A survey of the acoustic output of ultrasonic Doppler equipment.

Measurements of the acoustic output generated by a variety of clinical ultrasonic Doppler instruments have been carried out. The instruments surveyed include continuous wave Doppler units for cardiovascular investigations, fetal monitors, stand-alone pulsed Doppler equipment and 'duplex' scanners working in Doppler mode. Acoustic measurements have been made using a calibrated PVDF membrane hydrophone, and a milliwatt radiation force balance. Almost all the pulsed Doppler and duplex systems investigated could generate spatial-peak, temporal-average intensities in water which exceeded 100 mW cm-2, with a maximum of 825 mW cm-2 measured. These intensities were also reached by some continuous-wave Doppler systems, particularly those incorporated into duplex scanning systems. Fetal monitoring equipment was found to operate typically at lower intensities. Doppler units have been found to show a very wide variation in pulse length, repetition frequency and spatial peak pressure, and a wider range of pulse average intensities than previously reported. Units were found to vary considerably in their control of total acoustic power whilst operating parameters such as gate width and range were altered.