Are hydrophones of diameter 0.5 mm small enough to characterise diagnostic ultrasound equipment?

A basic requirement for making measurements of medical ultrasonic fields using small sensors is that the sensor should be smaller than the ultrasonic wavelength. Until recently, the smallest commercially-available PVDF membrane hydrophone sensor had a diameter of 0.5 mm, which is larger that the wavelength in water for frequencies above 3 MHz. Thus many measurements have been made with hydrophones which are strictly too large. In this situation, averaging of the acoustic pressure over the active element can cause an underestimate of the spatial-peak acoustic pressure level. In the past, this error was estimated using theoretical models of the beam profile. However, these models make basic assumptions about both the ultrasonic field and the directional response of the hydrophone--assumptions which may not be valid in all diagnostic ultrasonic fields. GEC-Marconi membrane hydrophones with diameters as small as 0.1 mm have now been used to check these theories for diagnostic fields. This paper shows that the error resulting from the use of too large a hydrophone can be up to three times that predicted by current theories. Possible new correction methods are discussed for use in some situations. In other cases the errors can only be reduced by using these new hydrophones, particularly when the acoustic waveform is distorted by nonlinear propagation.