EXPERIMENTAL VALIDATION OF PREDICTED TEMPERATURE RISES IN TISSUE-MIMICKING MATERIALS

Increasingly, it is recognised that diagnostic ultrasound is capable of causing temperature rises sufficient to damage tissue. Predictions of this heating are often based on simplified models of both the in vivo conditions and the relevant physical effects. Few measurements have been made to verify these predictions, however, particularly for the tightly focused beams often employed in diagnosis. Furthermore, non-linear effects in both the acoustic field and the surrounding medium have largely been ignored in calculations. To provide an alternative way to estimate the beating, NPL has developed a measurement system to determine directly the temperature rise in tissue-mimicking materials. If necessary, the measurement results can be processed to model the effects of blood perfusion. The temperature is measured using thin-film thermocouples, which have essentially no interaction with the ultrasound or thermal fields. Measurements were made on transducers operating in the frequency range 2-10 MHz and with focal beam-widths from 1 to 3 mm. The measurements agree with theoretical predictions that use either the measured beam-profile or a simplified (Gaussian) model. The results are also compared with the standard soft-tissue models developed by the National Council on Radiation Protection and Measurements (NCRP) and jointly between the American Institute of Ultrasound in Medicine and the National Electrical Manufacturers Association (AIUM/NEMA). On average, the predictions of the NCRP formula are 15% higher than the measurements, confirming its validity as a worst-case model. The predictions based on the AIUM/NEMA formula, however, are typically 30% lower than the measured values.