratory motion-resolved whole-heart anatomy in a single continuous noncontrast MR scan. METHODS: A non-electrocardiograph (ECG)-triggered 3D golden-angle radial balanced steady-state free precession sequence was used for data acquisition. The acquired 3D k-space data were sorted into a 5D dataset containing separated cardiac and respiratory dimensions using a self-extracted respiratory motion signal and a recorded ECG signal. Images were then reconstructed using XD-GRASP, a multidimensional compressed sensing technique exploiting correlations/sparsity along cardiac and respiratory dimensions. 5D whole-heart imaging was compared with respiratory motion-corrected 3D and 4D whole-heart imaging in nine volunteers for evaluation of the myocardium, great vessels, and coronary arteries. It was also compared with breath-held, ECG-gated 2D cardiac cine imaging for validation of cardiac function quantification. RESULTS: 5D whole-heart images received systematic higher quality scores in the myocardium, great vessels and coronary arteries. Quantitative coronary sharpness and length were always better for the 5D images. Good agreement was obtained for quantification of cardiac function compared with 2D cine imaging. CONCLUSION: 5D whole-heart sparse imaging represents a robust and promising framework for simplified comprehensive cardiac MRI without the need for breath-hold and motion correction. Magn Reson Med, 2017. (c) 2017 International Society for Magnetic Resonance in Medicine.

PURPOSE: The proposed reconstruction framework addresses the reconstruction accuracy, noise propagation and computation time for magnetic resonance fingerprinting. METHODS: Based on a singular value decomposition of the signal evolution, magnetic resonance fingerprinting is formulated as a low rank (LR) inverse problem in which one image is reconstructed for each singular value under consideration. This LR approximation of the signal evolution reduces the computational burden by reducing the number of Fourier transformations. Also, the LR approximation improves the conditioning of the problem, which is further improved by extending the LR inverse problem to an augmented Lagrangian that is solved by the alternating direction method of multipliers. The root mean square error and the noise propagation are analyzed in simulations. For verification, in vivo examples are provided. RESULTS: The proposed LR alternating direction method of multipliers approach shows a reduced root mean square error compared to the original fingerprinting reconstruction, to a LR approximation alone and to an alternating direction method of multipliers approach without a LR approximation. Incorporating sensitivity encoding allows for further artifact reduction. CONCLUSION: The proposed reconstruction provides robust convergence, reduced computational burden and improved image quality compared to other magnetic resonance fingerprinting reconstruction approaches evaluated in this study. Magn Reson Med, 2017. (c) 2017 International Society for Magnetic Resonance in Medicine.

Non-Cartesian magnetic resonance imaging (MRI) sequences have shown great promise for abdominal examination during free breathing, but break down in the presence of bulk patient motion (i.e. voluntary or involuntary patient movement resulting in translation, rotation or elastic deformations of the body). This work describes a data-consistency-driven image stabilization technique that detects and excludes bulk movements during data acquisition. Bulk motion is identified from changes in the signal intensity distribution across different elements of a multi-channel receive coil array. A short free induction decay signal is acquired after excitation and used as a measure to determine alterations in the load distribution. The technique has been implemented on a clinical MR scanner and evaluated in the abdomen. Six volunteers were scanned and two radiologists scored the reconstructions. To show the applicability to other body areas, additional neck and knee images were acquired. Data corrupted by bulk motion were successfully detected and excluded from image reconstruction. An overall increase in image sharpness and reduction of streaking and shine-through artifacts were seen in the volunteer study, as well as in the neck and knee scans. The proposed technique enables automatic real-time detection and exclusion of bulk motion during MR examinations without user interaction. It may help to improve the reliability of pediatric MRI examinations without the use of sedation.

OBJECTIVES: The aim of this study was to assess the applicability of Dixon radial volumetric encoding (Dixon-RAVE) for comprehensive dynamic contrast-enhanced 3D magnetic resonance imaging (MRI) of the breast using a combination of radial sampling, model-based fat/water separation, compressed sensing, and parallel imaging. MATERIALS AND METHODS: In this Health Insurance Portability and Accountability Act-compliant prospective study, 24 consecutive patients underwent bilateral breast MRI, including both conventional fat-suppressed and non-fat-suppressed precontrast T1-weighted volumetric interpolated breath-hold examination (VIBE). Afterward, 1 continuous Dixon-RAVE scan was performed with the proposed approach while the contrast agent was injected. This scan was immediately followed by the acquisition of 4 conventional fat-saturated VIBE scans. From the comprehensive Dixon-RAVE data set, different image contrasts were reconstructed that are comparable to the separate conventional VIBE scans.Two radiologists independently rated image quality, conspicuity of fibroglandular tissue from fat (FG), and degree of fat suppression (FS) on a 5-point Likert-type scale for the following 3 comparisons: precontrast fat-suppressed (pre-FS), precontrast non-fat-suppressed (pre-NFS), and dynamic fat-suppressed (dyn-FS) images. RESULTS: When scores were averaged over readers, Dixon-RAVE achieved significantly higher (P < 0.001) degree of fat suppression compared with VIBE, for both pre-FS (4.25 vs 3.67) and dyn-FS (4.10 vs 3.46) images. Although Dixon-RAVE had lower image quality score compared with VIBE for the pre-FS (3.56 vs 3.67, P = 0.490), the pre-NFS (3.54 vs 3.88, P = 0.009), and the dyn-FS images (3.06 vs 3.67, P < 0.001), acceptable or better diagnostic quality was achieved (score >/= 3). The FG score for Dixon-RAVE in comparison to VIBE was significantly higher for the pre-FS image (4.23 vs 3.85, P = 0.044), lower for the pre-NFS image (3.98 vs 4.25, P = 0.054), and higher for the dynamic fat-suppressed image (3.90 vs 3.85, P = 0.845). CONCLUSIONS: Dixon-RAVE can serve as a one-stop-shop approach for comprehensive T1-weighted breast MRI with diagnostic image quality, high spatiotemporal resolution, reduced overall scan time, and improved fat suppression compared with conventional imaging.

PURPOSE: The inhomogeneity of flip angle distribution is a major challenge impeding the application of high-field MRI. We report a method combining spatially selective excitation using generalized spatial encoding magnetic fields (SAGS) with radiofrequency (RF) shimming to achieve homogeneous excitation. This method can be an alternative approach to address the challenge of B1+ inhomogeneity using nonlinear gradients. METHODS: We proposed a two-step algorithm that jointly optimizes the combination of nonlinear spatial encoding magnetic fields and the combination of multiple RF transmitter coils and then optimizes the locations, RF amplitudes, and phases of the spokes. RESULTS: Our results show that jointly designed SAGS and RF shimming can provide a more homogeneous flip angle distribution than using SAGS or RF shimming alone. Compared with RF shimming alone, our approach can reduce the relative standard deviation of flip angle by 56% and 52% using phantom and human head data, respectively. CONCLUSION: The jointly designed SAGS and RF shimming method can be used to achieve homogeneous flip angle distributions when fully parallel RF transmission is not available. Magn Reson Med, 2016. (c) 2016 International Society for Magnetic Resonance in Medicine.