Hydrophone measurements in diagnostic ultrasound fields.

Some of the methods, calculations, and problems associated with making hydrophone measurements in diagnostic ultrasound fields are considered. Several organizations have proposed definitions of various peak and average intensities that need to be specified when characterizing medical ultrasound fields. All of these can be determined from hydrophone measurements, but the bandwidths encountered (>50 MHz), along with the small focal diameters achievable ( approximately 1 mm), can place great demands on hydrophone performance. Two general types of hydrophones are available-the spot-poled membrane and needle types. Both employ the piezopolymer polyvinylidine fluoride (PVDF), and for the most part they have effective dimensions in the 0.5-1.0-mm range. Hydrophones can be made with useful bandwidths extending beyond 50 MHz. However, above approximately 15 MHz the nature of the response becomes highly dependent on the method of construction and PVDF film thickness used, as well as the characteristics of any associated preamplifier circuitry. Other factors that can affect measurement accuracy are discussed.