Knowledge of electrical tissue conductivity is necessary to determine deposition of electromagnetic energy and can further be used to diagnostically differentiate between normal and neoplastic tissue. We measured 17 rats with a total of 24 tumours of the K12/TRb rat colon cancer cell line. In each animal we measured in vivo hepatic tumour and normal tissue conductivity at seven frequencies from 10 Hz to 1 MHz, at different tumour stages between 6 and 12 weeks after induction. Conductivity of normal liver tissue was 1.26 +/- 0.15 mS cm(-1) at 10 Hz, and 4.61 +/- 0.42 mS cm(-1) at 1 MHz. Conductivity of tumour was 2.69 +/- 0.91 mS cm(-1) at 10 Hz, and 5.23 +/- 0.82 mS cm(-1) at 1 MHz. Conductivity was significantly different between normal and tumour tissue (p < 0.05). We determined the percentage of necrosis and fibrosis at the measurement site. We fitted the conductivity data to the Cole-Cole model. For the tumour data we determined Spearman's correlation coefficients between the Cole-Cole parameters and age, necrosis, fibrosis and tumour volume and found significant correlation between necrosis and the Cole-Cole parameters (p < 0.05). We conclude that necrosis within the tumour and the associated membrane breakdown is likely responsible for the observed change in conductivity.

Abstract-Radio-frequency (RF) hepatic ablation, offers an alternative method for the treatment of hepatic malignancies. We employed finite-element method (FEM) analysis to determine tissue temperature distribution during RF hepatic ablation. We constructed three-dimensional (3-D) thermal-electrical FEM models consisting of a four-tine RF probe, hepatic tissue, and a large blood vessel (10-mm diameter) located at different locations. We simulated our FEM analyses under temperature-controlled (90 C) 8-min ablation. We also present a preliminary result from a simplified two-dimensional (2-D) FEM model that includes a bifurcated blood vessel. Lesion shapes created by the four-tine RF probe were mushroom-like, and were limited by the blood vessel. When the distance of the blood vessel was 5 mm from the nearest distal electrode 1) in the 3-D model, the maximum tissue temperature (hot spot) appeared next to electrods A. The location of the hot spot was adjacent to another electrode 2) on the opposite side when the blood vessel was 1 mm from electrode A. The temperature distribution in the 2-D model was highly nonuniform due to the presence of the bifurcated blood vessel. Underdosed areas might be present next to the blood vessel from which the tumor can regenerate.

Finite element (FE) analysis has been utilised as a numerical tool to determine the temperature distribution in studies of radio frequency (RF) cardiac ablation. However, none of the previous FE analyses clarified such computational aspects as software requirements, computation time or convergence test. In addition, myocardial properties included in the previous models vary greatly. A process of FE modelling of a system that included blood, myocardium, and an ablation catheter with a thermistor embedded at the tip is described. The bio-heat equation is solved to determine the temperature distribution in myocardium using a commercial software application (ABAQUS). A Cauchy convergence test (epsilon = 0.1 degree C) was performed and it is concluded that the optimal number of elements for the proposed system is 24610. The effects of changes in myocardial properties (+/- 50% electric conductivity, +100%/-50% thermal conductivity, and +100%/-50% specific heat capacity) in both power-controlled (PCRFA) and temperature-controlled RF ablation (TCRFA) were studied. Changes in myocardial properties affect the results of the FE analyses of PCRFA more than those of TCRFA, and the maximum changes in lesion volumes were -58.6% (-50% electric conductivity), -60.7% (+100% thermal conductivity), and +43.2% (-50% specific heat).

The resistivity of swine liver tissue was measured in vivo, during induced ischaemia and post-mortem, so that associated changes in resistivity could be quantified. Plunge electrodes, the four-terminal method and a computer-automated measurement system were used to acquire resistivities between 10Hz and 1 MHz. Liver resistivity was measured in vivo in three animals at 11 locations. At 10 Hz, resistivity was 758 +/- 170 ohm x cm. At 1 MHz, the resistivity was 250 +/- 40 ohm x cm. The resistivity time course was measured during the first 10 min after the liver blood supply in one animal had been occluded. Resistivity increased steadily during occlusion. The change in resistivity of an excised tissue sample was measured during the first 12h after excision in one animal. Resistivity increased during the first 2h by 53% at 10 Hz and by 32% at 1 MHz. After 2h, resistivity decreased, probably owing to membrane breakdown. The resistivity data were fitted to a Cole-Cole circle, from which extracellular resistance Re, intracellular resistance Ri and cell membrane capacitance Cm were estimated. Re increased during the first 2h by 95% and then decreased, suggesting an increase in extracellular volume. Cm increased during the first 4 h by 40%, possibly owing to closure of membrane channels, and then decreased, suggesting membrane breakdown. Ri stayed constant during the initial 6h and then increased.

Radio-frequency (RF) ablation has become an important means of treatment of nonresectable primary and metastatic liver tumors. Major limitations are small lesion size, which make multiple applications necessary, and incomplete killing of tumor cells, resulting in high recurrence rates. We examined a new bipolar RF ablation method incorporating two probes with hooked electrodes (RITA model 30). We performed monopolar and bipolar in vivo experiments on three pigs. The electrodes were 2.5 cm apart and rotated 45 degrees relative to each other. We used temperature-controlled mode at 95 degrees C. Lesion volumes were 3.9+/-1.8 cm3 (n=7) for the monopolar case and 12.2 +/- 3 cm3 (n=10) for the bipolar case. We generated finite-element models (FEMs) of monopolar and bipolar configurations. We analyzed the distribution of temperature and electric field of the finite element model. The lesion volumes for the FEM are 7.95 cm3 for the monopolar and 18.79 cm3 for the bipolar case. The new bipolar method creates larger lesions and is less dependent on local inhomogenities in liver tissue-such as blood perfusion-compared with monopolar RF ablation. A limitation of the new method is that the power dissipation of the two probes cannot be controlled independently in response to different conditions in the vicinity of each probe. This may result in nonuniform lesions and decreased lesion size.