Assessment of liver viscoelasticity using multifrequency MR elastography

MR elastography (MRE) allows the noninvasive assessment of the viscoelastic properties of human organs based on the organ response to oscillatory shear stress. Shear waves of a given frequency are mechanically introduced and the propagation is imaged by applying motion-sensitive gradients. An experiment was set up that introduces multifrequency shear waves combined with broadband motion sensitization to extend the dynamic range of MRE from one given frequency to, in this study, four different frequencies. With this approach, multiple wave images corresponding to the four driving frequencies are simultaneously acquired and can be evaluated with regard to the dispersion of the complex modulus over the respective frequency. A viscoelastic model based on two shear moduli and one viscosity parameter was used to reproduce the experimental wave speed and wave damping dispersion. The technique was applied in eight healthy volunteers and eight patients with biopsy-proven high-grade liver fibrosis (grade 3-4). Fibrotic liver had a significantly higher (P < 0.01) viscosity (14.4 +/- 6.6 Pa . s) and elastic moduli (2.91 +/- 0.84 kPa; 4.83 +/- 1.77 kPa) than the viscosity (7.3 +/- 2.3 Pa . s) and elastic moduli (1.16 +/- 0.28 kPa; 1.97 +/- 0.30 kPa) of normal volunteers. Multifrequency MRE is well suited for the noninvasive differentiation of normal and fibrotic liver as it allows the measurement of rheologic material properties.