ACOUSTIC PERFORMANCE AND CLINICAL USE OF A FIBEROPTIC HYDROPHONE

Initial clinical experience with the use of an optical fibre hydrophon e for in vivo ultrasound dosimetry is reported. The hydrophone, origin ally described by Beard and Mills (1997), operates as an extrinsic, lo w-finesse Fabry-Perot optical sensor where acoustically-induced thickn ess changes in a polymer film modulate the phase difference between li ght beams reflected from the two surfaces of the him, The pressure wav eforms from the sensor are compared with those from a calibrated piezo electric polymer membrane hydrophone. The sensor is found to have a fr equency resonance at around 12 MHz, corresponding to the thickness mod e of the 50-mu m polymer film, The directional responses at 0.16 MHz, 1.0 MHz and 5.0 MHz are found to be similar to those predicted for a p lane piston receiver with the same diameter as that of the polymer fil m (400 mu m). The performance of the sensor as a broad-band hydrophone is degraded by the relatively low acoustical impedance of the adhesiv e used in the fibre-film bond. The hydrophone was used in the clinic f or measurement of acoustic pressures within the ureter of 4 patients u ndergoing clinical extracorporeal shock-wave lithotripsy on a Dornier HM3 lithotripter, Pressures in the range 0.5 to 5.0 MPa were recorded in the ureter at positions over 10 cm from the renal pelvis. Problems related to the clinical use of the sensor, including instability in th e sensitivity of the sensor following handling and its mechanical stre ngth in high-amplitude acoustic fields, are discussed. (C) 1998 World Federation for Ultrasound in Medicine & Biology.