Quantitative impact of tissue layer composition in transcranial focused ultrasound modeling

Transcranial focused ultrasound is widely used for treating brain diseases; however, skull-induced attenuation and aberration require numerical acoustic simulations for safe application. These simulations predominantly focus on skull modeling, whereas the soft tissues are often simplified or neglected despite their relevance under clinical conditions. In this study, we systematically investigated how head model compositions influence ultrasound propagation by incorporating the scalp and non-skull soft tissues into skull-centered models. Accordingly, four head model configurations were devised from a common MRI-based head model with anatomically detailed structure and tissue-specific acoustic properties that differed in the inclusion of tissues (skull, scalp–skull, skull–tissue, and scalp–skull–tissue). Across the 13 stimulation positions, the peak pressure and focal volume were quantified from numerical simulations at 250 kHz and statistically compared across model configurations using repeated-measures ANOVA and two-tailed paired t-tests. Consequently, the non-skull soft tissues produced greater changes in peak pressure and focal volume, with significant differences across a wider stimulation position, varying by subject and target location. In addition, this significance generally tended to be associated with decreases in pressure and an increase in focal volume. Correlation analysis demonstrated that skull thickness was the primary factor related to pressure attenuation (r = −0.5334) and focal region enlargement (r = 0.5106), whereas scalp thickness showed weaker associations with pressure (r = −0.1630) and focal regions (r = 0.1239). In the spatial beam pattern analysis, non-skull soft tissues altered the pressure distribution substantially (intersection over union, IoU: 0.5–0.9) compared to the scalp (IoU: 0.7–0.98). These results indicate that non-skull soft tissues have a greater impact on ultrasound propagation than the scalp. Consequently, this work provides essential quantitative insights into head model composition, emphasizing the critical role of non-skull soft tissues in accurate modeling.