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

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Abstract
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 than 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. The author 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.

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