Acoustic shock generation by ultrasonic imaging equipment.
The pulses generated by ultrasonic imaging equipment have been observed to form acoustic shocks in water within a range of a few centimetres under normal operating conditions. The commonly held view of pulse propagation from ultrasonic imaging equipment is that the acoustic pulse has the form of a damped sine wave which will project largely unchanged in waveform. Any waveform changes which do occur result from diffraction effects and from the scattering and attenuation properties of tissue. The theory on which this understanding is based assumes that propagation laws are linear. This paper presents experimental evidence that this assumption is quite invalid at the pressures generated by commercial pulse-echo imaging equipment in common use. Measurements in water of the pulse waveforms using a calibrated broad-band polymer hydrophone have demonstrated that pulse distortion and shock formation commonly occur due to the inherent non-linearity of the propagation medium. This fact must be considered during the calibration of pulse-echo equipment. In addition, the conditions under which shock formation might occur during normal clinical procedures should be reviewed and any associated biological effects assessed.