Book Description

To improve treatment planning of thermal therapies, a better understanding of the effects of blood flow on temperature distributions is required. This thesis describes the experiments performed to measure temperature profiles of perfused heated tissues. The results were compared to predictions of mathematical models of tissue heat transfer which were then used to estimate lesion dimensions created during high intensity focused ultrasound (HIFU) therapy. An experimental system was built to provide accurate and reproducible high spatial resolution steady state and transient temperature profiles of heated tissues. By analyzing the temporal evolution of the temperature distribution of a fixed heated kidney as a function of flow we were able to demonstrate that predictions of the Bioheat Transfer Equation model (BHTE) were in good agreement with the experimental data but predictions of the scalar Effective Thermal Conductivity Equation model (ETCE) could not model the data. Using a similar experimental system, we investigated steady state and transient temperature distributions of heated tissues near large vessels and how they change as a function of flow. Temperature gradients of 6°C/mm were measured close to large vessels which could produce either excess heating or cooling in the target region. We demonstrated that the temperature gradients caused by large vessels were dependent on whether the heating source was highly localized or more distributed and that convective heat transfer by large vessels can heat regions distal to the treatment area. Optimal tissue heating strategies were proposed based on these results. It has been suggested that short duration high temperature treatments may overcome the effects of blood flow on the temperature distribution. A novel finite difference model was developed to examine how blood flow modifies tissue lesions created by HIFU. It was shown that the effects of tissue microvascular perfusion on lesion formation were significant for exposure times greater than 2 s for highly perfused tissues, but were not significant for up to 12 s in moderately perfused tissues such as muscle. Predictions of lesion size using the BHTE were in better agreement with the experimental lesion data than predictions of the ETCE.