Predicting skull lesions after clinical transcranial MRI-guided focused ultrasound with acoustic and thermal simulations

Abstract: Transcranial MRI-guided focused ultrasound (TcMRgFUS) thermal ablation is a noninvasive functional neurosurgery technique. Previous reports have shown that bone marrow damage in the skull can occur at high acoustic energies. While this damage is asymptomatic, it would be desirable to avoid it. Here we examined whether acoustic and thermal simulations can predict where the thermal lesions in the skull might occur. Post-treatment imaging was obtained at 3-15 months after 40 clinical TcMRgFUS procedures, and skull lesions were observed after 16/40 treatments. The presence for lesions was predicted by the acoustic energy with a threshold of 18.1-21.1 kJ (maximum acoustic energy) and 97-112 kJ (total acoustic energy). The size and degree of the lesions was not always predicted by the acoustic en-ergy used during treatment alone. In contrast, the heating estimated by the acoustic and thermal simulations was pre-dictive of the lesion extent. The lesions appeared in areas that were predicted to have high temperatures, and when thin slice T2-weighted imaging was obtained at 3-15 months, an excellent agreement was observed between the predictions and lesion locations. While more work is needed to validate the absolute temperatures measurements in and around the skull, being able to predict the locations and onset for skull lesions could allow for better distribution of the acoustic energy over the skull. Understanding skull ab-sorption characteristics of TcMRgFUS could also be useful in optimizing transcranial focusing.