  In the framework of algebraic inversion, Magnetic Resonance Elastography
(MRE) repeatability, reproducibility and robustness were evaluated on extracted
shear velocities (or elastic moduli). The same excitation system was
implemented at two sites equipped with clinical MR scanners of 1.5 T and 3 T. A
set of four elastic, isotropic, homogeneous calibrated phantoms of distinct
elasticity representing the spectrum of liver fibrosis severity was
mechanically characterized. The repeatability of the measurements and the
reproducibility between the two platforms were found to be excellent with mean
coefficients of variations of 1.62% for the shear velocity mean values and
1.95% for the associated standard deviations. MRE velocities were robust to the
amplitude and pattern variations of the displacement field with virtually no
difference between outcomes from both magnets at identical excitation
frequencies even when the displacement field amplitude was 6 times smaller.
However, MRE outcomes were very sensitive to the number of voxels per
wavelength, s, of the recorded displacement field, with relative biases
reaching 62% and precision losing up to a factor 23.5. For both magnetic field
strengths, MRE accuracy and precision were largely degraded outside of
established conditions of validity ($6 \lesssim s \lesssim 9$) resulting in
estimated shear velocity values not significantly different between phantoms of
increasing elasticity. When fulfilling the spatial sampling conditions, either
prospectively in the acquisition or retrospectively before the reconstruction,
MRE produced quantitative measurements that allowed to unambiguously
discriminate, with infinitesimal p-values, between the phantoms mimicking
increasing severity of liver fibrosis.
