This work proposes development and implementation of acoustic and optic measurement methods capable of determining absolute sensitivity of miniature ultrasonic hydrophone probes over a wide, 100 M}Iz bandwidth. Absolutely calibrated probes are needed to determine and monitor acoustic output of ultrasound imaging devices, which presently account for about 30 percent of all imaging procedures. Although a majority of clinical imaging is performed at frequencies less than 15 MHz, due to the nonlinear propagation effect in tissue, which constitutes the basis of harmonic imaging, diagnostic pulses can have spectral content from 100 kHz to 100 MHz. Also catheter based systems routinely use frequencies beyond 20 MHz. Moreover, to properly measure mechanical bioeffects exposure index (MI) and to comply with regulatory guidelines, the hydrophone probes should be calibrated in the frequency range extending to 8 times center frequency of the imaging transducer. In addition, dermatological, ophthalmological and pre-clinical applications of ultrasound utilize imaging transducers in the frequency range 20-100 MHz. However, currently used calibration technology is usually limited to approximately 20 MHz and therefore, there is a need for research and development of high frequency calibration techniques capable of determining frequency response of the miniature ultrasound hydrophone probes. In the United States, the national laboratory traceable data are limited to approximately 21 MHz and are available at discrete frequencies only. The outcome of this work will facilitate characterization of high frequency medical ultrasonic fields and assessment of the safety of diagnostic ultrasound devices operating at frequencies beyond 20 MHz or containing harmonics in the signal due to nonlinear propagation phenomena. Globally, no reference laboratory succeeded so far in providing virtually continuous calibration data that are expected to be available as the result of this research. The proposed research will also provide a tool to calculate Mechanical and Thermal Indices in case of conventional 3-12 MHz equipment operating at high pressure amplitudes. The indices are widely accepted as predictors of potential bioeffects. Also, the results of this research are applicable to design and optimization of ultrasound imaging transducers. PIs have succeeded in fabricating fiber optic tips having diameters equal to a fraction of a wavelength at 100 MHz and are hopeful that these fiber optic probes will be suitable to replace piezoelectric hydrophones in measurements where spatial averaging errors are unacceptable.
