Optimal MR-ARFI encoding parameters for a range of contemporary magnetic field gradient systems
Purpose: To optimize motion encoding gradients in MR-Acoustic Radiation Force Imaging (ARFI) experiments under a range of experimental conditions, and to provide a general simulation tool. Methods: In MR-ARFI, Transcranial focused Ultrasound Stimulation (TUS) induced tissue displacement is encoded by motion encoding gradients as a phase change. We extended previously proposed equations for phase SNR to account for finite gradient switching times. We simulated the phase SNR while varying the strength and slew rate of motion encoding gradients for monopolar and bipolar encoding schemes in order to identify optimal configurations for various gradient hardware specifications. Phantom MR-ARFI experiments were performed to validate the simulations. Results: Optimized gradient durations are derived for a range of gradient strengths and slew rates. Gradient slew-rate is shown to influence the optimal parameters. Phantom MR-ARFI results showed good agreement with the simulations. Conclusion: The availability of modern gradient hardware is shown to dramatically improve the sensitivity of the ARFI experiment. Our openly available calculation tool allows ready calculation of optimal timings for any experimental conditions.