Evidence for ultrasonic finite‐amplitude distortion in muscle using medical equipment

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.