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Multiresolution MR elastography using nonlinear inversion.

ABSTRACT: Nonlinear inversion (NLI) in MR elastography requires discretization of the displacement field for a finite element (FE) solution of the "forward problem", and discretization of the unknown mechanical property field for the iterative solution of the "inverse problem". The resolution requirements for these two discretizations are different: the forward problem requires sufficient resolution of the displacement FE mesh to ensure convergence, whereas lowering the mechanical property resolution in the inverse problem stabilizes the mechanical property estimates in the presence of measurement noise. Previous NLI implementations use the same FE mesh to support the displacement and property fields, requiring a trade-off between the competing resolution requirements.This work implements and evaluates multiresolution FE meshes for NLI elastography, allowing independent discretizations of the displacements and each mechanical property parameter to be estimated. The displacement resolution can then be selected to ensure mesh convergence, and the resolution of the property meshes can be independently manipulated to control the stability of the inversion.Phantom experiments indicate that eight nodes per wavelength (NPW) are sufficient for accurate mechanical property recovery, whereas mechanical property estimation from 50 Hz in vivo brain data stabilizes once the displacement resolution reaches 1.7 mm (approximately 19 NPW). Viscoelastic mechanical property estimates of in vivo brain tissue show that subsampling the loss modulus while holding the storage modulus resolution constant does not substantially alter the storage modulus images. Controlling the ratio of the number of measurements to unknown mechanical properties by subsampling the mechanical property distributions (relative to the data resolution) improves the repeatability of the property estimates, at a cost of modestly decreased spatial resolution.Multiresolution NLI elastography provides a more flexible framework for mechanical property estimation compared to previous single mesh implementations.

SUBMITTER: McGarry MD

PROVIDER: S-EPMC3477197 | biostudies-other | 2012 Oct

REPOSITORIES: biostudies-other

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Project description:In MR elastography (MRE), zeroth moment balanced motion-encoding gradients (MEGs) are incorporated into MRI sequences to induce a phase shift proportional to the local displacement caused by external actuation. To maximize the signal-to-noise ratio (SNR), fractional encoding is employed, i.e., the MEG duration is reduced below the wave period. Here, gradients encode primarily the velocity of the motion-reducing encoding efficiency. Thus, in GRE-MRE, T2 * decay and motion sensitivity have to be balanced, imposing a lower limit on repetition times (TRs). We propose to use a single trapezoidal gradient, a "unipolar gradient", to directly encode spin displacement. Such gradients cannot be used in conventional sequences as they exhibit a large zeroth moment and dephase magnetization. By time-reversing a spoiled SSFP sequence, the spoiling gradient becomes an efficient unipolar MEG. The proposed "unipolar MRE" technique benefits from this approach in three ways: first, displacement encoding is split over multiple TRs increasing motion sensitivity; second, spoiler and MEG coincide, allowing a reduction in TR; third, motion sensitivity of a typical unipolar lobe is of an order of magnitude higher than a bipolar MEG of equal duration. In this work, motion encoding using unipolar MRE is analyzed using the extended phase graph (EPG) formalism with a periodic motion propagator. As an approximation, the two-transverse TR approximation for diffusion-weighted SSFP is extended to incorporate cyclic motion. A complex encoding efficiency metric is introduced to compare the displacement fields of unipolar and conventional GRE-MRE sequences in both magnitude and phase. The derived theoretical encoding equations are used to characterize the proposed sequence using an extensive parameter study. Unipolar MRE is validated against conventional GRE-MRE in a phantom study showing excellent agreement between measured displacement fields. In addition, unipolar MRE yields significantly increased octahedral shear strain-SNR relative to conventional GRE-MRE and allows for the recovery of high stiffness inclusions, where conventional GRE-MRE fails.

Project description:Purpose To investigate the utility of magnetic resonance (MR) elastography-derived mechanical properties in the discrimination of hepatic inflammation and fibrosis in the early stages of chronic liver diseases. Materials and Methods All studies were approved by the institutional animal care and use committee. A total of 187 animals were studied, including 182 mice and five pigs. These animals represented five different liver diseases with a varying combination and extent of hepatic inflammation, fibrosis, congestion, and portal hypertension. Multifrequency three-dimensional MR elastography was performed, and shear stiffness, storage modulus, shear loss modulus, and damping ratio were calculated for all animals. Necroinflammation, fibrosis, and portal pressure were either histologically scored or biochemically and physically quantified in all animals. Two-sided Welch t tests were used to evaluate mean differences between disease and control groups. Spearman correlation analyses were used to evaluate the relationships between mechanical parameters and quantitative fibrosis extent (hydroxyproline concentration) and portal pressure. Results Liver stiffness and storage modulus increased with progressively developed fibrosis and portal hypertension (mean stiffness at 80 Hz and 48-week feeding, 0.51 kPa ± 0.12 in the steatohepatitis group vs 0.29 kPa ± 0.01 in the control group; P = .02). Damping ratio and shear loss modulus can be used to distinguish inflammation from fibrosis at early stages of disease, even before the development of histologically detectable necroinflammation and fibrosis (mean damping ratio at 80 Hz and 20-week feeding, 0.044 ± 0.012 in the steatohepatitis group vs 0.014 ± 0.008 in the control group; P < .001). Damping ratio and liver stiffness vary differently with respect to cause of portal hypertension (ie, congestion- or cirrhosis-induced hypertension). These differentiation abilities have frequency-dependent variations. Conclusion Liver stiffness and damping ratio measurements can extend hepatic MR elastography to potentially enable assessment of necroinflammatory, congestive, and fibrotic processes of chronic liver diseases. © RSNA, 2017 Online supplemental material is available for this article.

Project description:Background Stiffness thresholds for liver MR elastography in children vary between studies and may differ from thresholds in adults. Normative liver stiffness data are needed to optimize diagnostic thresholds for children. Purpose To determine normal liver stiffness, and associated normal ranges for children, as measured with MR elastography across vendors and field strengths. Materials and Methods This was a prospective multicenter cohort study (ClinicalTrials.gov identifier: NCT03235414). Volunteers aged 7-17.9 years without a known history of liver disease were recruited at four sites for a research MRI and blood draw between February 2018 and October 2019. MRI was performed on three vendor platforms and at two field strengths (1.5 T and 3.0 T). All MRI scans were centrally analyzed; stiffness, proton density fat fraction (PDFF), and R2* values were expressed as means of means. Mean and 95% confidence intervals (CIs) for liver stiffness were calculated. Pearson correlation coefficient (r), two-sample t test, or analysis of variance was used to assess univariable associations. Results Seventy-one volunteers had complete data and no documented exclusion criterion (median age, 12 years; interquartile range [IQR], 10-15 years; 39 female participants). Median body mass index percentile was 54% (IQR, 32.5%-69.5%). Mean liver stiffness was 2.1 kPa (95% CI: 2.0, 2.2 kPa) with mean ± 1.96 kPa standard deviation of 1.5-2.8 kPa. Median liver PDFF was 2.0% (IQR, 1.7%-2.6%). There was no association between liver stiffness and any patient variable or MRI scanner factor. Conclusion Mean liver stiffness measured with MR elastography in children without liver disease was 2.1 kPa (similar to that in adults). The 95th percentile of normal liver stiffness was 2.8 kPa. Liver stiffness was independent of sex, age, or body mass index and did not vary with MRI scanner vendor or field strength. © RSNA, 2020 Online supplemental material is available for this article. See also the editorial by Yin in this issue.

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Multiresolution MR elastography using nonlinear inversion.

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