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Numerical prediction of frequency dependent 3D maps of mechanical index thresholds in ultrasonic brain therapy

Therapeutic ultrasound has been used in brain therapy for thrombolysis and thermal ablation. Two approaches are used to transmit acoustic energy through the skull to the brain: low frequency ultrasound, with a wavelength that is larger than the skull thickness, and high frequency ultrasound, that is sensitive to aberrations and must use corrective techniques. At high frequency the Mechanical Index (MI) is lower, which translates to a higher cavitation threshold. The goal of this study is to determine if there is a comparable acoustic environment during high intensity focused ultrasound (HIFU) brain ablation at a high frequency by evaluating the volume that is above various MI thresholds for different device geometries that correspond to each treatment. The acoustic field was modeled with a 3D finite difference fullwave acoustic code developed by the authors that has been previously validated in a general acoustic context and with registered skull experiments. In addition to the non focused geometry of a 300 kHz blood clot treatment device two types of focused therapeutic transducers were modeled: a low frequency 300 kHz transducer, and a 1 MHz transducer that required aberration correction with a time reversal approach. It is shown that the brain volume above a range of MI is over three orders of magnitude larger at 300 kHz compared to 1 MHz.