Comparison of the output display standard's TIS estimates with independently determined maximum temperature increase calculations ~diagnostic ultrasound equipment\

The monopole-source solution was used to calculate the three-dimensional complex acoustic pressure field for focused circular apertures (diameters: 1, 2, 4 cm) and transmit f-numbers (radius of curvature/diameter values: 0.7, 1, 1.3, 1.6, 2, 3, 4, 5) for various ultrasonic frequencies (1, 2, 3, 4, 5, 7, 9, 12 MHz) from which the three-dimensional temperature distribution was calculated using the bio-heat transfer equation in homogeneous, perfused media (attenuation=absorption=0.3 dB/cm-MHz; perfusion length: 1.0 cm). For the 184 cases, the acoustic field was normalized to the derated spatial peak, temporal average intensity (I/sub SPTA.3/) of 720 mW/cm/sup 2/, the maximum value allowed by the FDA 510(k) diagnostic ultrasound equipment approval process, from which the axial temperature increase profile and the maximum temperature increase (/spl Delta/T/sub max/) were determined. Also, from the normalized acoustic field, the soft-tissue thermal index (TIS) was determined according to the procedures of the Output Display Standard from the normalized acoustic field. In general, TIS and /spl Delta/T/sub max/ increase with increasing transmit f-number and TIS tracks /spl Delta/T/sub max/ which allows for As Low As Reasonably Achievable (ALARA) Principle implementation. TIS mostly underestimates /spl Delta/T/sub max/ (TIS/spl les//spl Delta/T/sub max/) for f-numbers/spl les/1 and mostly overestimates /spl Delta/T/sub max/ (TIS/spl ges//spl Delta/T/sub max/) for f-numbers/spl ges/2. For the lower transmit f-numbers /spl les/2, /spl Delta/T/sub max/ does not exceed a value of 0.30/spl deg/C (0.0060/spl les//spl Delta/T/sub max//spl les/0.30/spl deg/C for these 115 cases) and the maximum value of TIS does not exceed a value of 0.40 (0.00010/spl les/TIS/spl les/ 0.40 for these 115 cases). Therefore, for f-numbers /spl les/2, the TIS display would never be available to the system operator since the TIS value does not equal or exceed 1.0. For the higher transmit f-numbers/spl ges/3, TIS is generally within a factor of 2 of /spl Delta/T/sub max/ for predicting /spl Delta/T/sub max/. These results generally confirm the applicability of the TIS estimation procedure but question the FDA I/sub S/PTA.3 intensity limit of 720 mW/cm/sup 2/.