Ultrasound hyperthermia in clotted blood and ex vivo cranial bone

Abstract

Ultrasound thermal effects have been hypothesized to contribute moderately to ultrasound-assisted thrombolysis. Transcranial ultrasound adjuvant to thrombolytic treatment of ischemic stroke requires a detailed investigation of the heating produced by ultrasound exposure to the skull bone, brain, and blood clots. Thermocouple measurements of ex vivo cranial bone and blood clot heating were compared to theoretical predictions of ultrasound hyperthermia, which assumed adiabatic conditions and provided an estimate for the worst-case scenario. The in vitro experiments were conducted at two temperatures, 20<th>°C and 37<th>°C. Ex vivo human skull, and porcine or human blood clots were exposed to 120 kHz, 1 MHz, or 3.5 MHz pulsed ultrasound with an 80% duty cycle and an acoustic spatial peak intensity of 0.5 W/cm2. The thermal elevation in human cranial temporal bone was below 1<th>°C for all three frequencies. The ultrasound hyperthermia in porcine and human clots increased with an increase in center frequency, but did not exceed (0.33±0.06)<th>°C. These results provide the information necessary to quantify the contribution of a thermal mechanism for ultrasound-enhanced thrombolysis.