Assessment of advanced hepatic MR elastography methods for susceptibility artifact suppression in clinical patients.

PURPOSE: To assess the success rate, image quality, and the ability to stage liver fibrosis of a standard 2D gradient-recalled echo (GRE) and four different spin-echo (SE) magnetic resonance elastography (MRE) sequences in patients with different liver iron concentrations. MATERIALS AND METHODS: A total of 332 patients who underwent 3T MRE examinations that included liver fat and iron quantification were enrolled, including 136 patients with all five MRE techniques. Thirty-four patients had biopsy results for fibrosis staging. The liver stiffness, region of interest area, image quality, and success rate of the five sequences were compared in 115/136 patients. The area under the receiver operating characteristic curves (AUCs) and the accuracies for diagnosing early-stage fibrosis and advanced fibrosis were compared. The effect of BMI (body mass index), the R2* relaxation time, and fat fraction on the image quality and liver stiffness measurements were analyzed. RESULTS: The success rates were significantly higher in the four SE sequences (99.1-100%) compared with GRE MRE (85.3%) (all P < 0.001). There were significant differences of the mean ROI area between every pair of sequences (all P < 0.0001). There were no significant differences in the AUC of the five MRE sequences for discriminating advanced fibrosis (10 P-values ranging from 0.2410-0.9171). R2* had a significant effect on the success rate and image quality for the noniron 2D echo-planar imaging (EPI), 3D EPI and 2D GRE (all P < 0.001) sequences. BMI had a significant effect on the iron 2D EPI (P = 0.0230) and iron 2D SE (P = 0.0040) sequences. CONCLUSION: All five techniques showed good diagnostic performance in staging liver fibrosis. The SE MRE sequences had higher success rates and better image quality than GRE MRE in 3T clinical hepatic imaging. LEVEL OF EVIDENCE: 3 Technical Efficacy: Stage 5 J. Magn. Reson. Imaging 2018;47:976-987.

General review of magnetic resonance elastography.

Magnetic resonance elastography (MRE) is an innovative imaging technique for the non-invasive quantification of the biomechanical properties of soft tissues via the direct visualization of propagating shear waves in vivo using a modified phase-contrast magnetic resonance imaging (MRI) sequence. Fundamentally, MRE employs the same physical property that physicians utilize when performing manual palpation - that healthy and diseased tissues can be differentiated on the basis of widely differing mechanical stiffness. By performing "virtual palpation", MRE is able to provide information that is beyond the capabilities of conventional morphologic imaging modalities. In an era of increasing adoption of multi-parametric imaging approaches for solving complex problems, MRE can be seamlessly incorporated into a standard MRI examination to provide a rapid, reliable and comprehensive imaging evaluation at a single patient appointment. Originally described by the Mayo Clinic in 1995, the technique represents the most accurate non-invasive method for the detection and staging of liver fibrosis and is currently performed in more than 100 centers worldwide. In this general review, the mechanical properties of soft tissues, principles of MRE, clinical applications of MRE in the liver and beyond, and limitations and future directions of this discipline -are discussed. Selected diagrams and images are provided for illustration.

Measuring the effects of aging and sex on regional brain stiffness with MR elastography in healthy older adults.

Changes in tissue composition and cellular architecture have been associated with neurological disease, and these in turn can affect biomechanical properties. Natural biological factors such as aging and an individual's sex also affect underlying tissue biomechanics in different brain regions. Understanding the normal changes is necessary before determining the efficacy of stiffness imaging for neurological disease diagnosis and therapy monitoring. The objective of this study was to evaluate global and regional changes in brain stiffness as a function of age and sex, using improved MRE acquisition and processing that have been shown to provide median stiffness values that are typically reproducible to within 1% in global measurements and within 2% for regional measurements. Furthermore, this is the first study to report the effects of age and sex over the entire cerebrum volume and over the full frontal, occipital, parietal, temporal, deep gray matter/white matter (insula, deep gray nuclei and white matter tracts), and cerebellum volumes. In 45 volunteers, we observed a significant linear correlation between age and brain stiffness in the cerebrum (P<.0001), frontal lobes (P<.0001), occipital lobes (P=.0005), parietal lobes (P=.0002), and the temporal lobes (P<.0001) of the brain. No significant linear correlation between brain stiffness and age was observed in the cerebellum (P=.74), and the sensory-motor regions (P=.32) of the brain, and a weak linear trend was observed in the deep gray matter/white matter (P=.075). A multiple linear regression model predicted an annual decline of 0.011 ± 0.002 kPa in cerebrum stiffness with a theoretical median age value (76 years old) of 2.56 ± 0.08 kPa. Sexual dimorphism was observed in the temporal (P=.03) and occipital (P=.001) lobes of the brain, but no significant difference was observed in any of the other brain regions (P>.20 for all other regions). The model predicted female occipital and temporal lobes to be 0.23 kPa and 0.09 kPa stiffer than males of the same age, respectively. This study confirms that as the brain ages, there is softening; however, the changes are dependent on region. In addition, stiffness effects due to sex exist in the occipital and temporal lobes.

Magnetic resonance elastography of the brain.

The purpose of this study was to obtain normative data using magnetic resonance elastography (MRE) (a) to obtain estimates of the shear modulus of human cerebral tissue in vivo and (b) to assess a possible age dependence of the shear modulus of cerebral tissue in healthy adult volunteers. MR elastography studies were performed on tissue-simulating gelatin phantoms and 25 healthy adult volunteers. The data were analyzed using spatiotemporal filters and a local frequency estimating algorithm. Statistical analysis was performed using a paired t-test. The mean shear stiffness of cerebral white matter was 13.6 kPa (95% CI 12.3 to 14.8 kPa); while that of gray matter was lower at 5.22 kPa (95% CI 4.76 to 5.66 kPa). The difference was statistically significant (p<0.0001).

Evaluation of renal parenchymal disease in a rat model with magnetic resonance elastography.

Alterations in the mechanical properties or "hardness" of tissues allow physicians to detect disease by palpation. Recently, attempts have been made to quantitate and image these tissue properties with the use of magnetic resonance elastography (MRE). This technique has been validated in ex vivo specimens, including kidney, breast, and prostate. In this study, in vivo MRE imaging of rat renal cortex is demonstrated and validated with a disease model that will facilitate further studies. Normal rats and rats with nephrocalcinosis induced with either 2 or 4 weeks of ethylene glycol exposure were studied with MRE. Histology in the diseased rats documented the presence of nephrocalcinosis. MRE measurements and images of shear stiffness were highly reproducible in individual rats. The shear stiffness of the renal cortex in normal rats was 3.87 kPa (95% CI 2.84-4.90 kPa). The shear stiffness increased to 5.02 kPa (95% CI 3.34-6.70 kPa) after 2 weeks of exposure, and to 6.49 kPa (95% CI 4.84-8.14 kPa) after 4 weeks of exposure (P = 0.0302, alpha < 0.05). MRE is capable of detecting alterations in the tissue mechanical properties of kidneys in vivo. It is a promising noninvasive technique that might have pathologic and prognostic significance.