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Multipolar Radiofrequency Ablation with Internally Cooled Electrodes: Experimental Study in ex Vivo Bovine Liver with Mathematic Modeling¹

¹ From the Department of Diagnostic Radiology, Eberhard-Karls-University, Hoppe-Seyler-Strasse 3, 72076 Tübingen, Germany (S.C., D.S., A.B., C.D.C., P.L.P.); and Department of Medical Biometry (K.D.) and Institute of Pathology (S.M.K.), University of Tübingen, Tübingen, Germany. From the 2004 RSNA Annual Meeting. Received April 6, 2005; revision requested June 3; revision received June 30; final version accepted July 20. Address correspondence to S.C. (e-mail: stephan.clasen{at}med.uni-tuebingen.de).

View larger version (16K): [in a new window]   Figure 1a: Schematic drawings demonstrate orientation of electric field for (a) single bipolar applicator and (b–f) the simultaneous use of a triangular array of bipolar applicators activated in multipolar mode. By using a single bipolar applicator with both electrodes located on the same applicator shaft, electric current is directed parallel to the applicator (a). If three bipolar applicators are used, they are activated in multipolar mode. In this mode, every possible pair of electrodes, which are not necessarily located on the same shaft, is activated one after the other for a short period of time. The electric current may pass between electrodes on the same or on different applicator shafts. Drawings show possible combinations between (b) pairs of electrodes located on the same shaft, (c) applicators 1 and 2, (d) applicators 1 and 3, and (e) applicators 2 and 3. (f) Drawing demonstrates that, altogether, activation can be switched between 15 different combinations in multipolar mode.

View larger version (44K): [in a new window]   Figure 1b: Schematic drawings demonstrate orientation of electric field for (a) single bipolar applicator and (b–f) the simultaneous use of a triangular array of bipolar applicators activated in multipolar mode. By using a single bipolar applicator with both electrodes located on the same applicator shaft, electric current is directed parallel to the applicator (a). If three bipolar applicators are used, they are activated in multipolar mode. In this mode, every possible pair of electrodes, which are not necessarily located on the same shaft, is activated one after the other for a short period of time. The electric current may pass between electrodes on the same or on different applicator shafts. Drawings show possible combinations between (b) pairs of electrodes located on the same shaft, (c) applicators 1 and 2, (d) applicators 1 and 3, and (e) applicators 2 and 3. (f) Drawing demonstrates that, altogether, activation can be switched between 15 different combinations in multipolar mode.

View larger version (38K): [in a new window]   Figure 1c: Schematic drawings demonstrate orientation of electric field for (a) single bipolar applicator and (b–f) the simultaneous use of a triangular array of bipolar applicators activated in multipolar mode. By using a single bipolar applicator with both electrodes located on the same applicator shaft, electric current is directed parallel to the applicator (a). If three bipolar applicators are used, they are activated in multipolar mode. In this mode, every possible pair of electrodes, which are not necessarily located on the same shaft, is activated one after the other for a short period of time. The electric current may pass between electrodes on the same or on different applicator shafts. Drawings show possible combinations between (b) pairs of electrodes located on the same shaft, (c) applicators 1 and 2, (d) applicators 1 and 3, and (e) applicators 2 and 3. (f) Drawing demonstrates that, altogether, activation can be switched between 15 different combinations in multipolar mode.

View larger version (37K): [in a new window]   Figure 1d: Schematic drawings demonstrate orientation of electric field for (a) single bipolar applicator and (b–f) the simultaneous use of a triangular array of bipolar applicators activated in multipolar mode. By using a single bipolar applicator with both electrodes located on the same applicator shaft, electric current is directed parallel to the applicator (a). If three bipolar applicators are used, they are activated in multipolar mode. In this mode, every possible pair of electrodes, which are not necessarily located on the same shaft, is activated one after the other for a short period of time. The electric current may pass between electrodes on the same or on different applicator shafts. Drawings show possible combinations between (b) pairs of electrodes located on the same shaft, (c) applicators 1 and 2, (d) applicators 1 and 3, and (e) applicators 2 and 3. (f) Drawing demonstrates that, altogether, activation can be switched between 15 different combinations in multipolar mode.

View larger version (36K): [in a new window]   Figure 1e: Schematic drawings demonstrate orientation of electric field for (a) single bipolar applicator and (b–f) the simultaneous use of a triangular array of bipolar applicators activated in multipolar mode. By using a single bipolar applicator with both electrodes located on the same applicator shaft, electric current is directed parallel to the applicator (a). If three bipolar applicators are used, they are activated in multipolar mode. In this mode, every possible pair of electrodes, which are not necessarily located on the same shaft, is activated one after the other for a short period of time. The electric current may pass between electrodes on the same or on different applicator shafts. Drawings show possible combinations between (b) pairs of electrodes located on the same shaft, (c) applicators 1 and 2, (d) applicators 1 and 3, and (e) applicators 2 and 3. (f) Drawing demonstrates that, altogether, activation can be switched between 15 different combinations in multipolar mode.

View larger version (55K): [in a new window]   Figure 1f: Schematic drawings demonstrate orientation of electric field for (a) single bipolar applicator and (b–f) the simultaneous use of a triangular array of bipolar applicators activated in multipolar mode. By using a single bipolar applicator with both electrodes located on the same applicator shaft, electric current is directed parallel to the applicator (a). If three bipolar applicators are used, they are activated in multipolar mode. In this mode, every possible pair of electrodes, which are not necessarily located on the same shaft, is activated one after the other for a short period of time. The electric current may pass between electrodes on the same or on different applicator shafts. Drawings show possible combinations between (b) pairs of electrodes located on the same shaft, (c) applicators 1 and 2, (d) applicators 1 and 3, and (e) applicators 2 and 3. (f) Drawing demonstrates that, altogether, activation can be switched between 15 different combinations in multipolar mode.

View larger version (27K): [in a new window]   Figure 2a: (a) Applicator tip with two electrodes separated by insulation. (b) Three bipolar applicators placed in a triangular array in ex vivo bovine liver.

View larger version (134K): [in a new window]   Figure 2b: (a) Applicator tip with two electrodes separated by insulation. (b) Three bipolar applicators placed in a triangular array in ex vivo bovine liver.

View larger version (142K): [in a new window]   Figure 3a: Two zones of coagulation after multipolar RF ablation. (a) Cross section of liver that was sliced in a plane containing one of three applicator tracks and the center of the triangular array. Diameters parallel (a) and perpendicular (b) to the applicator track were measured macroscopically. (b) Cross section perpendicular to applicator tracks shows confluent zone of coagulation. Line in b represents plane shown in a.

View larger version (140K): [in a new window]   Figure 3b: Two zones of coagulation after multipolar RF ablation. (a) Cross section of liver that was sliced in a plane containing one of three applicator tracks and the center of the triangular array. Diameters parallel (a) and perpendicular (b) to the applicator track were measured macroscopically. (b) Cross section perpendicular to applicator tracks shows confluent zone of coagulation. Line in b represents plane shown in a.

View larger version (27K): [in a new window]   Figure 4a: Graphs show the volume of coagulation and duration of multipolar RF ablation in relation to preselected power outputs and bipolar applicator distances. (a) Data points represent geometric mean of confluent volumes of coagulation at preselected power outputs and interapplicator distances. Line graph represents mathematic model for Equation (1). (b) Data points represent geometric mean of RF ablation time until an increase in tissue resistance occurred at preselected power outputs and interapplicator distances. Line graph represents mathematic model for Equation (2). Error bars indicate 95% confidence intervals. Coagulation volume and RF ablation time increased at lower power output levels and larger applicator distances.

View larger version (26K): [in a new window]   Figure 4b: Graphs show the volume of coagulation and duration of multipolar RF ablation in relation to preselected power outputs and bipolar applicator distances. (a) Data points represent geometric mean of confluent volumes of coagulation at preselected power outputs and interapplicator distances. Line graph represents mathematic model for Equation (1). (b) Data points represent geometric mean of RF ablation time until an increase in tissue resistance occurred at preselected power outputs and interapplicator distances. Line graph represents mathematic model for Equation (2). Error bars indicate 95% confidence intervals. Coagulation volume and RF ablation time increased at lower power output levels and larger applicator distances.

View larger version (28K): [in a new window]   Figure 5: Graph demonstrates the volume of coagulation in relation to the duration of multipolar RF ablation and applicator distance. Data points represent geometric mean of confluent volumes of coagulation at different preselected power outputs and interapplicator distances in relation to the duration of RF ablation. Line graph represents mathematic model for Equation (3). Error bars indicate 95% confidence intervals. During multipolar RF ablation, the volume of coagulation as a function of RF ablation time had a continuously decreasing slope.

View larger version (31K): [in a new window]   Figure 6a: Graphs present (a) expected volume of coagulation and (b) duration of RF ablation. Graphs were calculated on the basis of mathematic models for Equations (1) and (2) and were plotted in relation to preselected power outputs and applicator distances.

View larger version (31K): [in a new window]   Figure 6b: Graphs present (a) expected volume of coagulation and (b) duration of RF ablation. Graphs were calculated on the basis of mathematic models for Equations (1) and (2) and were plotted in relation to preselected power outputs and applicator distances.

View larger version (49K): [in a new window]   Figure 7: Graph presents relationship between the volume and fusion of coagulation and the duration of multipolar RF ablation as a function of preselected power output and applicator distance. The three-dimensional surface represents predicted volume of coagulation. Expected duration of RF ablation is color coded. The dark red line, which separates the color-coded area from the gray surface, corresponds to a probability that the zone of coagulation is 99% confluent, as predicted by Equation (4). The gray surface reflects the probability that the zone of coagulation is less than 99% confluent. For these combinations of power output and applicator distance, the predicted duration of RF ablation is not color coded; only the border of the different durations, as used for color coding, is given by the black lines.

View larger version (29K): [in a new window]   Figure 8: Graph presents the geometry of confluent zones of coagulation, which is described by the ratio between diameters parallel (diameter a) and perpendicular (diameter b) to the applicator shaft. Data points represent the mean a/b ratio for multipolar RF ablation at different preselected power outputs and applicator distances. An a/b ratio of close to 1.0 indicates a more spherical volume of coagulation. Line graph represents mathematic model for Equation (5), and error bars indicate standard deviations. Induced coagulation during multipolar RF ablation is more spherical at lower power outputs and larger interapplicator distances.