Evaluation of nonscanned mode soft-tissue thermal index in the presence of the residual temperature rise.

Previously, the temperature rise (deltaT) caused by diagnostic ultrasound and the AUIM/NEMA-defined thermal indices were examined to evaluate whether these indices were reasonable indicators of potential bioeffects due to ultrasound heating in the absence of a residual temperature rise (RTR). In our study, deltaT induced by diagnostic ultrasound exposures was estimated in the presence of an RTR using the Bioheat Transfer Equation. To evaluate deltaT/TIS in the presence of an RTR, 11 frequencies, eight cooling times, eight insonation times for the second ultrasound examination, and three source powers for a circular aperture (A(aprt)< or = 1 cm2) were investigated. In our comparison of the ratios of deltaT/TIS in the absence and presence of an RTR, a higher deltaT/TIS value was obtained in the examination with the RTR. We showed that the deltaT/TIS value is equal to 2.88 in the presence of an RTR, whereas the deltaT/TIS value without the RTR equals 1.90. In the presence of the RTR, although the TIS does not inform the user of higher ultrasound heating due to TIS values that do not exceed 1.00, deltaT reaches 2.62 degrees C, and the deltaT without the RTR reaches 1.68 degrees C in the case of a TIS value that does not exceed 1.00. These results suggest that, for nonscanned mode situations where soft tissue is insonated, the TIS should not be regarded as a reliable indicator of potential bioeffects due to ultrasound heating in the presence of the RTR. Our study also indicates the necessity for a new indicator that provides the clinical user with accurate in vivo temperature rise feedback (possibly even true deltaT), and includes adding an exposure time component to the Bio-Heat Equation model.