We propose to design, develop and experimentally verify the performance of a highly innovative, acoustooptic measurement system with sub-millimeter resolution, appropriate for characterization of ultrasound fields encountered in clinical applications. Such a system with spatial and temporal resolutions proposed here is not currently available. The innovative elements of the proposed research include the development of a tapered fiber optic (FO) sensor with an active diameter on the order of 5-7 mu m (microns) that operates in the frequency range from 0.1 - 100 MHz and is sufficiently robust to measure fields generated by High Frequency Focused Ultrasound (HIFU) transducers used for treatment of malignant tissues. The small physical dimensions of the sensor will obviate the need for spatial averaging corrections so that true pressure-time (p-t) waveforms can be faithfully recorded. The recording of these waveforms is essential in order to determine all clinically relevant safety indicators, such as Mechanical and Thermal Indices. The sensitivity of the FO sensor will be comparable with that typical of currently used hydrophone probes. As optical fibers are inherently immune to non-optical electromagnetic fields stimuli, this proposed system has a potential of being nearly immune to Electromagnetic Field Interference (EMI). In addition, the intrinsically rugged characteristics of the fiber constitute an attractive feature as the existing ultrasound hydrophone probes are fragile and, in practice, cannot be used in therapeutic HIFU fields. Also, the miniature physical dimensions of the fiber optic sensor make it potentially well suited for in vivo measurements, virtually impossible with the currently available ultrasound hydrophone probes. Preliminary data indicate that once fully developed and calibrated, the acousto-optic system will form an important breakthrough in acoustic measurements of both diagnostic and therapeutic fields. Such outcome will also accelerate further advances of ultrasound applications in medicine. These advances are important because ultrasound offers nonionizing interaction with tissue and is economically favorable in comparison with other imaging and interventional modalities. [unreadable] [unreadable] [unreadable]