PREDICTION OF NONLINEAR PROPAGATION IN WATER DUE TO DIAGNOSTIC MEDICAL ULTRASOUND EQUIPMENT

There is a well established requirement for making output measurements on medical ultrasound equipment, in particular for safety and calibration purposes. In addition there is a need for non-linear propagation models to allow predictions to be made for in vivo pressure fields of systems ranging from lithotripters to diagnostic imaging sets. In the past it has been shown that good agreement can be found between experiment and theory for transducers that behave as perfect pistons but little work has been published on comparisons for 'real' medical ultrasound systems. This paper compares experiment with theory for a medical ultrasound system operating in water. Pressure field measurements are presented for a Philips 3.5 MHz, 13 mm diameter, 'long internal focus' (LIF) transducer. The measurements were made using a membrane hydrophone in water. Initially the transducer was driven continuously with a function generator at low amplitude so that the effective aperture radius and focal length could be determined. The transducer was then driven by a Philips 'sono-Diagnost B' imaging system which produced a short finite amplitude pulse. The results are compared with a finite difference model based on the parabolic approximation to the non-linear wave equation. Reasonable agreement (typically 10-15%) is shown for spectral magnitudes although some difficulty was encountered in accurately characterizing the transducer in terms of its geometry and drive waveform.