Surface heating of diagnostic ultrasound transducers

Surface temperatures of a variety of transducers used with common commercial ultrasonic diagnostic equipment have been measured. Transducers operating in imaging mode, in both continuous and pulsed Doppler modes, and in mixed modes were investigated. A total of 30 transducers and scan-heads used with equipment from 10 manufacturers were examined, including a range of array types, mechanical sectors and continuous-wave Doppler transducers. Measurements were made using an infrared radiometer, or a thermocouple probe, with the transducers operating in air. Surface temperatures of 13 transducers operating in imaging mode were found to be in the range 0.0-13.1 degrees C above ambient after 5 min operation. Some transducers operating in pulsed Doppler mode reached considerably higher temperatures. The most extreme example increased the surface temperature by 36.5 degrees C after 1 min and reached a steady-state temperature of almost 80 degrees C. Transducers operating at these temperatures cannot be retained on the skin of a conscious subject without pain, and will cause skin burns within a brief period of time. A linear relationship has been demonstrated between temperature increase and spatial-average acoustic intensity. The rate of increase in air was found to be about 10 times greater for pulsed arrays than for continuous-wave Doppler transducers.