MR-ARFI for transcranial ultrasound neuromodulation: The effect of varying frequency and depth.

Motivation: Previous MR-ARFI optimizations have incorporated tissue response times, but assumed those were constant in a homogenous medium. Goal(s): To demonstrate that tissue response times vary with ultrasound parameters such as frequency and depth, and to use those to optimize MR-ARFI. Approach: We measured displacement response time in a tissue-mimicking phantom at 0.5MHz and 1.0MHz ultrasound frequencies and depths of 40- and 60-mm. Results: With our tissue-mimicking phantom, the time constants were approximately 3ms and 6ms for 1.0MHz and 0.5MHz ultrasound frequencies, respectively. Impact: Ultrasound parameters of frequency and depth affect tissue response and can be used to optimize the acquisition to maximize SNR and minimize ultrasound dose.