Prediction of temperature rise in layered media from measured ultrasonic intensity data

It is now widely recognized that under some circumstances exposure to ultrasound at current diagnostic levels may result in undesirable heating in tissue. The National Council on Radiation Protection and Measurements (NCRP) and the American Institute of Ultrasound in Medicine (AIUM) in conjunction with the National Electrical Manufacturers Association (NEMA) have suggested relatively simple methods for estimating the in situ temperature rise based on simplified ultrasonic intensity profiles. In this paper, the measured spatial intensity distribution from three unscanned ultrasonic transducers is used in an integral solution to the bioheat equation to calculate the temperature increase in layered media, and these calculations are compared with the simple predictions under similar conditions. The transducers included weak and strong focusing devices and one device with a non-cylindrical beam. The ratio between the NCRP and AIUM/NEMA predictions varied from 0.5 to 2.3 in equivalent situations. The ratio between the simple predictions and the more detailed calculations varied from 0.7 to 3.2 for NCRP, and from 0.3 and 3.5 for AIUM/NEMA. These results highlight the need, firstly, to establish simple 'standard' methods for estimating likely temperature increase during clinical examination, and secondly to be clear about whether these methods are intended to give 'worst case' or 'typical' estimates.