title: Microwave ablation energy delivery: influence of power pulsing on ablation results in an ex vivo and in vivo liver model.
abstract: OBJECTIVE
  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.
authors.affiliation: Department of Medical Physics, University of Wisconsin-Madison, Madison, Wisconsin 53705 and Department of Radiology, University of Wisconsin-Madison, Madison, Wisconsin 53705.
authors.forename: Mariajose
authors.initials: M
authors.lastname: Bedoya
authors.affiliation: Department of Medical Physics, University of Wisconsin-Madison, Madison, Wisconsin 53705 and Department of Radiology, University of Wisconsin-Madison, Madison, Wisconsin 53705.
authors.forename: Alejandro Muñoz
authors.initials: AM
authors.lastname: del Rio
authors.affiliation: Department of Biomedical Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53705 and Department of Radiology, University of Wisconsin-Madison, Madison, Wisconsin 53705.
authors.forename: Jason
authors.initials: J
authors.lastname: Chiang
authors.affiliation: Department of Medical Physics, University of Wisconsin-Madison, Madison, Wisconsin 53705; Department of Biomedical Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53705; and Department of Radiology, University of Wisconsin-Madison, Madison, Wisconsin 53705.
authors.forename: Christopher L
authors.initials: CL
authors.lastname: Brace
mesh_terms: D000818:Animals; D055592:Biophysical Phenomena; D002417:Cattle; D005260:Female; D006979:Hyperthermia, Induced / Q000379:methods; D066298:In Vitro Techniques; D008099:Liver / Q000473:pathology; D008113:Liver Neoplasms / Q000473:pathology / Q000628:therapy*; D008872:Microwaves / Q000627:therapeutic use*; D023421:Models, Animal; D034421:Sus scrofa