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Microwave Ablation: Results with a 2.45-GHz Applicator in ex Vivo Bovine and in Vivo Porcine Liver¹

¹ From the Department of Radiology, Beth Israel Deaconess Medical Center, Harvard Medical School, 1 Deaconess Rd, WCC 308B, Boston, MA 02215 (A.U.H., N.P., Z.L., S.N.G.); Department of Physics, University of Bath, Bath, England (P.C., N.C.); and Microsulis Americas, Waltham, Mass (P.C., N.C., T.P.R.). From the 2004 RSNA Annual Meeting. Received February 15, 2005; revision requested April 12; revision received June 2; accepted June 21; final version accepted September 1. Supported by Microsulis. Address correspondence to S.N.G. (e-mail: sgolber{at}caregroup.harvard.edu).

Purpose: To characterize the relationship between applied power and treatment duration in their effect on extent of coagulation produced with a 2.45-GHz microwave applicator in both an ex vivo and a perfused in vivo liver model.

Materials and Methods: All experimentation was approved by the Institute of Animal Care and Use Committee. Multiple tissue ablations were performed in ex vivo bovine liver (120 ablations) and in vivo porcine liver (45 ablations) with a 5.7-mm-diameter 2.45-GHz microwave applicator. The applied power was varied from 50 to 150 W (in 25-W increments ex vivo and 50-W increments in vivo), while treatment duration varied from 2 to 20 minutes (in eight time increments for ex vivo and five for in vivo liver). Three-dimensional contour maps of the resultant short- and long-axis coagulation diameters were constructed to identify the optimal parameters to achieve maximum coagulation in both ex vivo and in vivo models. Multivariate analysis was performed to characterize the relationship between applied power and treatment duration.

Results: Power and treatment duration were both associated with coagulation diameter in a sigmoidal fashion (ex vivo, R² = 0.78; in vivo, R² = 0.74). For ex vivo liver, the maximum short-axis coagulation diameter (7.6 cm ± 0.2 [standard deviation] by 12.3 cm ± 0.8) was achieved at greatest power (150 W) and duration (20 minutes). In vivo studies revealed a sigmoidal relationship between duration and coagulation size, with a relative plateau in coagulation size achieved within 8 minutes duration at all power levels. After 8 minutes of treatment at 150 W, the mean short-axis coagulation diameter for in vivo liver was 5.7 cm ± 0.2 by 6.5 cm ± 1.7, which was significantly larger than the corresponding result for ex vivo liver (P < .05).

Conclusion: Large zones of ablation can be achieved with the 2.45-GHz microwave applicator used by the authors. For higher-power ablations, larger zones of coagulation were achieved for in vivo liver than for ex vivo liver with short energy applications, a finding previously not seen with other ablation devices, to the authors' knowledge.

© RSNA, 2006