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

Abstract

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.

Fig. 1

During RF ablation, the RF electrode is inserted into the target tissue, and ground pads are placed on the patient's thighs, equidistant from the RF electrode. If four pads are used, they are placed on top and bottom of each thigh (reproduced with permission from [28]).

Fig. 2

Current density distribution from a previous 2-D modeling study. In this collinear pad arrangement, current flows preferentially to the leading edge of the proximal pad, resulting in little heating elsewhere at the proximal pad or at the distal pad. In this example, each pad was 8.5 cm long and the pads were spaced 1.5 cm apart. (reproduced with permission from [28]).

Fig. 3

Ground pad model setup (not to scale). Voltage was applied along the right boundary of the model, 50 cm from the leading edge of the electrode/gap layer. A temperature boundary condition (37 °C) was applied to the left, right, and bottom edges of the model.

Fig. 4

Sequential activation algorithm: Example with three ground electrodes. During each part of the activation cycle, tissue heating occurs primarily at the closest activated ground electrode (in this case, proximal during t₁ , middle during t₂, and distal during t₃).

Fig. 5

Two sample electrode/gap configurations (not to scale). In the top case (a), the three ground electrodes are each 4 cm in length and the two gaps are each 1 cm (14 cm total). In the bottom case (b), the four ground electrodes are each 2 cm in length and the three gaps are each 2 cm (14 cm total). In all cases, the gel layer extended 1 cm in front of the proximal electrode.

Fig. 6

Flowchart of simulation procedure for a two-ground-electrode configuration. T_2pads and T_1pad represent the temperature at the control nodes for the front (both electrodes subset) and rear (distal electrode only subset) ground electrodes, respectively. After each simulation time step, the elapsed time is checked to determine if the simulation should continue. If so, the control node temperature differential is checked to determine which electrode subset should be activated during the subsequent time step.

Fig. 7

Computation of reported temperature values. The graph displays the temperature at the control (i.e., hottest) nodes during the last cycle for a sample three-electrode case (4-cm-long electrodes, 1-cm-long gaps). The time periods t₁, t₂, and t₃ correspond to the switching periods in Fig. 4. The dotted line (40.24 °C) represents the calculated average temperature for the distal electrode control node (blue), while the dashed line (40.27 °C) shows the calculated average temperature for the middle and proximal electrodes hottest nodes (red and green). The overall average maximum temperature rise reported for this simulation is the average of these three values, or 40.26 °C (Table II).

Fig. 8

Example of “short circuit” effect in a two-electrode case. When only the rear electrode is activated, some of the total current passes through the inactive front electrode (arrows) instead of the surrounding tissue on its way to the rear electrode. This leads to increased leading edge heating at the front electrode, since current is flowing into the electrode even when it is inactive. This phenomenon becomes more pronounced as the distance between the electrodes is decreased (Table III).

Fig. 9

Maximum temperature (averaged over the final cycle) versus gap length for all simulations. The dashed line at the top represents the maximum temperature achieved in the control case (one electrode). The color curves represent the 2nd-order polynomial approximations for each number of electrodes.

Fig. 10

Temperature distribution after 12 min for (a) the single-electrode case, (b) a two-electrode simulation (5.5 cm electrodes, 3 cm gap), and (c) a four-electrode simulation (2 cm electrodes, 2 cm gaps). The temperatures at the control nodes (black dots) are used to control the switching periods. Temperature rise occurs primarily at the leading edges of the electrodes (gray) underneath the gel layer (white). Sequential activation of the electrodes leads to a significant decrease in maximum temperature.