Ultrasonic measurement of the temperature distribution due to absorption of diagnostic ultrasound: potential and limitations

Diagnostic ultrasound can cause hazardous temperature rises under certain imaging conditions. Thermocouples have been used to measure temperature, but they are invasive and it is difficult to localise the peak value. This paper examines the potential and limitations of using ultrasonic thermography to measure the temperature distribution due to a diagnostic ultrasound field. The technique exploits the principles that the sound speed in a medium is a function of temperature and that an ultrasound scanner interprets heat-induced sound speed changes as displacements. These apparent displacements can be measured by echo tracking and used to produce an image of the temperature rise. We have demonstrated in vitro that this is a sensitive technique that can portray temperature rises of <1oC above room temperature. The temperature rise can be measured in two, and potentially three dimensions, with a spatial resolution similar to that of diagnostic ultrasound. A possible limitation to the accuracy of the method is the need to know the temperature dependence of sound speed for the medium. Furthermore, some media may incur sound speed changes that are too small to be measured. The technique will not usually be possible beyond gas or bone. Finally, there are sources of error (e.g., thermoacoustic refraction and mechanical motion) that are likely to require correction. We conclude that ultrasonic measurement of temperature distributions due to diagnostic fields is worthy of study.