Impact of skull modeling in spectral element simulations of transcranial focused ultrasound

Transcranial focused ultrasound (tFUS) has various applications in non-invasive treatments like neuromodulation, opening the blood-brain barrier, and tissue ablation. The objective of this work is to accurately model the propagation of waves in tFUS using the spectral-element method (SEM) and to better understand the types of physics that affect the wavefield within the laboratory measurements. Simulating transcranial ultrasound is non-trivial because of the skull’s high acoustic impedance, the P-S conversions at the skull boundary, and significant attenuation in the diplo¨e layer. Within this framework, we aim to qualitatively and quantitatively compare different skull model simulations. A coupled acoustic-viscoelastic wave equation is solved using the SEM. First, the reference simulation is carried out with a heterogeneous attenuating solid skull. Then, three simpler simulations are carried out by (1) eliminating heterogeneities in the skull, (2) neglecting the effects of shear waves, and (3) ignoring the effects of attenuation. We quantified the differences between each simulation and the reference model, with the acoustic skull simulation showing the highest percentage deviation of around 35% while the homogeneous and non-attenuating model shows deviation of around 20%. The higher deviation in the acoustic model is seen because of the prominent shear wave mode conversion at the elastic skull boundary. This comparison helps identify potential errors from omitting specific skull properties in simulations and guides researchers on which properties can be safely simplified based on their application.