RF ablation at low frequencies for targeted tumor heating: in vitro and computational modeling results.

RF ablation uses RF current to heat and kill cancer applied via an electrode inserted under image guidance. Tumor has about half the electrical resistivity of normal tissue below 20 kHz, but similar resistivity above 500 kHz. We placed normal porcine liver tissue in contact with agar gel having similar resistivity as tumor within 20-450 kHz. A needle electrode was placed with half of the electrically active tip in each layer. We performed ablation with electric current applied for 12 min at 30 W, either at 20 or 450 kHz (n = 7 each), while measuring temperature via thermocouples 4 and 8 mm from the electrode. Mathematical heat-transfer models were created of an equivalent configuration and temperature profile determined at both frequencies. At 8-mm distance, at 450 kHz, tumor gel phantom and normal tissue obtained similar temperatures (57.5 ± 1.4 versus 58.7 ± 2.5 (°)C); at 20 kHz, tumor phantom obtained significantly higher temperatures than normal tissue (65.6 ± 2.0 versus 57.2 ± 5.6 (°)C, p < 0.01). Computer models confirm these results, and show the ablation zone diameter to be larger within the tumor phantom at 20 kHz compared to 450 kHz. Heating at low RFs may thus allow targeted heating of tumor tissue and reduced heating of normal tissue.

Sequential activation of ground pads reduces skin heating during radiofrequency tumor ablation: in vivo porcine results.

Skin burns below ground pads during monopolar RF ablation are increasingly prevalent, thereby hindering the development of higher power RF generators capable of creating larger tumor ablation zones in combination with multiple or new applicators. Our goal was to evaluate reduction in skin temperatures via additional ground pads in an in vivo porcine model. Three ground pads placed on the animal's abdomen were activated either simultaneously or sequentially, where activation timing was adjusted to equilibrate skin temperature below each pad. Thirteen RF ablations (n = 4 simultaneous at 300 W, n = 5 sequential at 300 W, and n = 4 sequential at 375 W) were performed for 12 min via two internally cooled cluster electrodes placed in the gluteus maximus of domestic swine. Temperature rise at each pad and burn degree as determined via histology were compared. Ablation zone size was determined via T2-weighted MRI. Maximum temperature rise was significantly higher with simultaneous activation than with either of the sequential activation group (21.4 degrees C versus 8.1 degrees C or 9.6 degrees C, p < 0.01). Ablation zone diameters during simultaneous (300 W) and sequential activations (300 and 375 W) were and 6.9 +/- 0.3, 5.6 +/- 0.3, and 7.5 +/- 0.6 cm, respectively. Sequential activation of multiple ground pads results in significantly lower skin temperatures and less severe burns, as measured by histological examination.

Sequential activation of ground pads reduces skin heating during radiofrequency ablation: initial in vivo porcine results.

PURPOSE: Radiofrequency (RF) ablation is a common treatment modality for inoperable liver cancer. Skin burns below ground pads during RF ablations are increasingly prevalent, hindering the development of higher-power RF generators capable of creating larger ablation zones. MATERIALS AND METHODS: 9 RF ablations (n=4 simultaneous, n=5 sequential) were performed with 300 W for 12 min via two internally cooled cluster electrodes placed in the gluteus maximus of domestic swine. Three ground pads placed on the animal's abdomen were activated either simultaneously, or sequentially where activation timing was adjusted to equilibrate skin temperature below each pad. Temperature rise at each pad was compared. Ablation zone dimensions were determined via MRI. RESULTS: Maximum temperature rise was significantly higher with simultaneous activation than with sequential activation (21.4 vs 8.1 degress C, p<0.01). Ablation zone diameters during simultaneous and sequential activation were 6.9+/-0.3 and 5.6+/-0.3, respectively. CONCLUSION: Sequential activation of multiple ground pads resulted in significantly lower skin temperatures during highpower RF ablation.

Electrical conductivity measurement of excised human metastatic liver tumours before and after thermal ablation.

We measured the ex vivo electrical conductivity of eight human metastatic liver tumours and six normal liver tissue samples from six patients using the four electrode method over the frequency range 10 Hz to 1 MHz. In addition, in a single patient we measured the electrical conductivity before and after the thermal ablation of normal and tumour tissue. The average conductivity of tumour tissue was significantly higher than normal tissue over the entire frequency range (from 4.11 versus 0.75 mS cm(-1) at 10 Hz, to 5.33 versus 2.88 mS cm(-1) at 1 MHz). We found no significant correlation between tumour size and measured electrical conductivity. While before ablation tumour tissue had considerably higher conductivity than normal tissue, the two had similar conductivity throughout the frequency range after ablation. Tumour tissue conductivity changed by +25% and -7% at 10 Hz and 1 MHz after ablation (0.23-0.29 at 10 Hz, and 0.43-0.40 at 1 MHz), while normal tissue conductivity increased by +270% and +10% at 10 Hz and 1 MHz (0.09-0.32 at 10 Hz and 0.37-0.41 at 1 MHz). These data can potentially be used to differentiate tumour from normal tissue diagnostically.

Effects of variation in perfusion rates and of perfusion models in computational models of radio frequency tumor ablation.

PURPOSE: Finite element method (FEM) models are commonly used to simulate radio frequency (RF) tumor ablation. Prior FEM models of RF ablation have either ignored the temperature dependent effect of microvascular perfusion, or implemented the effect using simplified algorithms to reduce computational complexity. In this FEM modeling study, the authors compared the effect of different microvascular perfusion algorithms on ablation zone dimensions with two commercial RF electrodes in hepatic tissue. They also examine the effect of tissue type and inter-patient variation of perfusion on ablation zone dimensions. METHODS AND MATERIALS: The authors created FEM models of an internally cooled and multi-tined expandable electrode. RF voltage was applied to both electrodes (for 12 or 15 min, respectively) such that the maximum temperature in the model was 105 degrees C. Temperature dependent microvascular perfusion was implemented using three previously reported methodologies: cessation above 60 degrees C, a standard first-order Arrhenius model with decreasing perfusion with increasing degree of vascular stasis, and an Arrhenius model that included the effects of increasing perfusion at the ablation zone boundary due to hyperemia. To examine the effects of interpatient variation, simulations were performed with base line and +/-1 standard deviation values of perfusion. The base line perfusion was also varied to simulate the difference between normal and cirrhotic liver tissue. RESULTS: The ablation zone volumes with the cessation above 60 degrees C perfusion algorithm and with the more complex Arrhenius model were up to 70% and 25% smaller, respectively, compared to the standard Arrhenius model. Ablation zone volumes were up to 175% and approximately 100% different between the simulations where -1 and +1 standard deviation values of perfusion were used in normal and cirrhotic liver tissue, respectively. CONCLUSIONS: The choice of microvascular perfusion algorithm has significant effects on final ablation zone dimensions in FEM models of RF ablation. The authors also found that both interpatient variation in base line tissue perfusion and the reduction in perfusion due to cirrhosis have considerable effect on ablation zone dimensions.

Sequential activation of a segmented ground pad reduces skin heating during radiofrequency tumor ablation: optimization via computational models.

Radiofrequency (RF) ablation has become an accepted treatment modality for unresectable tumors. The need for larger ablation zones has resulted in increased RF generator power. Skin burns due to ground pad heating are increasingly limiting further increases in generator power, and thus, ablation zone size. We investigated a method for reducing ground pad heating in which a commercial ground pad is segmented into multiple ground electrodes, with sequential activation of ground electrode subsets. We created finite-element method computer models of a commercial ground pad (14 x 23 cm) and compared normal operation of a standard pad to sequential activation of a segmented pad (two to five separate ground electrode segments). A constant current of 1 A was applied for 12 min in all simulations. Time periods during sequential activation simulations were adjusted to keep the leading edge temperatures at each ground electrode equal. The maximum temperature using standard activation of the commercial pad was 41.7 degrees C. For sequential activation of a segmented pad, the maximum temperature ranged from 39.3 degrees C (five segments) to 40.9 degrees C (two segments). Sequential activation of a segmented ground pad resulted in lower tissue temperatures. This method may reduce the incidence of ground pad burns and enable the use of higher power generators during RF tumor ablation.

Tumor ablation at low frequencies for preferential tumor heating: initial ex-vivo tissue studies.

PURPOSE: Radiofrequency (RF) ablation is a common treatment modality for inoperable liver cancer. Several studies have demonstrated that significant differences exist between the electrical properties of tumor and normal tissue, especially at lower frequencies. In this study, we investigated in an ex vivo setting whether the use of lower frequencies during ablation results in preferential heating of tumor tissue. MATERIALS AND METHODS: We created a setup consisting of adjacent layers (3 cm thick each) of tumor phantom (agar gel) and freshly excised normal porcine liver tissue in a saline bath. A standard RF needle electrode was placed such that half of the active electrode was in each layer. We applied 25 W of power at a frequency of either 20 or 325 kHz to the electrode for 12 minutes. Temperatures were recorded 6 and 10 mm from the electrode in both the phantom and normal tissue layers. RESULTS: The ratio of the temperature rise in the tumor phantom to the temperature rise in the normal tissue was significantly higher in the 20 kHz trials after 12 minutes at both 6 mm (1.50+/-0.27 vs. 1.02+/-0.16) and 10 mm (1.34+/-0.28 vs. 0.90+/-0.11) from the electrode (p<0.01). CONCLUSION: Tumor ablation at frequencies lower than currently used may preferentially heat tumor tissue, preserving normal tissue at the treatment site.

In vitro measurements of temperature-dependent specific heat of liver tissue.

We measured the specific heat of liver tissue in vitro by uniformly heating liver samples between two electrodes. We insulated the samples by expanded polystyrene, and corrected for heat loss and water loss. The specific heat of the liver is temperature-dependent, and increases by 17% at 83.5 degrees C (p < 0.05), compared to temperatures below 65 degrees C. The average specific heat was 3411 J kg(-1)K(-1) at 25 degrees C, and 4187 J kg(-1)K(-1) at 83.5 degrees C. Water loss from the samples was significant above 70 degrees C, with approximately 20% of reduction in sample mass at 90 degrees C.

Measurement of temperature-dependent specific heat of biological tissues.

We measured specific heat directly by heating a sample uniformly between two electrodes by an electric generator. We minimized heat loss by styrofoam insulation. We measured temperature from multiple thermocouples at temperatures from 25 degrees C to 80 degrees C while heating the sample, and corrected for heat loss. We confirm method accuracy with a 2.5% agar-0.4% saline physical model and obtain specific heat of 4121+/-89 J (kg K)(-1), with an average error of 3.1%.

Large-volume radiofrequency ablation of ex vivo bovine liver with multiple cooled cluster electrodes.

Three methods of creating large thermal lesions with cool-tip cluster electrodes were compared. Three cluster electrodes were arranged 4 cm apart in a triangular array. Eight lesions were created ex vivo in fresh bovine liver (from a butcher) with each method: sequential ablation (three electrodes, 12 minutes each); simultaneous activation of electrodes (12 minutes); and rapid switching of power between electrodes (12 minutes), for which an electronic computer-controlled switch was developed. For sequential, rapid switching, and simultaneous methods, lesion volumes were 137.5 cm(3)+/- 22.2, 116.4 cm(3)+/- 15.2, and 22.3 cm(3)+/- 6.4 (P < .05), respectively, and final temperatures at lesion center were 80 degrees C +/- 5, 97 degrees C +/- 8, and 41 degrees C +/- 3 (P < .001), respectively. Because of electrical interference between electrodes, simultaneous method led to little heating at the center between the electrodes and created small discontinuous lesions. Rapid switching created large round lesions by employing multiple electrodes concurrently, which substantially reduced treatment time and resulted in more effective heating between electrodes.