Tissue Contraction Caused by Radiofrequency and Microwave Ablation: A Laboratory Study in Liver and Lung

PURPOSE: To determine the amount of tissue contraction during radiofrequency (RF) and microwave ablation. MATERIALS AND METHODS: Markers were inserted into explanted bovine liver and lung 10 mm (inner), 20 mm (middle; not used in lung), and 30 mm (peripheral) diametrically around an ablation applicator. Aside from unablated controls, RF and microwave ablations 25-30 mm in diameter were then created and sectioned to measure the distance between markers (n = 12, liver RF; n = 8, other). Total contraction was calculated by subtracting postablation measurements from controls at each position. Relative contraction was calculated by subtracting the nearest more central measurement. Sample water content was measured to determine the relationship between dehydration and relative contraction. A mixed-effects model tested for differences in diameters, total and relative contraction, and water content with energy, tissue, and marker position as independent variables. RESULTS: Total contractions at the inner, middle, and peripheral positions in liver were 2.9 mm (31%), 4.8 mm (24%), and 4.5 mm (15%) for RF and 3.6 mm (38%), 6.6 mm (33%), and 9.0 mm (30%) for microwave, respectively. Significantly more contraction was noted in lung (P < .001): 5.1 mm (55%) and 14.2 mm (49%) for RF and 4.8 mm (52%) and 13.7 mm (47%) for microwave at the inner and peripheral positions, respectively. Microwaves produced more contraction than RF in liver (P < .05) but not in lung. A positive correlation between dehydration and relative contraction was observed in all cases. CONCLUSIONS: Ablation-induced tissue contraction is substantial and influenced by dehydration. Contraction should be considered when testing devices and computer models or comparing pre- and postablation images.