Shear wave group velocity inversion in MR elastography of human skeletal muscle

Abstract

In vivo quantification of the anisotropic shear elasticity of soft tissue is an appealing objective of elastography techniques because elastic anisotropy can potentially provide specific information about structural alterations in diseased tissue. Here a method is introduced and applied to MR elastography (MRE) of skeletal muscle. With this method one can elucidate anisotropy by means of two shear moduli (one parallel and one perpendicular to the muscle fiber direction). The technique is based on group velocity inversion applied to bulk shear waves, which is achieved by an automatic analysis of wave-phase gradients on a spatiotemporal scale. The shear moduli are then accessed by analyzing the directional dependence of the shear wave speed using analytic expressions of group velocities in k-space, which are numerically mapped to real space. The method is demonstrated by MRE experiments on the biceps muscle of five volunteers, resulting in 5.5 ؎ 0.9 kPa and 29.3 ؎ 6.2 kPa (P < 0.05) for the medians of the perpendicular and parallel shear moduli, respectively. The proposed technique combines fast steadystate free precession (SSFP) MRE experiments and fully automated processing of anisotropic wave data, and is thus an interesting MRI modality for aiding clinical diagnosis. Magn Reson Med 56:489 -497, 2006.

Non‐invasive measurement of brain viscoelasticity using magnetic resonance elastography

“…Regions in the vicinity of the brain surface appear to be softer than central brain parenchyma, which supports observations (32,33) that white brain matter is stiffer than gray matter. However, according to the literature, shear waves near the surface are affected by diffraction or non-evanescent (non-propagating) waves, eventually leading to biases in the estimated local wavelengths (43)(44)(45)(46). This conclusion justifies our approach of focusing on global measures, as shown in Figs Table 1.…”

Three‐parameter shear wave inversion in MR elastography of incompressible transverse isotropic media: Application to in vivo lower leg muscles

“…To overcome this limitation, future studies should include diffusion tensor imaging in order to derive the local frame of elasticity in each voxel (24,33). Of note, the curl-operator accounts for the directionality of shear waves by separating those curl components related to all three TI material parameters m In this way, Equation [13] includes the directional filters, which are applied separately otherwise (20,24). It is encouraging that the ratio between the shear moduli parallel and perpendicular to the fiber direction reported in a study combining MRE and DTI by Green et al (21) are similar to our ratios m More studies are warranted on the interaction of anisotropic viscoelasticity parameters with muscle load as well as on the observed laterality.…”

“…Explicit expressions for the shear wave modes in an incompressible TI material can be found in the literature (20):…”

Congruence of Imaging Estimators and Mechanical Measurements of Viscoelastic Properties of Soft Tissues

“…The potential of dynamic MRE clinical implementation has been proven in preliminary human studies in which the data of human prostate, breast, brain, muscle, and liver were presented (Bensamoun et al 2006;Bishop et al 1998;Kemper et al 2004;Kruse et al 2000;Papazoglou et al 2006;Rouviere et al 2006;Sinkus et al 2005). In particular, Kruse et al (2000) evaluated porcine livers with MRE at multiple shear wave frequencies and reported that the wave velocity and the shear stiffness increased with frequency.…”