A novel sensor for determining ultrasonic intensity

Quantifying the acoustic output of diagnostic and therapeutic medical ultrasound devices is an established practice, using measurement methods based on applying miniature hydrophones to determine acoustic pressure distributions. However, specification standards require manufacturers to describe and declare acoustic output information in the form of intensity values, as these are more relevant to the possibility of adverse bioeffects. Simple relationships exist to calculate intensities from pressure data, but under many circumstances, such as away from the acoustic axis and in the transducer near-field, the underpinning assumptions break down. This paper describes the design, development and testing to proof-of-concept of a novel design of ultrasound sensor which can determine intensity directly. The sensor uses the pyroelectric properties of the piezoelectric polymer PVDF, and takes the form of a conventional membrane hydrophone backed with a highly attenuating polyurethane-based material. Ultrasound incident on the backing material is quickly absorbed, and the rate of temperature increase over a short timescale is proportional to the intensity in the beam, and produces a pyroelectric voltage response in the PVDF film. The new sensor also behaves as a conventional hydrophone, and can be used to derive acoustic pressure profiles. Intensity results are compared with pressure-squared data obtained from beam-plotting a range of simple transducer fields, and suggest differences in the beam profiles, particularly in the acoustic near field.