Influence of shear waves on transcranial ultrasound propagation in cortical brain regions

TL;DR

This study investigates how shear waves influence transcranial ultrasound propagation in cortical brain regions, revealing that ignoring shear waves leads to significant overestimations and inaccuracies, especially at oblique incidence angles.

Contribution

The paper introduces a solid skull model incorporating shear waves, demonstrating improved accuracy over traditional fluid models in simulating transcranial ultrasound in cortical regions.

Findings

Fluid skull models overestimate intracranial pressure by ~40%.

Ignoring shear waves causes up to 125% deviation in focal area predictions.

Solid models with shear waves provide more stable and accurate simulations.

Abstract

Transcranial ultrasound applications require accurate simulations to predict intracranial acoustic pressure fields. The current gold standard typically consists of calculating a longitudinal ultrasound wave propagation using a fluid skull model, which is based on full head CT images for retrieving the skull's geometry and elastic constants. Although this approach has extensively been validated for deep brain targets and routinely used in transcranial ultrasound ablation procedures, its accuracy in shallow cortical regions remains unexplored. In this study, we explore the shear wave effects associated with transcranial focused ultrasound propagation, both numerically and experimentally. The intracranial acoustic pressure was measured at different incidence angles at the parietal and frontal regions in an ex vivo human skull. The fluid-like skull model was then compared to the solid model comprising both longitudinal and shear waves. The results consistently show a larger error and variability for both models when considering an oblique incidence, reaching a maximum of 125% mean deviation of the focal area when employing the fluid skull model. Statistical assessments further revealed that ignoring shear waves results in an average ~40% overestimation of the intracranial acoustic pressure and inability to obtain an accurate intracranial acoustic pressure distribution. Moreover, the solid model has a more stable performance, even when small variations in the skull-transducer relative position are introduced. Our results could contribute to the refinement of the transcranial ultrasound propagation modeling methods thus help improving the safety and outcome of transcranial ultrasound therapy in the cortical brain areas.