Sub-acute changes in lesion conspicuity and geometry following MR-guided radiofrequency ablation

Purpose: To evaluate MR signal and lesion zone volume evolution through the sub-acute phase following image-guided radiofrequency (RF) thermal ablation. Materials and Methods: For many tissues, including muscle and liver, thermal lesions that result from RF heating have a characteristic two-boundary appearance featuring an inner core (zone I) surrounded by a hyper-intense margin (zone II) and normal tissue (zone III), found in both T-2 and contrast enhanced (CE) T-1-weighted MR images, both immediately post-ablation and four days later. First, we compared corresponding points between manually segmented zone boundaries apparent in T-2- and CE T-1-weighted images. Second, we examined the contrast-to-noise ratio (CNR) between all zone combinations. Third, we quantified the volume of zone I, zone II, and the entire E lesion using a three-dimensional lesion geometry model fitted to segmented images. Results: On a slice-by-slice basis; no statistically significant differences were found between zone boundaries apparent in T-2 and CE T-1-weighted images. The contrast to noise ratio (CNR) of zone I vs. zone II, zone I vs: background muscle, and zone II vs. background muscle was always equal or greater for T-2-weighted images than for CE T-1-weighted images. In addition, by day four, zone II significantly increased in intensity compared to background muscle. The median Zone I volume increase was 44.2% (42.6%) using T-2 weighted images and 55.5% (68.7% interquartile range) using CE T-1- weighted images. This expansion likely corresponds to an enlargement of the ablated, coagulative necrosis, region. The median Zone)I volume increase was 15.0% (42:6%) using T-2- weighted images 1.5% (38.8%) using CE T-1-weighted images. Conclusions: 1) There are no significant differences between the apparent zone boundaries in T-2- and CE T-1-weighted images; 2) CNR is equal or greater for T-2-weighted images as compared to CE T-1-images; and 3) both the inner and outer lesion zone volumes typically increase several days post-ablation.