Evaluation of tissue deformation during radiofrequency and microwave ablation procedures: Influence of output energy delivery

Abstract

Purpose: The purpose of this study was to quantitatively analyze tissue deformation during radiofrequency (RF) and microwave ablation for varying output energy levels.

Methods: A total of 46 fiducial markers which were classified into outer, middle, and inner lines were positioned into a single plane around an RF or microwave ablation applicator in each ex vivo bovine liver sample (8 cm × 6 cm × 4 cm, n = 18). Radiofrequency (500 kHz; ~35 W average) or microwave (2.4 GHz; 50-100 W output, ~35-70 W delivered) ablation was performed for 10 min (n = 4-6 each setting). CT images were acquired over the entire liver volume every 15 s. Principle strain magnitude and direction were determined from fiducial marker displacement. Normal and shear strain were then calculated such that negative strain denoted contraction and positive strain denoted expansion. Temporal variations, the final magnitudes, and angles of the strain were compared across energy delivery settings, using one-way ANOVA with post hoc Tukey's tests.

Results: On average, tissue strain rates peak at around 1 min and decayed exponentially over time. No evidence of tissue expansion was observed. The tissue strains from RF and 50 W, 75 W, and 100 W microwave ablation at 10 min were -8.5%, -38.9%, -54.4%, and -65.7%, respectively, from the inner region and -3.6%, -23.7%, -41.8%, and -44.3%, respectively, from the outer region. Negative strain magnitude was positively correlated to energy delivery in the inner region (Spearman's ρ = -0.99). Microwaves at higher powers (75-100 W) induced significantly more strain than at lower power (50 W) or after RF ablation (P < 0.01). Principal strain angles ranged from 0.8° to -8.1°, indicating that tissue deformed more in the direction transverse to the applicator than along the direction of the applicator.

Conclusions: The influence of output energy on tissue deformation during RF and microwave ablation was analyzed. Microwave ablation created significantly greater contraction than RF ablation with similar energy delivery. During microwave ablation, more contraction was noted at higher power levels and in proximity to the antenna. Contraction primarily transverse to the antenna produces ablation zones that are more elongated than the original tissue volume.

Keywords: microwave ablation; radiofrequency ablation; thermal ablation; tissue contraction; tissue deformation.

Conflict of interest statement

The authors have no conflicts to disclose.

© 2019 American Association of Physicists in Medicine.

Figures

Figure 1

Experimental setup for fiducial setting during CT imaging and RF & microwave ablation. RF, radiofrequency; CT, computed tomography. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 2

The deformation of triangles of contralateral middle (yellow) and inner (red) fiducials at pre‐ablation and post‐ablation: with fiducials moving toward antenna, the shape of triangles changed during ablation. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 3

Pathology images during RF and microwave ablation. Ablation zone measured in RF, microwave 50 W, 75 W, and 100 W were 3.5 cm, 3.6 cm, 5.0 cm, and 6.5 cm in length, respectively, and 1.8 cm, 2.7 cm, 3.8 cm, and 4.0 cm in diameter, respectively. RF, radiofrequency. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 4

Inner strains vs time. Data were shown as mean strain of the triangles at the inner ablative regions with error bars representing the standard errors of the mean. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 5

Strains of the inner triangles at 10 min of RF ablation, microwave ablations of 50 W, 75 W, and 100 W. Data were shown as box plots including median strains, 25% and 75% percentages, outliers. The ANOVA test showed a significant difference between these four ablations (P < 0.001). **denotes P < 0.01, *denotes P < 0.05, and n is the total number of triangles during different ablations. RF, radiofrequency. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 6

Strains of inner triangles vs output microwave energies. The strains of inner triangles were given in mean values of inner triangles at each time point. The red line is a quadratic fitted curve, which described a close correlation between strains and output energies (Spearman correlation coefficient ρ= −0.99) [Color figure can be viewed at wileyonlinelibrary.com]

Figure 7

Middle strains vs time. Data were shown as mean strain of the triangles at the middle ablative region with error bars representing the standard errors of the mean. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 8

Strains of the middle triangles at 10 min of RF ablation, microwave ablations of 50 W, 75 W, and 100 W. Data were shown as box plots including median strains, 25% and 75% percentages, outliers. The AVONA test showed a significant difference between these four ablations (P < 0.001). NS denotes statistical nonsignificance (P > 0.05), **denotes P < 0.01, and n is the number of triangles during different ablations. RF, radiofrequency. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 9

Outer strains vs Time. Data were shown as mean strain of the triangles at the outer ablative region with error bars representing the standard errors of the mean. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 10

Strains of the outer triangles at 10 min of RF ablation, microwave ablations of 50 W, 75 W, and 100 W. Data were shown as box plots including median strains, 25% and 75% percentages, outliers (red +). The AVONA test showed significant difference between these four ablations (P < 0.001). NS denotes statistical nonsignificance (P > 0.05), **denotes P < 0.01, and n is the number of triangles during different ablations. RF, radiofrequency. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 11

Principal angles of the outer triangles at 10 min of RF ablation, Microwave ablation with 50 W, 75 W, and 100 W. Data were shown as the box plots including median angles in degree, 25% and 75% percentages, outliers. The AVONA test showed four ablations had significant different mean values (P < 0.001). NS denotes statistical nonsignificance (P > 0.05), * and **denote P < 0.05 and P < 0.01, respectively, and n is the number of triangles during different ablations. RF, radiofrequency. [Color figure can be viewed at wileyonlinelibrary.com]