Only one High Intensity Focused Ultrasound device has been clinically approved for transcranial brain surgery at the time of writing. The device operates within 650 kHz and 720 kHz and corrects the phase distortions induced by the skull of each patient using a multi-element phased array. Phase correction is estimated adaptively using a proprietary algorithm based on computed-tomography (CT) images of the patient's skull. In this paper, we assess the performance of the phase correction computed by the clinical device and compare it to (i) the correction obtained with a previously validated full-wave simulation algorithm using an open-source pseudo-spectral toolbox and (ii) a hydrophone-based correction performed invasively to measure the aberrations induced by the skull at 650 kHz. For the full-wave simulation, three different mappings between CT Hounsfield units and the longitudinal speed of sound inside the skull were tested. All methods are compared with the exact same setup thanks to transfer matrices acquired with the clinical system for N=5 skulls and T=2 different targets for each skull. We show that the clinical ray-tracing software and the full-wave simulation restore respectively 84±5% and 86±5% of the pressure obtained with hydrophone-based correction for targets located in central brain regions. On the second target (off-center), we also report that the performance of both algorithms degrades when the average incident angles of the acoustic beam at the skull surface increases. When incident angles are higher than 20°, the restored pressure drops below 75% of the pressure restored with hydrophone-based correction.

The phase correction necessary for transcranial ultrasound therapy requires numerical simulation to noninvasively assess the phase shift induced by the skull bone. Ideally the numerical simulations need to be fast enough for clinical implementation in a brain therapy protocol and to provide accurate estimation of the phase shift to optimize the refocusing through the skull. In this paper, we experimentally performed transcranial ultrasound focusing at 900kHz on N=5 human skulls. To reduce the computation time, we propose here to perform the numerical simulation at 450kHz and use the corresponding phase shifts experimentally at 900kHz. We demonstrate that a 450kHz simulation restores 94.2% of the pressure as compared to a simulation performed at 900kHz and 85.0% of the gold standard pressure obtained by an invasive time reversal procedure based on the signal recorded by a hydrophone placed at the target. From a 900kHz simulation to a 450kHz simulation, the grid size is divided by eight and the computation time is divided by ten.