Numerical and experimental characterization of radiofrequency ablation in perfused kidneys.

We develop a three-dimensional finite element model in order to predict the resulting temperature distribution of a radiofrequency ablation (RFA) treatment in human kidneys. Here, a strong cooling effect results from a high degree of blood perfusion, which is modeled via two different approaches. The influence of big blood vessels for treatments close to renal hilus is modeled by including a cylindrical cooling tube based on the renal artery (or vein) in the kidney model. The influence of the perfusion of small arterioles and capillaries is represented by Pennes' approach in the bioheat equation. The experimental validation is performed by an in vivo RFA treatment on porcine kidney. Prior to the in vivo measurements several ex vivo experiments on fresh kidneys are carried out as a plausibility check for the model. During the treatments temperature profiles are measured using thermocouples which are radially arranged around the RFA applicator trocar. The evaluated data for each sensor show a deviation between 0.01 and 12 % from the simulation results. The approach serves for the design of a preplanning tool for RFA treatment in the future.

Radiofrequency ablation of renal tumors improved by preoperative ex-vivo computer simulation model.

BACKGROUND AND PURPOSE: Radiofrequency ablation (RFA) of small renal tumors has emerged as an option for patients with contraindications to surgery. The technique usually is limited to dorsolateral tumors because of the unknown pattern of heat distribution. The aim of this in-vitro experiment was to develop a planning computer simulation model of the heat distribution in order to calculate needle placement correctly. MATERIALS AND METHODS: Our measurement system enables capture of the temperature pattern of a linear arrangement of thermocouples in a porcine cadaver kidney. A high-frequency sinusoidal electric current (460 kHz) was introduced via a unipolar umbrella-shaped needle electrode. The results of the RFA thermocoagulation were compared with the computer model using the finite element method. RESULTS: Comparison between ex-vivo simulation and in-vivo measurement of destruction in three porcine cadaver kidneys showed excellent agreement. The discoloring of the necrotic region represented a temperature of 42 degrees C, where the tissue begins to denaturate. The 42 degrees C isodose temperature could be simulated precisely starting 6 mm from the center of the needle electrode. Using several MATLAB programs, simulation models were created to predict the space and time correlation of the necrotic regions. CONCLUSION: This experimental model allows successful planning of the expected necrotic region using RFA in a cadaver kidney. Future planning can be improved by using a perfused in-vivo kidney model, thus allowing much more precise computer simulation before applying RFA to kidney tumors at critical locations.