The effects of residual temperature rise on ultrasound heating

A generic bioheat transfer thermal model for a perfused tissue

A thermal model was needed to predict temperatures in a perfused tissue, which satisfied the following three criteria. One, the model satisfied conservation of energy. Two, the heat transfer rate from blood vessels to tissue was modeled without following a vessel path. Three, the model applied to any unheated and heated tissue. To meet these criteria, a generic bioheat transfer model (BHTM) was derived here by conserving thermal energy in a heated vascularized finite tissue and by making a few simplifying assumptions. Two linear coupled differential equations were obtained with the following two variables: tissue volume averaged temperature and blood volume averaged temperature. The generic model was compared with the widely employed empirical Pennes' BHTM. The comparison showed that the Pennes' perfusion term wC(p)(1-epsilon) should be interpreted as a local vasculature dependent heat transfer coefficient term. Suggestions are presented for further adaptations of the general BHTM for specific tissues using imaging techniques and numerical simulations.

Evaluation of biological effects induced by diagnostic ultrasound in the rat foetal tissues

In recent years, there has been growing interest in estimating the degree of heating caused by the diagnostic ultrasound in clinical practice. Both theoretical and experimental methods have been suggested for estimating the heating potential, or thermal hazard, of diagnostic ultrasound. Aim of this study was to evaluate in vivo effects of ultrasound exposure of variable duration (from 10 up to 20 min) with commercially available imaging systems commonly used for diagnostic imaging. Numerical results related to the thermal effect are obtained by simulation program based on B-mode (scanning) and Doppler (non-scanning). To investigate the biological effects of the ultrasound exposure to the brain and liver tissues, the antioxidant enzyme activity and thiobarbituric acid reactive substances (TBARS) of the tissues were evaluated. In liver tissue, as a lipid peroxidation index, TBARS levels very significantly increase in Doppler group compared to control. However, in B-mode, TBARS levels are the same with the control group. Use of B-mode in foetal tissue is more reliable than Doppler mode because temperature rise is very small compared to the Doppler mode. On the other hand, the antioxidant enzyme activities tend to increase in B-mode and Doppler groups compared to the control group as a defensive mechanism. In the brain tissue, lipid peroxidation is increased slightly in B-mode compared to the control group. This situation is related to the molecular structure of the brain tissue because of its high lipid concentration. In brain tissue, the antioxidant enzyme activities and lipid peroxidation were significantly increased, such as liver tissue in Doppler groups. Doppler ultrasound may produce harmful effects in rat foetus liver and brain tissues as a result of the high temperature rises.

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.