Effects of skull properties on long-pulsed transcranial focused ultrasound transmission

Abstract: Transcranial low-intensity focused ultrasound can deliver energy to the brain in a minimally invasive manner for neuromodulation applications. However, continuous sonication through the skull introduces significant wave interactions, complicating precise energy delivery to the target. We present a comprehensive examination of intracranial acoustic fields generated by focused ultrasound transducers and assess the characteristics of cranial bone that affect acoustic transmission. Acoustic field maps were generated at 88 regions of interest across 10 historical and 2 Thiel-embalmed human skull specimens with sonication at frequencies of 220 kHz, 650 kHz, and 1000 kHz. The average peak pressure insertion losses for historical were 3.6$\pm$3.4 dB, 9.3$\pm$3.3 dB, and 14.8$\pm$5.8 dB, respectively, and for Thiel skulls, the respective losses were 2.9$\pm$1.8 dB, 9.4$\pm$2.6 dB, and 17.0$\pm$5.5 dB. The effect of skull thickness, skull density ratio, and skull curvature on intracranial peak pressure, power and focal area was investigated and linear fits produced. Several unfavorable focusing performances were observed in regions with excessive thickness variation. The effects of angulation and spacing between the transducer and the skull were also investigated. Preliminary findings indicate that wave superposition resulting from skull and transducer spacing could lead to a 30-40% uncertainty in peak recorded intracranial pressure.