Preußer, Tobias
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Preußer, Tobias
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Preußer, Tobias
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Item type:Publication, Identification of Material Parameters from Temperature Measurements in Radio Frequency Ablation(2011-11-01); ; ; The mathematical simulation of the method of radio frequency ablation (RFA) offers an opportunity to improve the success of the RFA. The results of the RFA depend highly on the experience of the radiologist. A simulation will offer a prediction of the results which can be used to adapt the setting and enable a complete destruction of the tumor, e.g. by adapting the probe's position. A good simulation needs as much information of the reality as possible. Especially the material properties pose a challenge since they vary from patient to patient, they can not be measured in vivo and they additionally change during the ablation. The aim of this thesis is to develop a mathematical model for the identification of the material parameters from temperature measurements and apply it to appropriate data sets. At first a minimization problem is formulated, where the difference between the measured temperature and the calculated temperature is minimized with respect to the material parameters. The temperature distribution is calculated with a coupled system of partial differential equations. Different approaches are considered which depend on the diverse modeling of the material parameters. The parameters are modeled as constant values as well as temperature dependent, tissue dependent and also spatially distributed. The advantages and disadvantages of the diverse models are illustrated by the numerical results for the identification with artificial temperature distributions as well as real temperature measurements.doctoral thesis551 159 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Optimization of the Probe Placement for Radiofrequency Ablation(2009-04-27); ; ; Radiofrequency (RF) ablation is a widely used, minimally invasive technique for the treatment of liver cancer. Within this method, an RF current is used to heat the tumor tissue up to high temperatures which are lethal to the tissue. The RF current is generated by a high frequency generator and induced into the tissue via so-called needle- or umbrella probes, which contain one or more electrodes. Especially, in situations where a surgical resection is not possible due to the patient's physical condition and state of the tumors, the RF ablation technique offers a powerful but less invasive alternative. However, the success of an RF ablation, i. e. the completeness of tumor destruction with minimum amount of effected native tissue, considerably depends on the accuracy of needle insertion and control of the energy supply, as well as on the cooling effects of blood perfusion. The aim of this work is to develop a three-dimensional model for the optimization of the RF probe placement. The model is based on a numerical finite element computation of the electric potential and heat distribution inside the malignant and surrounding native tissue during an RF ablation. The optimization is performed by minimizing a temperature based objective functional under these constraining equations. Moreover, since the tissue properties of the individual patient cannot be determined exactly in advance, also a model based on stochastically distributed tissue parameters is developed, in order to investigate the sensitivity of an optimal probe placement found by the presented algorithm, with respect to changes in these quantities. A further well-known difficulty associated with RF ablation is the cooling influence of blood perfusion on the ablation result. For this reason, a method to quickly estimate the cooling effect of large blood vessels based on a precalculation of all patient-independent quantities, is introduced. Finally, a first approach towards an optimal control of the electric energy which is induced into the tissue via the RF generator, is presented and discussed. The results show that the simulation and optimization of an RF ablation is of essential importance to yield the best possible outcome and thus present a helpful tool for assisting the interventional radiologist.doctoral thesis299 127