Quantifying local stiffness variations in radiofrequency ablations with dynamic indentation

Elastographic imaging can be used to monitor ablation procedures, and confident determination of the ablation boundary is essential to ensure complete treatment of the pathological target. To investigate the potential for ablation boundary representation on elastographic images, local variations in the viscoelastic properties in radiofrequency ablated regions of ex vivo porcine liver were quantified using dynamic indentation. Regions of interest in eleven radiofrequency ablation samples were indented at 18-24 locations each, including the central zone of complete necrosis and peripheral transition zones including normal tissue. Storage modulus and rate of stiffening were both greatest in the central ablation zone and decreased with radial distance away from the center. The storage modulus and modulus contrast at the ablation outer transition zone boundary were 3.1 ± 1.0 kPa and 1.6 ± 0.4, respectively, and 36.2 ± 9.1 kPa and 18.3 ± 5.5 at the condensation boundary within the ablation zone. Elastographic imaging modalities were then compared to gross pathology in ex vivo bovine liver tissue. Area estimated from strain, shear wave velocity, and gross pathology images were 470 mm², 560 mm², and 574 mm², respectively, and ablation widths were 19.4 mm, 20.7 mm, and 23.0 mm. This study provided insights into spatial stiffness distributions within radiofrequency ablations, increasing our understanding of the correlation of elastographic images with gross pathology.