A novel method for acoustic modeling of cranial bone based on the porosity index

Transcranial ultrasound stimulation (TUS) has emerged as a clinically validated neuromodulation technique. Particularly, phased array ultrasound can be applied in TUS to focus on the cortex or deep brain non-invasively, such as the ventral intermediate thalamic nucleus (VIM) and Precuneus (PCu) region for the treatment of essential tremor (ET) and Alzheimer Disease (AD). Current TUS treatment planning relies on computed tomography (CT)-derived skull porosity measurements, which involve patient radiation exposure and potential registration errors. This study proposes a Porosity Index (PI)-based method, derived from Ultrashort Echo Time (UTE) Magnetic Resonance (MR) images, for establishing skull acoustic models as a viable alternative, aiming to eliminate these limitations. Acoustic simulations using the K-Wave open-source platform were performed to validate the PI method's accuracy in predicting skull porosity and simulating focal distributions compared to CT. Focal spot characteristics were quantified using five metrics: peak intensity, target intensity, focal positioning error, Dice similarity coefficient, and Pearson correlation coefficient between CT- and PI-based simulation results. Statistical differences between these metrics were assessed using Tukey's multiple comparisons test. Quantitative comparisons against the gold-standard CT approach demonstrated comparable performance in peak focal intensity (deviation < 5%) and spatial pressure distribution patterns (Dice coefficient > 0.82). No significant differences (p > 0.05) were observed for any of the evaluated metrics. Our findings demonstrate that both the sound pressure distribution and prediction of the porosity are comparable with those from the reference CT. Using the PI to replace the traditional CT porosity has high feasibility and can achieve the purpose of reducing unnecessary radiation exposure and registration error for patients.