Evaluation of the unscanned soft-tissue thermal index

The monopole-source solution was used to calculate the three-dimensional co mplex acoustic pressure field for focused circular apertures in a medium ha ving homogeneous acoustic and thermal properties. The three source diameter s were 1, 2, and 4 cm and the eight transmit f-numbers (ratio of the radius of curvature/source diameter) were 0.7, 1.0, 1.3, 1.6, 2.0, 3.0, 4.0, and 5.0. For these focused field geometries, eight ultrasonic frequencies were evaluated (1, 2, 3, 4, 5, 7, 9, and 12 MHz) from which the three-dimensiona l temperature distribution was calculated using the bio-heat transfer equat ion in homogeneous, perfused media (attenuation = absorption = 0.3 dB/cm-MH z; perfusion length: 1.0 cm). For each of the 192 cases, the acoustic field was normalized to the derated spatial peak, temporal average intensity (I- SPTA.3) of 720 mW/cm(2), the maximum value allowed by the FDA 510(k) diagno stic ultrasound equipment approval process. Using the normalized acoustic f ield, the axial temperature increase profiles and the maximum temperature i ncreases (Delta T-max) were determined for each case. Also, from the normal ized acoustic field, the unscanned soft-tissue thermal index (TIS) was dete rmined. In general, Delta T-max, TIS, and source power increase with increa sing transmit f-number, source diameter, or frequency. The TIS generally un derestimates (is less than) Delta T-max for f-numbers less than or equal to 2, conditions for which Delta T-max less than or equal to 0.30 degrees C a nd TIS less than or equal to 0.40. This suggests that, for transmit f-numbe rs I 2, TIS would not need to be displayed according to the ODS display req uirements. With the exception of the longer-focus, larger-diameter, higher- frequency sources, TIS generally tracks Delta T-max for f-numbers greater t han or equal to 3. These exceptions suggest a breakdown of the ODS procedur es for calculating TIS.