915MHz Implanted Internally Cooled Microwave Ablation

Purposes: (1) To explore the ablation shapes and lesions of 915MHz implanted internally cooled microwave in vitro and vivo porcine livers, and the corresponding pathological changes of ablation lesions in vivo. (2) To obtain computer-simulated three-dimensional thermal fields of 915MHz implanted internally cooled microwave with the experiments in equivalent microwave phantom in three microwave outputs, then detect and correct these thermal fields based on the ablation specimens and temperature curves in vitro porcine livers. Materials and Methods: (1) 915MHz rigid implanted internally cooled microwave antennae were directly inserted into liver tissues during ablation experiments, and temperature curves of selected spots were simultaneously recorded by iron-constantan couple temperature-measuring system. 70 ablation experiments with 237 temperature curves and 9 ones with 25 curves were performed in vitro and vivo porcine livers, respectively. Continuous and pulsed wave irradiation patterns were used. (2) Based on Pennes' formulation of bio-heat transfer, 48 pre-designed spots whose temperatures measured by thermal couple were precisely arranged in microwave phantom. According to the slopes of initial temperature curves, the distributions of special absorption rate (SAR) of 915MHz microwave in phantom were solved in two quadrants (z > 0 and z < 0). In quadrant of z > 0, SAR showed an exponential decay in r axis, and a three-order polynomial in z direction. Two-dimensional fitting method was applied in quadrant of z < 0. Three-dimensional thermal fields were simulated by computer calculated by finite element method (FEM) in three diflFerent energy outputs and repeatedly detected and corrected combined with temperature curves actually measured in phantom experiments. (3) Thermal physical parameters of in vitro porcine liver were applied to the thermal fields obtained in phantom experiments and contrasted and corrected repeatedly with 73 temperature curves actually measured.Results: (1) The shapes of 915MHz microwave ablation lesions were all elliptical and radial symmetry about the antenna, but the distribution of z axis was asymmetrical. The temperatures of spots in direction of z < 0 were higher than the ones of corresponding spots in direction of z > 0. The coagulation areas of 54 °C were enlarged by increasing power in general. Gross manifestations of ablation lesions in vivo livers were corresponding with the pathological exhibition. (2) The distributions of SAR in three energy outputs were obtained through phantom experiments, and had no linear relationship with the powers. (3) Three-dimensional thermal fields simulated by computer in vitro porcine liver ablations were acquired. The shapes of these fields were coincided with the coagulation areas of 54 °C in vitro liver specimens. It was 80.8% of the 73 spots that the differences of simulated temperature curves and measured ones were between positive and negative 6 Celsius degrees.Conclusions: Computer-simulated thermal fields of 915MHz microwave in phantoms and in vitro liver were accurate and reliable. The elliptical ablation lesions can be enlarged by 915MHz implanted internally cooled microwave in vitro and vivo porcine livers, therefore, this technique will demonstrate remarkably applicable advantages and potential in clinical ablation treatments of larger liver tumors.