Simultaneous proton resonance frequency T - MR shear wave elastography for MR-guided focused ultrasound multiparametric treatment monitoring.

Purpose: To develop an efficient MRI pulse sequence to simultaneously measure multiple parameters that have been shown to correlate with tissue nonviability following thermal therapies.

Methods: A 3D segmented EPI pulse sequence was used to simultaneously measure proton resonance frequency shift (PRFS) MR thermometry (MRT), T relaxation time, and shear wave velocity induced by focused ultrasound (FUS) push pulses. Experiments were performed in tissue mimicking gelatin phantoms and ex vivo bovine liver. Using a carefully designed FUS triggering scheme, a heating duty cycle of approximately 65% was achieved by interleaving FUS ablation pulses with FUS push pulses to induce shear waves in the tissue.

Results: In phantom studies, temperature increases measured with PRFS MRT and increases in T correlated with decreased shear wave velocity, consistent with material softening with increasing temperature. During ablation in ex vivo liver, temperature increase measured with PRFS MRT initially correlated with increasing T and decreasing shear wave velocity, and after tissue coagulation with decreasing T and increasing shear wave velocity. This is consistent with a previously described hysteresis in T versus PRFS curves and increased tissue stiffness with tissue coagulation.

Conclusion: An efficient approach for simultaneous and dynamic measurements of PRSF, T , and shear wave velocity during treatment is presented. This approach holds promise for providing co-registered dynamic measures of multiple parameters, which correlates to tissue nonviability during and following thermal therapies, such as FUS.