Keywords: Elastography, Elastography

Motivation: Magnetic resonance elastography (MRE) data quality is susceptible to poor data quality from subject motion and long scan times. 

Goal(s): A novel sampling technique and estimation scheme was developed and implemented to improve flexibility and acquisition time of MRE.

Approach: The technique utilizes non-traditional sampling directions in an optimized encoding matrix to collect data efficiently to be used in a novel algorithm for estimating harmonic displacement fields. This allows for acquisition acceleration or flexibility in the data sampled to be rejected in post-processing if it is distorted.

Results: Scans were acquired 2.3x faster than standard methods with 95.2% multiscale structural image similarity.

Impact: A novel sampling and estimation scheme demonstrated here can be used to improve the application of magnetic resonance elastography. This is accomplished through prospective reduction in sampling, reducing acquisition time, and retrospective volume rejection, circumventing distortion introduced by subject motion.

Keywords: Artifacts, System Imperfections: Measurement & Correction, gradient impulse response function

Motivation: High-performance asymmetric gradient systems have the potential to produce high-spatial-order eddy-current-induced magnetic fields that can impact phase-based applications such as wavelet MRE using large-magnitude motion-encoding bipolar gradient pulses. 

Goal(s): Removal of the gradient-system-induced high-order phase deviation in the wavelet MRE phase map.

Approach: Gradient impulse response functions (GIRFs) can be used to characterize the gradient system. We introduce a GIRF-based phase compensation approach to alleviate the high-order phase deviations resulting from imperfections in the gradient system, along with practical recommendations, including the use of a tailored set of GIRFs.

Results: The phase inhomogeneity in wavelet-MRE phase map improved with the GIRF-based compensation.

Impact: The proposed GIRF-based phase compensation approach offers the potential to enhance phase image quality and accuracy, addressing imperfections in the gradient system, which is a challenge not fully resolved by alternative methods like concomitant field correction and pre-emphasis gradient modification.

Keywords: Elastography, Elastography

Motivation: Abnormal changes in myocardial stiffness during the cardiac cycle are a potential biomarker for cardiac diseases. However, there is currently no established MR-method for time-resolved mapping of cardiac stiffness.

Goal(s): To demonstrate the reproducibility of time-resolved stiffness mapping of the in-vivo human heart over the cardiac cycle by MR elastography (MRE). 

Approach: Eleven healthy volunteers were examined twice using multifrequency cardiac-triggered, segmented gradient echo spiral MRE for stiffness mapping of the left ventricular myocardium in a short-axis view. 

Results: Excellent, good, and moderate reproducibility was achieved for isovolumetric contraction, systolic, and diastolic phases, respectively. Significant differences among all phases were apparent.

Impact: The achieved high reproducibility of cardiac multifrequency MRE, together with its ability to identify distinct time-resolved stiffness levels during the cardiac phases of isovolumetric contraction, systole and diastole, could potentially detect stiffness related cardiac pathology in early disease stages.

Keywords: Elastography, Brain

Motivation: There exists a need for a comprehensive method to analyze regional brain tissue mechanics that accounts for variability across subject populations.

Goal(s): Here we aimed to implement a multivariate data-driven technique to capture brain mechanical properties across a wide population while preserving small-scale differences between subjects.

Approach: Non-negative matrix factorization was used to reduce mechanical properties derived from magnetic resonance elastography (MRE) into a low-dimensional form to generate unconfined regions of the brain that demonstrate high covariance across all subjects.

Results: This technique was able to capture recognizable anatomical regions in the brain without structural input to determine weightings on the population average.

Impact: This low-dimensional representation of brain tissue mechanics acquired from non-negative matrix factorization and MRE will help define baseline properties that accurately represent a wide range of subject populations while minimizing variability across imaging studies and contributing to improved statistical models.

Keywords: Elastography, Elastography

Motivation: Previous studies using MR Elastography (MRE) have suggested that gliomas exhibit reduced stiffness and viscosity. However, the interplay between micromechanical tumor changes that determine the macroscopic mechanical properties measured by MRE remains unclear.

Goal(s): This study aims to investigate the relationship between viscoelastic parameters measured MRE in-vivo and histopathologically quantified parameters in glioma.

Approach: High-resolution multifrequency MRE with quantified histopathology was prospectively performed in 23 patients with glioma. 

Results: Stiffness and viscosity in gliomas are associated with increased cell elongation, micro-vessel density, and apoptotic rate suggesting unjamming, neovascularization and cell proliferation as biomechanically sensitive tumor hallmarks for clinical diagnosis.

Impact: In this study, we demonstrated that viscoelastic parameters, quantified by MR Elastography, provide insights into cell mobility, cellularity, mitotic and apoptotic rates, as well as vascularization of gliomas in-vivo. This technique holds promise for future clinical diagnosis of neurotumors.  

Keywords: Other Musculoskeletal, Elastography

Motivation: The purpose of study was integration of Dixon and X-ray CT like image technique in the GRE-MultiEcho-MRE sequence.

Goal(s): The additional visualization of bone damage and water / fat compornents on MR elastography would yield benefits in evaluating radiologic skeletal muscle evaluation, since soft tissue injuries can be assessed directly related to the osseous injuries using one image modality.

Approach: The effectiveness of this method was evaluated by volunteer studies using the original MRE pulse sequence and vibration system.

Results: This method allows simultaneous acquisition of elastograms, wave images, water/fat component images and X-ray CT like images.

Impact: For the patient, a reduction in total imaging time is beneficial. For the clinicians, images such as elastograms, wave images, water/fat component images, and X-ray CT are available simultaneously for multimodal diagnosis.

Keywords: Elastography, Pancreas, Elastography

Motivation: There are discrepancies in pancreatic MRE outcomes. Biomechanical validation of MRE in pancreatic ductal adenocarcinoma (PDAC) poses substantial challenges.

Goal(s): This study aims to conduct biomechanical analysis on the surgically removed specimen and correlate this to preoperative MRE outcomes.

Approach: Six PDAC patients underwent MRE prior to surgical intervention, resulting in shear wave speed (SWS) and phase angle (ф) for whole-tumor and tumor-center. Compression testing was performed on specimen at 5%-strain resulting in Young’s moduli (E). Correlation analysis was done with MRE outcomes and E.

Results: Pearson’s correlation revealed a significant correlation between SWS_tumor-whole and E (r(6)=.857, p=.029) and ф_tumor-center (r(6)=.844, p=.035).

Impact: Validation using ex vivo compression mechanical testing of pancreatic MRE outcomes gives more insight in the accuracy of the measured elastic properties in complex and inhomogeneous tumorous tissue, which is imperative for clinical application of MRE as a predictive biomarker.

Keywords: Elastography, Elastography

Motivation: Estimating the nonlinear coefficient in MR elastography could provide a relevant mechanical parameter for tumor characterization.

Goal(s): The goal of this study was to develop a method for assessing the nonlinear coefficient in MR elastography.

Approach: We developed a specific MR elastography setup and a post-processing pipeline to quantify nonlinear coefficients in phantoms and mice with a tumor implanted subcutaneously.

Results: In phantoms, we observed that nonlinear coefficient was able to provide a higher contrast than storage modulus to distinguish different structure. In addition, in mice, nonlinear coefficient was correlated to the entropy which is a marker of collagen distribution irregularity.

Impact: The estimation of the nonlinear coefficient provides a new biomarker to assess tissue mechanical parameters. The relation between this parameter and tissue structure could be relevant in tumor investigation, as tissue microarchitecture is an important marker of tumor severity.

Keywords: fMRI Analysis, Elastography

Motivation: Identifying patients with chronic hepatitis B (CHB) who are at high risk of HCC development for close monitoring to improve prognosis is crucial.

Goal(s): We aimed to develop a 3D MRE-based risk score to predict HCC development in CHB patients.

Approach: The novel HCC risk score was developed using liver shear stiffness and LSS, age, platelet count, and albumin.

Results: Our results showed that the risk score has excellent performance for the identification of patients of HCC development, the cutoff value 55 of our risk score provided the high negative predictive value for HCC development at 3 and 5 years.

Impact: MRE- based HCC risk score may be a non-invasively accurate tool for predicting HCC, which may provide a useful reference for decision-making in HCC surveillance strategy for CHB patients.