A comparative study of experimental and simulated ultrasound beam propagation through cranial bones.

Objective: Transcranial ultrasound is used in a variety of treatments, including neuromodulation, opening the blood-brain barrier (BBB), and high intensity focused ultrasound (HIFU) therapies. To ensure safety and efficacy of these treatments, numerical simulations of the ultrasound field within the brain are used for treatment planning and evaluation. This study investigates the accuracy of numerical modelling of the propagation of focused ultrasound through cranial bones. Approach: Holograms of acoustic fields after propagation through four human skull specimens were measured for frequencies ranging from 270 kHz to 1 MHz, using both quasi-continuous and pulsed modes. The open-source k-Wave toolbox was employed for simulations, using an equivalent-source hologram and a uniform bowl source with parameters that best matched the measured free-field pressure distribution. Main results: The average absolute error in k-Wave simulations with sound speed and density derived from CT scans compared to measurements was 15\% for the spatial-peak acoustic pressure amplitude, 2.7 mm for the position of the focus, and 35\% for the focal volume. Optimised uniform bowl sources achieved calculation accuracy comparable to that of the hologram sources. Significance: This method is demonstrated as a suitable tool for prediction of focal position, size and overall distribution of transcranial ultrasound fields. The accuracy of the shape and position of the focal region demonstrate the suitability of the sound speed and density mapping used here. However, large errors in pressure amplitude and transmission loss in some individual cases show that alternative methods for mapping individual skull attenuation are needed and the possibility of considerable errors in pressure amplitude should be taken into account when planning focused ultrasound studies or interventions in the human brain, and appropriate safety margins should be used.