Fractional encoding of harmonic motions in MR elastography.

In MR elastography (MRE) shear waves are magnetically encoded by bipolar gradients that usually oscillate with the same frequency fv as the mechanical vibration. As a result, both the repetition time (TR) and echo time (TE) of such an MRE sequence are greater than the vibration period 1/fv. This causes long acquisition times and considerable signal dephasing in tissue with short transverse relaxation times. Here we propose a reverse concept with TR<or=1/fv which we call "fractional" MRE, i.e., only a fraction of one vibration cycle per TR, can be used for motion sensitization. The benefit of fractional MRE is twofold: 1) acquisition times in seconds can be achieved for a single-phase difference wave image, and 2) materials that combine low elasticity, high viscosity, and short T2* relaxation times show an increased phase-to-noise ratio (PNR). A twofold increase of the phase signal is predicted for liver-like materials. Volunteer studies performed in liver and biceps show the benefit of fractional MRE. Furthermore, we demonstrate the feasibility of the technique for in vivo myocardial MRE by visualizing transverse wave propagation in the interventricular septum (IVS).