Sui, Yi

ABSTRACT Purpose Magnetic resonance elastography (MRE) of the heart has predominantly utilized breath‐held acquisitions with limited anatomic coverage. This work investigates the feasibility of 3D, free‐breathing cardiac MRE. Methods A 3D hybrid radial and EPI acquisition is utilized and combined with retrospective binning of k ‐space via physiologic monitoring, iterative reconstruction, MRE processing and inversions, and post‐processing specialized for cardiac imaging. Results Feasibility of free‐breathing stiffness mapping of the left ventricular myocardium was demonstrated in 11 participants. Myocardial stiffness estimates were obtained throughout the left ventricle for seven temporal states of the cardiac cycle. Stiffness estimates were presented in participant‐specific images, 17‐segment bullseye plots, and stiffness plots throughout the cardiac cycle. Results indicate increased stiffness in systolic states, with localized stiffness heterogeneity observed between participants, illustrating a key benefit of 3D cardiac MRE acquisition. Mean stiffness estimates obtained through 11 subjects were 4.44 ± 0.68 kPa (begin‐systole), 5.74 ± 0.94, 5.42 ± 0.87 (end‐systole), 3.84 ± 0.71 (begin‐diastole), 3.51 ± 0.74, 3.54 ± 0.52, and 3.55 ± 0.59 kPa (end‐diastole). Statistical testing indicated significant differences in stiffness across cardiac states with one‐way, repeated‐measures ANOVA ( p < 0.001) and Friedman's test ( p < 0.001). Experiments in scanning three participants with and without applied motion indicated that stiffness changes were more robustly detected in systole than in diastole, although patterns were subject‐specific. Comparison of free‐breathing stiffness measurements against reference breath‐held measurements yielded similar systolic stiffness estimates. Conclusions This study establishes the feasibility of free‐breathing, 3D cardiac MRE in healthy volunteers. With the current framework, stiffness estimation appears more robust in systole, and the reliability of diastolic estimates is limited and subject‐specific.

ABSTRACT Purpose Abnormal restrictions at myofascial interfaces are a key factor in chronic low back pain (CLBP) with myofascial pain syndrome (MPS), but no established imaging method exists for quantitative assessment. This pilot study evaluated the clinical feasibility of MR elastography (MRE)‐based slip interface imaging (SII) for visualizing myofascial mobility in CLBP‐MPS patients versus age−/sex‐/BMI‐matched healthy participants. A new SII biomarker, maximal normalized displacement discontinuity (), was introduced, and the previously developed normalized octahedral shear strain (NOSS) map was used for comparison. Preliminary assessments of vibration‐frequency effects, repeatability, and group differences in inter‐muscular mobility were conducted. Methods MRE was conducted at 30, 60, and 90 Hz to compare with NOSS for visualizing inter‐muscular slip interfaces at the quadratus lumborum‐erector spinae (QL‐ES) and erector spinae‐multifidus (ES‐M) muscle boundaries at L3‐L4. Repeatability of and NOSS was assessed at 30 Hz (identified as the best‐performing frequency) using within‐subject coefficients of variation and a linear mixed‐effects model from within‐ and between‐day scans. Clinical feasibility was then assessed via Wilcoxon rank‐sum tests in six CLBP‐MPS patients and matched controls. Results at 30 Hz exhibited superior delineation of slip interfaces compared to higher frequencies and NOSS. demonstrated significantly lower variation and better repeatability than NOSS. Healthy participants showed continuous, well‐defined slip interfaces, whereas CLBP‐MPS patients displayed disrupted interfaces with significant mobility reduction at L4 ES‐M. Conclusion SII with lower vibration frequency and map enhances visualization of lower back myofascial mobility and shows potential for assessing differences between healthy individuals and CLBP‐MPS patients.

AbstractPurposeWhile standard MR elastography (MRE) uses harmonic mechanical waves, there are some applications in which imaging and analysis of transient mechanical motion are of interest. Wavelet MRE has been developed for detecting broadband motion from transient excitation. The goal of this study was to evaluate the accuracy and efficiency of wavelet MRE for transient displacement detection in brain MRE applications.MethodsTransient motion was induced in a gel phantom, while MRE images were acquired using bipolar motion‐encoding gradient (MEG) at multiple frequencies (20–200 Hz). Displacements were estimated using (1) combinations of multiple MEGs forming the wavelet MRE and (2) deconvolution from a single MEG. These estimated displacements were used to calculate the MRE phase for each MEG. Correlation (r2) between the calculated and acquired phases was evaluated.Three healthy volunteers were scanned twice in a clinical scanner using wavelet MRE with an occipital impact. Time‐resolved brain translation, rotation, and maximal principal strain were calculated. Repeatability was assessed both qualitatively and through Pearson correlation.ResultsWavelet MRE outperformed standard MRE in displacement estimation, showing higher correlations between calculated and acquired phase, even with fewer phase offsets. In the volunteer study, consistent temporal motion dynamics and spatial maximal principal strain distributions across repeated scans demonstrated the repeatability of wavelet MRE.ConclusionThis study validated the accuracy and efficiency of wavelet MRE for broadband motion detection and demonstrated its feasibility and repeatability in vivo. This technique shows promise for advancing our understanding of the injury risks and mechanisms associated with sports‐related head trauma.

AbstractPurposeAbnormal adherence at functional myofascial interfaces is hypothesized as an important phenomenon in myofascial pain syndrome. This study aimed to investigate the feasibility of MR elastography (MRE)–based slip interface imaging (SII) to visualize and assess myofascial mobility in healthy volunteers.MethodsSII was used to assess local shear strain at functional myofascial interfaces in the flexor digitorum profundus (FDP) and thighs. In the FDP, MRE was performed at 90 Hz vibration to each index, middle, ring, and little finger. Two thigh MRE scans were performed at 40 Hz with knees flexed and extended. The normalized octahedral shear strain (NOSS) maps were calculated to visualize myofascial slip interfaces. The entropy of the probability distribution of the gradient NOSS was computed for the two knee positions at the intermuscular interface between vastus lateralis and vastus intermedius, around rectus femoris, and between vastus intermedius and vastus medialis.ResultsNOSS map depicted distinct functional slip interfaces in the FDP for each finger. Compared to knee flexion, clearer slip interfaces and larger gradient NOSS entropy at the vastus lateralis–vastus intermedius interface were observed during knee extension, where the quadriceps are not passively stretched. This suggests the optimal position for using SII to visualize myofascial slip interface in skeletal muscles is when muscles are not subjected to any additional force.ConclusionThe study demonstrated that MRE‐based SII can visualize and assess myofascial interface mobility in extremities. The results provide a foundation for investigating the hypothesis that myofascial pain syndrome is characterized by changes in the mobility of myofascial interfaces.

AbstractPurposeTo demonstrate a novel MR elastography (MRE) technique, termed here wavelet MRE. With this technique, broadband motion sensitivity is achievable. Moreover, the true tissue displacement can be reconstructed with a simple inverse transform.MethodsA wavelet MRE sequence was developed with motion‐encoding gradients based on Haar wavelets. From the phase images' displacement was estimated using an inverse transform. Simulations were performed using a frequency sweep and a transient as ground‐truth motions. A PVC phantom was scanned using wavelet MRE and standard MRE with both transient (one and 10 cycles of 90‐Hz motion) and steady‐state dual‐frequency motion (30 and 60 Hz) for comparison. The technique was tested in a human brain, and motion trajectories were estimated for each voxel.ResultsIn simulation, the displacement information estimated from wavelet MRE closely matched the true motion. In the phantom test, the MRE phase data generated from the displacement information derived from wavelet MRE agreed well with standard MRE data. Testing of wavelet MRE to assess transient motion waveforms in the brain was successful, and the tissue motion observed was consistent with a previous study.ConclusionThe uniform and broadband frequency response of wavelet MRE makes it a promising method for imaging transient, multifrequency motion, or motion with unknown frequency content. One potential application is measuring the response of brain tissue undergoing low‐amplitude, transient vibrations as a model for the study of traumatic brain injury.

PurposeTo address the need for a method to acquire 3D data for MR elastography (MRE) of the whole brain with substantially improved spatial resolution, high SNR, and reduced acquisition time compared with conventional methods.MethodsWe combined a novel 3D spiral staircase data‐acquisition method with a spoiled gradient‐echo pulse sequence and MRE motion‐encoding gradients (MEGs). The spiral‐out acquisition permitted use of longer‐duration motion‐encoding gradients (ie, over two full oscillatory cycles) to enhance displacement SNR, while still maintaining a reasonably short TE for good phase‐SNR. Through‐plane parallel imaging with low noise penalties was implemented to accelerate acquisition along the slice direction. Shared anatomical information was exploited in the deblurring procedure to further boost SNR for stiffness inversion.ResultsIn vivo and phantom experiments demonstrated the feasibility of the proposed method in producing brain MRE results comparable to the spin‐echo–based approaches, both qualitatively and quantitatively. High‐resolution (2‐mm isotropic) brain MRE data were acquired in 5 minutes using our method with good SNR. Joint deblurring with shared anatomical information produced SNR‐enhanced images, leading to upward stiffness estimation.ConclusionA novel 3D gradient‐echo–based approach has been designed and implemented, and shown to have promising potential for fast and high‐resolution in vivo MRE of the whole brain.

PurposeTo develop a novel magnetic resonance elastography (MRE) acquisition using a hybrid radial EPI readout scheme (TURBINE), and to demonstrate its feasibility to obtain wave images and stiffness maps in a phantom and in vivo brain.MethodThe proposed 3D TURBINE‐MRE is based on a spoiled gradient‐echo MRE sequence with the EPI readout radially rotating about the phase‐encoding axis to sample a full 3D k‐space. A polyvinyl chloride phantom and 6 volunteers were scanned on a compact 3T GE scanner with a 32‐channel head coil at 80 Hz and 60 Hz external vibration, respectively. For comparison, a standard 2D, multislice, spin‐echo (SE) EPI‐MRE acquisition was also performed with the same motion encoding and resolution. The TURBINE‐MRE images were off‐line reconstructed with iterative SENSE algorithm. The regional ROI analysis was performed on the 6 volunteers, and the median stiffness values were compared between SE‐EPI‐MRE and TURBINE‐MRE.ResultsThe 3D wave‐field images and the generated stiffness maps were comparable between TURBINE‐MRE and standard SE‐EPI‐MRE for the phantom and the volunteers. The Bland–Altman plot showed no significant difference in the median regional stiffness values between the two methods. The stiffness measured with the 2 methods had a strong linear relationship with a Pearson correlation coefficient of 0.943.ConclusionWe demonstrated the feasibility of the new TURBINE‐MRE sequence for acquiring the desired 3D wave‐field data and stiffness maps in a phantom and in‐vivo brains. This pilot study encourages further exploration of TURBINE‐MRE for functional MRE, free‐breathing abdominal MRE, and cardiac MRE applications.

PurposeTo introduce newly developed MR elastography (MRE)‐based dual‐saturation imaging and dual‐sensitivity motion encoding schemes to directly measure in vivo skull–brain motion, and to study the skull–brain coupling in volunteers with these approaches.MethodsSix volunteers were scanned with a high‐performance compact 3T‐MRI scanner. The skull–brain MRE images were obtained with a dual‐saturation imaging where the skull and brain motion were acquired with fat‐ and water‐suppression scans, respectively. A dual‐sensitivity motion encoding scheme was applied to estimate the heavily wrapped phase in skull by the simultaneous acquisition of both low‐ and high‐sensitivity phase during a single MRE exam. The low‐sensitivity phase was used to guide unwrapping of the high‐sensitivity phase. The amplitude and temporal phase delay of the rigid‐body motion between the skull and brain was measured, and the skull–brain interface was visualized by slip interface imaging (SII).ResultsBoth skull and brain motion can be successfully acquired and unwrapped. The skull–brain motion analysis demonstrated the motion transmission from the skull to the brain is attenuated in amplitude and delayed. However, this attenuation (%) and delay (rad) were considerably greater with rotation (59 ± 7%, 0.68 ± 0.14 rad) than with translation (92 ± 5%, 0.04 ± 0.02 rad). With SII the skull–brain slip interface was not completely evident, and the slip pattern was spatially heterogeneous.ConclusionThis study provides a framework for acquiring in vivo voxel‐based skull and brain displacement using MRE that can be used to characterize the skull–brain coupling system for understanding of mechanical brain protection mechanisms, which has potential to facilitate risk management for future injury.

PurposeTo implement a reduced field of view (rFOV) technique for cardiac MR elastography (MRE) and to demonstrate the improvement in image quality of both magnitude images and post‐processed MRE stiffness maps compared to the conventional full field of view (full‐FOV) acquisition.MethodsWith Institutional Review Board approval, 17 healthy volunteers underwent both full‐FOV and rFOV cardiac MRE scans using 140‐Hz vibrations. Two cardiac radiologists blindly compared the magnitude images and stiffness maps and graded the images based on several image quality attributes using a 5‐point ordinal scale. Fisher's combined probability test was performed to assess the overall evaluation. The octahedral shear strain‐based signal‐to‐noise ratio (OSS‐SNR) and median stiffness over the left ventricular myocardium were also compared.ResultsOne volunteer was excluded because of an inconsistent imaging resolution during the exam. In the remaining 16 volunteers (9 males, 7 females), the rFOV scans outperformed the full‐FOV scans in terms of subjective image quality and ghosting artifacts in the magnitude images and stiffness maps, as well as the overall preference. The quantitative measurements showed that rFOV had significantly higher OSS‐SNR (median: 1.4 [95% confidence interval (CI): 1.2–1.5] vs. 2.1 [95% CI: 1.8–2.4]), P < 0.05) compared to full‐FOV. Although no significant change was found in the median myocardial stiffness between the 2 scans, we observed a decrease in the stiffness variation within the myocardium from 2.1 kPa (95% CI: [1.9, 2.3]) to 1.9 kPa (95% CI: [1.7, 2.0]) for full‐FOV and rFOV, respectively (P < 0.05) in a subgroup of 7 subjects with ghosting present in the myocardium.ConclusionThis pilot volunteer study demonstrated that rFOV cardiac MRE has the capability to reduce ghosting and to improve image quality in both MRE magnitude images and stiffness maps. Magn Reson Med 80:231–238, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.