Viscoelastic Properties of Living Human Brain by Local Frequency Estimation Applied to Distortional Wave Fields

The use of magnetic resonance imaging (MRI) to characterize the biomechanical material properties of tissue in vivo is an emerging application. Local frequency estimation (LFE) [1] has been used to locally extract the wavenumber κ from displacement wave fields in order to estimate the elastic shear modulus via the well-known phase velocity relation [2,3]. LFE-based inversion is attractive because it allows material parameters to be estimated without explicitly invoking the equations of motion, thus obviating the need to numerically compute the Laplacian. In this study, viscoelastic properties of brain tissue at multiple frequencies were obtained by local frequency estimation (LFE) applied to distortional wave fields. This avoids artifacts from the presence of unmodeled longitudinal waves. By use of the equation describing planar distortional wave propagation in a lossy medium, the viscoelastic shear moduli were approximated.