Optimization of the Probe Placement for Radiofrequency Ablation

Abstract

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.