In Vivo Wideband Multifrequency MR Elastography of the Human Brain and Liver

Purpose: To demonstrate the feasibility of in vivo wideband MR elastography (wMRE) using continuous, time-harmonic shear vibrations in the frequency range of 10-50 Hz. Theory and Methods: The method was tested in a gel phantom with marked mechanical loss. The brains and livers of eight volunteers were scanned by wMRE using multislice, single-shot MRE with optimized fractional encoding and synchronization of sequence acquisition to vibration. Multifrequency three-dimensional inversion was used to reconstruct compound maps of magnitude vertical bar G*vertical bar and phase phi of the complex shear modulus. A new phase estimation, phi*, was developed to avoid systematic bias due to noise. Results: In the phantom, G*-dispersion measured by wMRE agreed well with oscillatory shear rheometry. vertical bar G*vertical bar and phi* measured at vibrations of 10-25 HZ, 25-35 HZ, and 40-50 HZ were 0.62 +/- 0.08, 1.56 +/- 0.16, 2.18 +/- 0.20 kPa and 0.09 +/- 0.17, 0.39 +/- 0.16, 0.20 +/- 0.13 rad in brain and 0.89 +/- 0.11, 1.67 +/- 0.20, 2.27 +/- 0.35 kPa and 0.15 +/- 0.10, 0.24 +/- 0.05, 0.26 +/- 0.05 rad in liver. Elastograms including all frequencies showed the best resolution of anatomical detail with vertical bar G*vertical bar = 1.38 +/- 0.12 kPa, phi* = 0.24 +/- 0.10 rad (brain) and vertical bar G*vertical bar = 1.79 +/- 0.23 kPa, phi* = 0.24 +/- 0.05 rad (liver). Conclusion: wMRE reveals highly dispersive G* properties of the brain and liver, and our results suggest that the influence of large-scale structures such as fluid-filled vessels and sulci on the MRE-measured parameters increases at low vibration frequencies. (C) 2015 Wiley Periodicals, Inc.