Simultaneous proton resonance frequency T1 - 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-1 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-1 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-1 and decreasing shear wave velocity, and after tissue coagulation with decreasing T-1 and increasing shear wave velocity. This is consistent with a previously described hysteresis in T-1 versus PRFS curves and increased tissue stiffness with tissue coagulation.Conclusion: An efficient approach for simultaneous and dynamic measurements of PRSF, T-1, 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.

Automated summary

The purpose of this study was to develop an efficient MRI pulse sequence for simultaneously measuring multiple parameters correlated with tissue nonviability following thermal therapies. A 3D segmented EPI pulse sequence was used to measure PRFS MR thermometry, T-1 relaxation time, and shear wave velocity induced by FUS push pulses. The results showed that temperature increases and T-1 increases correlated with decreased shear wave velocity, indicating material softening with increasing temperature. This approach holds promise for providing co-registered dynamic measures of multiple parameters, which can help assess tissue nonviability during and after thermal therapies.