Radio-frequency-induced thermal lesions: Subacute magnetic resonance appearance and histological correlation

Purpose: To investigate the relationship between subacute magnetic resonance (MR) images of radio-frequency (RF) ablation lesions and tissue viability as determined from histological tissue samples. Materials and Methods: We generated lesions (N = 5) in a rabbit thigh model. Four days later, we obtained in vivo T₂- and contrast-enhanced (CE) T ₁-weighted images and ex vivo histological samples approximately perpendicular to the electrode path. Using the three-dimensional registration and warping we spatially compared manually segmented boundaries apparent on MR images to boundaries separating distinct histological zones determined from hematoxylin and eosin (H&E) and Masson trichrome (MT) stains as well as birefringence studies. Results: Lesions have a characteristic MR appearance: an outer hyperintense margin (M2) separating background tissue (M3) from an inner core (M1) in both T₂ and CE T₁ images. Histologically, there are two zones of damage: an outer zone of likely nonviable cells (H2) separating background tissue (H3) from an inner core of coagulated nonviable cells (H1). We measured distances between automatically-computed correspondence points along historical and MR boundaries. For T₂ and CE T ₁ images, respectively M1 vs. H1 distances were 0.72 ± 0.99 mm (mean ± SD) and 0.10 ± 0.95 mm, while outer M2 vs. H2 boundary distances were 0.26 ± 1.16 mm and 0.05 ± 1.08 mm. The discrepancy between histological and MR boundaries was larger than the variability in segmenting MR images, but probably within registration error. There were no significant differences between T₂ and CE T₁ boundaries. Conclusion: Lesion boundaries apparent in both T₂- and CE T ₁-weighted MR scans, performed several days postablation, similarly predict the histological response. That is the lesion core (M1) corresponds to nonviable coagulated cells (H1), while the hyperintense margin (M2) corresponds to likely nonviable cells undergoing necrotic changes (H2).