Towards high-resolution MRE
Background: Viscoelastic constants have been reported sensitive to various pathological processes and provide a new diagnostic marker for the assessment of tissue health. MRE is capable to measure tissue mechanical constants in vivo and non-invasively .
Problem: MRE still suffers from uncertainty regions in elastograms, which have to be masked in order to avoid biased stiffness estimates. Such artifacts are mainly due to the strong impact of boundaries and tissue interfaces on the appearance of wave fields including non-evanescent waves, wave nodes, and amplitude modulations deviating from ideal wave models implied by the majority of inversion routines.
Our approach: To reduce wave voids, multifrequency inversion was recently introduced, which adds wave fields of different drive frequencies to the solution of an overdetermined system of wave equations . As a result, the accuracy of elastograms can be greatly improved, depending on the amount of information combined into a single inversion step. In this approach, the major limiting factor for improving the quality of elastograms is the duration of image acquisition. We therefore use fast single-shot MRE with EPI (echo planar imaging)-readout and parallel imaging acceleration. Our current protocol for abdominal investigations requires three breath holds for a 3D full vector-field data set including 8 time-increments of the wave dynamics .
Figure 1: Example data of 3D multifrequency-MRE of the liver and the brain
 Guo J, Posnansky O, Hirsch S, Scheel M, Taupitz M, Braun J, Sack I. Fractal network dimension and viscoelastic powerlaw behavior: II. An experimental study of structure-mimicking phantoms by magnetic resonance elastography. Phys Med Biol 2012;57(12):4041-4053.
 Papazoglou S, Hirsch S, Braun J, Sack I. Multifrequency inversion in magnetic resonance elastography. Phys Med Biol 2012;57(8):2329-2346.
 Hirsch S, Guo J, Papazoglou S, Kroencke T, Braun J, Sack I. MR elastography of the liver and the spleen using a piezoelectric driver, single-shot wave-field acquisition, and multifrequency dual parameter reconstruction. Magn Reson Med 2013; accepted, in print.