MRI monitoring of heating produced by ultrasound absorption in the skull: in vivo study in pigs.

The purpose of this study was to test the utility of MR thermometry for monitoring the temperature rise on the brain surface and in the scalp induced by skull heating during ultrasound exposures. Eleven locations in three pigs were targeted with unfocused ultrasound exposures (frequency = 690 kHz; acoustic power = 8.2-16.5 W; duration = 20 s). MR thermometry (a chemical shift technique) showed an average temperature rise in vivo of 2.8 degrees C +/- 0.6 degrees C and 4.4 degrees C +/- 1.4 degrees C on the brain surface and scalp, respectively, at an acoustic power level of 10 W. The temperature rise on the scalp agreed with that measured with a thermocouple probe inserted adjacent to the skull (average temperature rise = 4.6 degrees C +/- 1.0 degrees C). Characterization of the transducer showed that the average acoustic intensity was 1.3 W/cm(2) at an acoustic power of 10 W. The ability to monitor the temperature rise next to the skull with MRI-based thermometry, as shown here, will allow for safety monitoring during clinical trials of transcranial focused ultrasound. Copyright 2004 Wiley-Liss, Inc.