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A survey of the powers, pressures and intensities generated by ultrasonic pulse-echo equipment in clinical use has been carried out. Thre conventional B-scanners, four linear-array scanners and four mechanically sectored scanners were included in the study. Measurements were made on a total of 22 transducers covering the nominal frequency range 2.25–7.5 MHz. On those instruments where an output power control was provided, two measurements were made: one at the maximum available power and a second at a lower power. On arrays with a variable transmit focus control, measurements were made at all available focussettings. In all, measurements were made on 38 separate focused pulsedultrasonic fields. The measurements were carried out using a calibrated ultrasonic force balance, and a calibrated polyvinylidene difiuoride (PVdF) membrane hydrophone. A very wide range of maximum powers, pressures and intensities were found. Powers from 0.5–80 mW were measured; spatialaverage temporal-peak positive pressures at the transducer varied between 30 kPa and 1.15 MPa, and spatial-peak pulseaverage intensities were in the range 3.6 × 103– 1.1 × 107Wm-2.

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°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°C after 1 min and reached a steady-state temperature of almost 80°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.