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Finite-amplitude distortion of ultrasonic waves from medical equipment has been observed to occur following transmission through calf muscle in human volunteers. Measurements were made using both dynamic pulse-echo imaging equipment and physiotherapy equipment. In both cases irradiation was carried out under operating conditions commonly used clinically. Pressure waveforms were measured at the skin surface using a broadband polyvinylidene difluoride membrane hydrophone. Using a pulsed, weakly focused 2.5-MHz beam with input peak pressure of 0.8 MPa and a pressure gain of 5.3 at the focus, the mean second harmonic peak magnitude (16 measurements) was 17 dB below the fundamental peak. A 1.1-MHz continuous wave therapy set with input peak pressure of 0.5 MPa showed mean second harmonic magnitude 23 dB below the fundamental. [ABSTRACT FROM AUTHOR]

Aim: Acoustic streaming in amniotic fluid has been investigated under a variety of conditions relevant to the diagnostic use of ultrasound., Method: An ultrasonic Doppler method has been used for measurement. Streaming velocities have been compared with those generated in water for the same exposure conditions. Beams were generated by laboratory equipment simulating beams from clinical systems. The fluids were insonated IN VITRO using 3.5 MHz, 5 MHz and 7.5 MHz transducers in continuous wave (CW) and pulsed mode., Results: Acoustic streaming was measured in both amniotic fluid and water at the power levels 50 mW and 140 mW. Enhancement of velocities due to non-linear effects in high amplitude pulses was demonstrated for amniotic fluid as well as for water. The potential and limitations of present numeric methods for the prediction of acoustic streaming were explored., Conclusion: Pulsed ultrasound caused similar streaming velocities in amniotic fluid and water while continuous wave beams induced significantly faster streaming in amniotic fluid than in water.

Streaming is shown to occur in water in the focused beams produced by a number of medical pulse-echo devices. The use of hot film anemometry to measure the streaming velocity is described and velocities measured in water using commercial equipment are quoted. The highest velocities occur in pulsed Doppler mode with a maximum velocity of 14 cm s-1 being observed. An experimental set-up was used to investigate the parameters affecting streaming and it was found that the harmonic content of the pulse waveform had a major effect on the streaming velocity. The time taken for a stream to become established at the focus of the acoustic beams studied was typically approximately 0.5 s.

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.

The effect of finite-amplitude distortion on the positions of peak pressures and peak intensities in beams generated in water by pulsed focused transducers has been investigated experimentally. The pulses generated by three single-element, circular, focused transducers with nominal frequencies of 2.25, 3.5, and 5.0 MHz have been investigated with pressures at the transducer P0 being varied over the range 10 kPa to 1.4 MPa. Measurements were made using a 9 micron thick polyvinylidene difluoride membrane hydrophone. In all cases the locations of peak pressures were not stationary as the field strength was altered. As P0 increased, the distance from the transducer to the positive peak initially increased, and then decreased. The distance to the negative, or decompression peak decreased monotonically with increasing P0. The location of peak pulse intensity integral was found to alter slightly with power, taking a position between the peak positive and negative pressures.