Aberration correction for trans-skull histotripsy

The skull represents a substantial barrier to performing histotripsy therapy within the brain causing focal pressure loss and shifts in focal location. At sub MHz frequencies, both effects are mostly due to wavefront aberration from localized variations in skull thickness and sound speed. These aberrations can be corrected using a phased array by varying element transmit timings to realign wavefronts at the focus. This talk presents results using propagation modeling and acoustic emissions from cavitation (as well as a combination of both) for determining appropriate element timings for aberration correction. Propagation models tested included forward and backward ray-tracing methods and full-wave simulations (k-wave). Acoustic emissions associated with the expansion and collapse of bubble clouds as well as the re-excitation of previously generated bubbles were considered for time-reversal-based corrections. Propagation modeling was found to be very effective at correcting focal shift errors, but generally provided only marginal increases in focal pressure amplitude. Aberration correction using acoustic emissions was found to significantly increase focal pressure amplitude, particularly when combined with propagation modeling, achieving roughly 90% of the pressure obtained using a hydrophone. Full-wave modeling did not show significant improvement over ray-tracing methods perhaps because of errors in sound speed estimation from CT data.