On modelling and simulation of perfused tissue during thermal treatment : thermal analysis of blood perfusion and lesioning during ablative neurosurgery

Abstract: Thermal treatment means therapy for which heat is the therapeutic agent. During treatment, the tissue temperature is increased to an injurious temperature (higher than 43 °C for the most widely used treatments). The aim is to reach necrotic temperature in the treatment area and non-lethal temperature outside the same. However, for several reasons this is not possible.The thesis deals with thermal treatment from a thermal point of view. The principle aim of the analysis is to obtain the entire temperature field in the treatment area, preferably before the treatment even takes place (temperature prediction). However, it is a complex task to obtain the entire temperature fields which usually varies substantially with both time and space. Parameters of importance are for instance the power supply needed for a certain treatment, the blood perfusion and the heat flux within the treatment area.The thesis comprises two different projects; the first project regards modelling and simulation of heat transfer in blood-perfused tissue. The second project concerns modelling and simulation of lesion growth and associated thermal problems during ablative neurosurgery. Throughout the thesis the focus is on modelling, but also experiments are carried out in order to enhance the thermal analysis in the second project.In the first project, an important aim is to increase the understanding about the equations (bio-heat equations, BHE's) used for modelling the effect from blood perfusion. A survey as well as a discussion of the equations used the last few decades is carried out. The core of the project is to the BHE's that are variants of the heat conduction equation, and therefore easily implemented in standard thermal simulation packages. An alternative model is proposed in the thesis as a more accurate and flexible tool compared with the most widely used models, the BHE of Pennes and the keff equation.In the second project, the focus is on the lesion growth together with the very important temperature measurement, which is used to monitor and control the lesioning process. A simulation model is developed by using input from the in vitro experiments. The model is considered to accurately describe the lesioning process, and very good agreement between experiments and simulations is obtained.Furthermore, simulations are used to analyse and evaluate the intra-electrode temperature measurement. The maximum temperature was always located outside the electrode, and therefore, there is always a difference in both time and level between the measured temperature and maximum temperature in the treatment area. The difference is important to quantify, since the lesioning process is directly dependent on the temperature measurement.