Microwave ablation energy delivery: influence of power pulsing on ablation results in an ex vivo and in vivo liver model

Abstract

Purpose: The purpose of this study was to compare the impact of continuous and pulsed energy deliveries on microwave ablation growth and shape in unperfused and perfused liver models.

Methods: A total of 15 kJ at 2.45 GHz was applied to ex vivo bovine liver using one of five delivery methods (n = 50 total, 10 per group): 25 W continuous for 10 min (25 W average), 50 W continuous for 5 min (50 W average), 100 W continuous for 2.5 min (100 W average), 100 W pulsed for 10 min (25 W average), and 100 W pulsed for 5 min (50 W average). A total of 30 kJ was applied to in vivo porcine livers (n = 35, 7 per group) using delivery methods similar to the ex vivo study, but with twice the total ablation time to offset heat loss to blood perfusion. Temperatures were monitored 5-20 mm from the ablation antenna, with values over 60 °C indicating acute cellular necrosis. Comparisons of ablation size and shape were made between experimental groups based on total energy delivery, average power applied, and peak power using ANOVA with post-hoc pairwise tests.

Results: No significant differences were noted in ablation sizes or circularities between pulsed and continuous groups in ex vivo tissue. Temperature data demonstrated more rapid heating in pulsed ablations, suggesting that pulsing may overcome blood perfusion and coagulate tissues more rapidly in vivo. Differences in ablation size and shape were noted in vivo despite equivalent energy delivery among all groups. Overall, the largest ablation volume in vivo was produced with 100 W continuous for 5 min (265.7 ± 208.1 cm(3)). At 25 W average, pulsed-power ablation volumes were larger than continuous-power ablations (67.4 ± 34.5 cm(3) versus 23.6 ± 26.5 cm(3), P = 0.43). Similarly, pulsed ablations produced significantly greater length (P ≤ 0.01), with increase in diameter (P = 0.09) and a slight decrease in circularity (P = 0.97). When comparing 50 W average power groups, moderate differences in size were noted (P ≥ 0.06) and pulsed ablations were again slightly more circular.

Conclusions: Pulsed energy delivery created larger ablation zones at low average power compared to continuous energy delivery in the presence of blood perfusion. Shorter duty cycles appear to provide greater benefit when pulsing.

FIG. 1.

Experimental power delivery protocols, each delivering a total of 15 kJ: 25 W continuous for 10 min (solid black), 100 W pulsed 30 s on and 90 s off for 10 min (dashed black), 50 W for 5 min (solid red or dark gray), 100 W pulsed 50 s on and 50 s off (dotted red or dark gray) and 100 W continuous for 2.5 min (solid blue or light gray).

FIG. 2.

Ex vivo experimental setup. Fiber-optic temperature sensors were placed 5–20 mm from the antenna. Ablation zone length and diameter were measured along and transverse to the antenna insertion tract, respectively.

FIG. 3.

Temperature profile over time at two different distances: (a) 5 mm from the antenna shaft: pulsed delivery of higher peak power created faster heating. While power was on, pulsed ablations had greater heating rates at all points than the continuous ablation. (b) 20 mm from the antenna shaft: the heat rate decreases with distance. The heating profile was similar for both pulsed and continuous groups.

FIG. 4.

Expected time required to reach a temperature increase and achieve necrosis during ablation with respect to distance from the antenna shaft. Distances less than 15 mm, pulsed power reached the lethal threshold temperature faster than continuous power with an average power of 25 W. As average power increases these differences diminished.

FIG. 5.

Cross-sectional area of the ablation zones from each experimental group in vivo. Different stages of tissue desiccation were observed within the ablation zone: little desiccation (orange or dark gray arrow), more complete desiccation or tissue necrosis (black arrow), and charring (white arrow).