Measurement of enhanced heating due to ultrasound absorption in the presence of nonlinear propagation

An important biophysical effect concerning the safety of diagnostic ultrasound is the heating caused by absorption of ultrasonic energy in tissue. It is shown that nonlinear propagation can increase this ultrasonic heating. The experimental arrangement incorporates a fluid path followed by a sample of tissue-mimicking gel, which models the scanning of a fetus through the full bladder. The measurements were made using a constant spatial-peak temporal-average intensity of 1 W/cm ², but the temporal-peak intensity was varied by altering the pulsing regime. Up to a threefold increase in the temperature rise of the gel was observed, with the maximum rise being about 2°C. This increased heating correlates well with the amount of nonlinear distortion present in the incident wave, as characterized by the shock parameter. At low amplitudes the results are compared with theoretical calculations. The experimental results are used to estimate the temperature rise that may be produced by diagnostic equipment when the acoustic path in the patient is composed mainly of fluid