The goal of this R21 application is to develop a hydrophone for characterizing broadband high frequency ultrasound transducers. Traditionally, clinical ultrasound imaging uses frequencies less than 10 MHz. The need for improved resolution has fueled the development of higher frequency transducers in the 50 MHz range. These high resolution systems have been useful for ophthalmology, dermatology, and small animal imaging. Transducers operating above 100 MHz can image tissue near the cellular level. Steady progress in high frequency transducer fabrication offers the potential of miniaturized, minimally invasive systems with unprecedented resolution. Accurately measuring the transmitted ultrasound is clearly an important procedure in developing high frequency transducers. Unfortunately, such basic information is difficult to obtain due to the lack of suitable hydrophones operating at frequencies higher than 40 MHz. No existing hydrophone combines large bandwidth, sufficient sensitivity, fine spatial resolution, and ease of use. We propose an optical technique based on a Sagnac interferometer. The common-path nature of this design requires no active stabilization and provides immunity to environmental perturbations. Our preliminary work with a free-space Sagnac interferometer measured ultrasound fields with extremely high spatial resolution and large bandwidth. Additional results strongly suggest that polydimethylsiloxane (PDMS), a transparent silicone, is an excellent material for high frequency ultrasound detection. The fundamental hypothesis of this proposal is that a fiber optic Sagnac interferometer with a PDMS sensing element can be integrated into an immersible hydrophone for characterizing 100 MHz transducers. Several technical challenges must be overcome to develop this device. These issues will be addressed with the following specific aims: 1) Design, construct, and characterize an all-fiber Sagnac interferometer for detecting 100 MHz broadband ultrasound. 2) Design, construct, and characterize an immersible probe incorporating a PDMS sensing element. 3) Measure the radiated fields of 100 MHz ultrasound sources. This research will produce an invaluable tool for developing miniaturized ultrasound devices for high resolution imaging. These devices will expedite minimally invasive diagnostic and interventional procedures. The end result is more efficient and streamlined medical care, an enormous benefit to public health. [unreadable] [unreadable] [unreadable]