1 Intrinsic Contrast for Characterization of Acute Radiofrequency Ablation Lesions Running title: Celik et al.; Intrinsic Contrast of Acute RF Ablation Lesions

2 Background- Both intrinsic contrast (T1 and T2 relaxation, and the equilibrium magnetization) and contrast-agent (gadolinium) enhanced magnetic resonance imaging (MRI) are used visualize and evaluate acute radiofrequency ablation (RFA) lesions. However, current methods are imprecise in delineating lesion extent shortly after the ablation. Methods and Results- Fifteen lesions were created in the endocardium of 13 pigs. A multi-contrast inversion-recovery steady state free precession (IR-SSFP) imaging method was used to delineate the acute ablation lesions, exploiting T1-weighted contrast. T2 and Mo * maps were also created from fast spin echo data in a subset of pigs (n = 5) to help characterize the change in intrinsic contrast in the lesions. Gross pathology was used as reference for the lesion size comparison and the lesion structures were confirmed with histological data. Additionally, a colorimetric iron-assay was utilized to measure ferric and ferrous iron content in the lesions and the healthy myocardium in a subset of pigs (n=2). The lesion sizes measured in IR-SSFP images were highly correlated with the extent of lesio