Design of a 30 um aperture membrane hydrophone for the measurement and characterization of High Frequency ultrasound

The field of High Frequency micro-ultrasound (ultrasound having centre frequencies greater than 15 MHz) is growing, with clinical and pre-clinical array based systems having centre frequencies of up to 50 MHz currently in production. Limitations in hydrophone technology related to the size of hydrophone apertures, limited bandwidth, and limited calibration range are problematic for clinical regulatory measurements. A method of making a membrane hydrophone with an active area as small as 10 um, having very low field fringing effects is presented. A new method of creating well aligned electrodes on opposite sides of a very thin polymer membrane have been developed and successfully implemented using proprietary FujiFilm VisualSonics Inc. (VSI) Excimer laser technology. Measurements made using newly developed 30 micron membrane hydrophones, including custom electronics and amplifiers, are presented. Calibration results to 110 MHz are presented along with a discussion of uncertainties and challenges related to measuring acoustic fields in water at 50 MHz centre frequency and high pressures.