Fractional encoding of harmonic motions in MR elastography

TLDR

MR elastography encodes shear waves with bipolar gradients at the vibration frequency, but the repetition and echo times exceed the vibration period, leading to long acquisition times and signal dephasing in tissues with short transverse relaxation times. The study proposes fractional MR elastography, using a repetition time no longer than one vibration period to sensitize motion with only a fraction of a cycle per TR. Fractional MRE achieves motion sensitization by encoding shear waves with a repetition time no longer than one vibration cycle, allowing only a fraction of the cycle per TR. Fractional MRE yields rapid single‑phase images, improves phase‑to‑noise ratio in soft, viscous tissues with short T2, predicts a two‑fold phase signal increase in liver‑like materials, and is validated in volunteer liver, biceps, and myocardial studies. Published in Magn Reson Med 57:388–395 (2007) © 2007 Wiley‑Liss, Inc.

Abstract

Abstract In MR elastography (MRE) shear waves are magnetically encoded by bipolar gradients that usually oscillate with the same frequency f v 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/ f v . This causes long acquisition times and considerable signal dephasing in tissue with short transverse relaxation times. Here we propose a reverse concept with TR ≤ 1/ f v 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 T relaxation times show an increased phase‐to‐noise ratio (PNR). In 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). Magn Reson Med 57:388–395, 2007. © 2007 Wiley‐Liss, Inc.