A numerical study of pulsed sonication for reducing thermal deposition in the skull during transcranial focused ultrasound surgery

Abstract

Non-uniform thermal deposition in skull during transcranial focused ultrasound thermal ablation of brain tumors may cause undesired temperature elevation and produce adverse effects in scalp, skull and the adjacent normal brain tissues. Instead of using continuous wave (CW) sonication scheme, a pulsed wave (PW) sonication scheme was proposed, intending to allow extra cooling between sonicating pulses so that the accumulated heat in skull would dissipate from the skull surface to the surrounding low temperature coupling water. Using a 0.6 MHz, 500-elemnt and 300-mm-diameter hemispherical phased array and taking into account of the speed of sound and acoustic attenuation coefficient variations in skull bone, the specific absorption rate distribution and the resulting thermal behaviors in the brain and skull were simulated from a large-scale 3D finite- difference time-domain (FDTD) acoustic and thermal simulation with distributed client/server computing techniques. An active exchange of 15°C chilling water was used to cool the skull. The CW case had a 20-s sonication followed by a 20-s cooling time. Two PW cases were simulated, with five identical pulses of 4-s period with 50% and 25% duty cycle respectively, and followed by a 20-s cooling time. To reach a 70°C peak temperature at focus in the brain tissue, the CW and two PW cases had peak temperatures of 51.5°C, 48.9°C and 48.5°C in the skull bone, respectively. The peak cumulative equivalent minutes at 43°C in the skull bone of the CW and PW cases were 13.5, 0.76 and 0.32 min, respectively. The peak thermal dose in skull of the PW cases were significantly lower than that of the CW case. The results suggest that the pulsed sonication scheme may be useful in reducing thermal deposition in the skull bone during the noninvasive transcranial focused ultrasound surgery of brain tumors.