Rapid FUS Focal Spot Localization based on MR-ARFI

Introduction For various therapeutic applications, focused ultrasound (FUS) is used in either thermal mode or in mechanical mode. Mechanical mode FUS has been used for temporary blood brain barrier disruption and for targeted drug delivery. Monitoring of such treatments remains a technical challenge since there should be no significant temperature increase in the focal spot, unlike in case of thermal mode FUS. As a potential monitoring method, it has been proposed to use MR acoustic radiation force imaging (MR-ARFI) to measure tissue displacement in the focal spot, rather than temperature. To do this, the ultrasound is applied during motion sensitizing gradients, which encode the displacement induced by the acoustic radiation force into the phase of the image. In previous work, the displacement maps were acquired with a diffusion-weighted (DW) line scan sequence,[1,2] and a set of repeated bipolar gradients were found to provide the optimal SNR and displacement sensitivity.[2] In this work, the repeated bipolar displacement encoding gradients are combined with a single shot EPI readout to localize the focal spot, while minimizing the scan time and possible heat deposition.