Magnetic resonance acoustic radiation force imaging in transcranial ultrasound stimulation

Transcranial ultrasound has many compelling applications, including brain stimulation and localized drug delivery. Variations in skull thickness and composition result in phase aberrations, attenuation, and absorption. Beam simulations using UTE MRI can be used to predict the intensity at the focus, and to correct for these effects. Then, at the time of the experiment, a measurement of the tissue displacement with MRI based on the acoustic radiation force (MR-ARFI) can be used to confirm focus location and focality. The measured displacement in theory will be a product of both the tissue stiffness and the beam intensity, with a correlation to tissue strain. In a large animal model, we demonstrated exactly this, that the MR-ARFI displacement was correlated to the neuromodulatory effect we were trying to achieve, a reduction in visual evoked potential to flashing lights in the sheep model. For human studies, we have focused on reducing the needed intensity for MR-ARFI, by improving the robustness to motion. We did this with a time series of single shot images, application of ultrasound in a block design, and removal of nuisance regressors. Ongoing work include characterization of MR-ARFI in white and grey matter and correlation to the neuromodulatory effect in humans.