Radiofrequency thermal ablation: Correlation of hyperacute MR lesion images with tissue response

Abstract

Purpose: To investigate the hypothesis that the outer boundary of the hyperintense region observed in hyperacute (several minutes post-ablation) T2 and gadolinium contrast-enhanced (CE) T1-weighted magnetic resonance (MR) lesion images is an accurate predictor of eventual cell death from radiofrequency (RF) thermal ablation. Materials and Methods: A low-field, open MR imaging system was used to guide an ablation electrode into a thigh muscle of five rabbits and acquire in vivo T2 and CE T1-weighted MR volumes. Ablation occurred by applying RF current for two minutes with the electrode's temperature maintained at 90°Ϯ 2°C. After fixation, we sliced and photographed the tissue at 3 mm intervals, using a specially designed apparatus, to obtain a volume of tissue images. Digital images of hematoxylin and eosin (H&E) and Masson trichrome-stained histologic samples were obtained, and distinct regions of tissue damage were labeled using a video microscopy system. After the MR and histology images were aligned using a three-dimensional registration method, we compared tissue damage boundaries identified in histology with boundaries marked in MR images. Results: The lesions have distinct zones of tissue damage histologically: a central zone of necrotic cells surrounded by an outer zone with cells that appeared non-viable and associated with marked interstitial edema. In 14 histology images from five lesions, the inner and outer boundaries of the outer zone were compared with the boundaries of a hyperintense rim that surrounds a central hypointense region in the T2 and CE T1-weighted MR images. For T2 and CE T1-weighted MR images, respectively, the mean absolute distance was 1.04 Ϯ 0.30 mm (mean Ϯ SD) and 1.00 Ϯ 0.34 mm for the inner boundaries, and 0.96 Ϯ 0.34 mm and 0.94 Ϯ 0.44 mm for the outer boundaries. The mean absolute distances for T2 and CE T1-weighted MR images were not sufficiently different to achieve statistical significance (P ϭ 0.745, 0.818, for the inner and outer boundary, respectively). Conclusion:In hyperacute T2 and CE T1-weighted MR lesion images, observations strongly suggest that the outer boundary of the hyperintense rim corresponds to the region of eventual cell necrosis within a distance comparable to our ability to measure. This is good evidence that during RF ablation procedures, MR lesion images can be used to accurately localize the zone of irreversible tissue damage at the lesion margin.

Magnetic resonance imaging and model prediction for thermal ablation of tissue

“…Despite the strong and consistent link between imaging and histology estimates of lesion area (9,16,24), it is not feasible to interleave histology or long-duration imaging for real-time monitoring of the ablation procedure. A PRF-shift method was developed to rapidly monitor an isotherm (e.g., ϳ48°C) (25).…”

“…For all experiments, tissue sectioning was performed approximately perpendicular to the RF electrode track. The processing of histological samples followed procedures described previously (9,16). Histological samples of the ablation zones were obtained from the tissue sections and stained using the hematoxylin and eosin (H&E) technique.…”

“…The accuracy of the lesion area estimated from the final GRE magnitude images (AЈ G ) was evaluated by comparison with lesion areas AЈ T and AЈ H estimated from post-treatment TSE images and histological analysis of tissue samples (9,16,27). The nonstatistical difference between the TSE or histology sections and the final GRE lesion areas can be tested by a nonparametric permutation method (20).…”

“…Validation of the post-ablation lesion size observed in GRE images was determined by comparison with TSE and histological images (9). The establishment of this relationship shows that real-time GRE images obtained during the ablation can serve as a surrogate for histology in assessing lesion size.…”

“…The ultimate lesion (coagulation) volume has been determined with T2-weighted turbo-spin-echo (TSE) or contrast-enhanced T1-weighted spin-echo (SE) images (9,10). To follow the dynamics of tissue temperature distribution during ablation, MR temperature imaging was developed based primarily on the proton resonance frequency (PRF) shift (11)(12)(13)(14).…”