Planar strain analysis of liver undergoing microwave thermal ablation using x-ray CT

Purpose: To study the planar strain effects in liver during microwave (MW) thermal ablation as a means for tracking tissue expansion and contraction as a method for improving ablation monitoring.

Methods: 1.4 mm circular metallic markers were inserted into 16 ex-vivo bovine fresh liver specimens, that were subsequently ablated (with the markers inside the specimen) by 40 W of microwave energy, for 1, 2, 3, 6, and 10 min. The markers were tracked during the ablation using an x-ray CT scanner. Images were acquired every 5-10 s enabling determination of the markers coordinates over time. The 2D principal strains were calculated for triangles formed by subgroups of three markers, and their planar strain index, ω, was plotted vs time. In addition, the radial distance of the markers from the antenna was measured at the end of each ablation. Subsequently, the tissue was sliced parallel to the imaged planes and the ablation zone was traced and digitized. The average ablation radius was then computed and compared to the radial distance.

Results: The planar strain, ω(t), profile demonstrated an ascending pattern until reaching a maximum at about 180 s, with a mean peak value (ω= 1.31±0.04) indicating tissue expansion. Thereafter, ω progressively declined over the remaining duration of the ablation treatment, indicating tissue contraction. Furthermore, when plotting the ablation size vs time and the markers mean radial distance vs time, it was found that the two curves intercepted at a time corresponding to the time of peak planar strain.

Conclusions: By detecting the point of maximal planar strain in tissues during MW application, it is possible to noninvasively identify the location of the ablation zone front. The fact that the liver tissue proximal to the ablated zone expands during the first part of the treatment and then contracts when the ablation front reaches it, may serve as an index for monitoring the thermal treatment.