Background MRI has been used to acutely visualize radiofrequency (RF) ablation lesions but its accuracy in predicting chronic lesion size is unknown. The main goal of this study was to characterize different areas of enhancement in late gadolinium enhancement (LGE) MRI done immediately after ablation to predict acute edema and chronic lesion size. Methods and Results In a canine model (n=10), ventricular RF lesions were created, using ThermoCool SmartTouch (Biosense Webster) catheter. All animals underwent MRI (LGE and T2-weighted (T2w) edema imaging), immediately after ablation and after 1, 2, 4 and 8 weeks. Edema, microvascular obstruction (MVO) and enhanced volumes were identified in MRI and normalized to chronic histological volume. Immediately after contrast administration, the MVO region was 3.2 +/− 1.1 times larger than the chronic lesion volume in acute MRI. Even 60 mins after contrast administration, edema was 8.73 +/− 3.31 times and the enhanced area 6.14 +/− 2.74 times the chronic lesion volume. Exponential fit to the MVO volume was found to be the best predictor of chronic lesion volume at 26.14 (95% prediction interval 24.35 – 28.11) mins after contrast injection. The edema volume in LGE correlated well with edema volume in T2w MRI with an R2 of 0.99. Conclusion MVO region on acute LGE images acquired 26.1 min after contrast administration can accurately predict the chronic lesion volume. We also show that T1-weighted MRI images acquired immediately after contrast injection accurately shows edema resulting from RF ablation.

Background: MRI has been used to visualize radiofrequency (RF) ablation lesions but the relationship between volumes that enhance in acute MRI and the chronic lesion size is unknown. Objectives: The main goal was to use non-contrast (native) T1 weighted (T1w) MRI and late gadolinium enhancement (LGE) MRI to visualize lesions acutely and chronically and correlate the acute area of enhancement with chronic lesion size in histology. Methods: In a canine (n=9) model RF ablation lesions were created in both ventricles. Native T1w MRI and LGE-MRI were acquired acutely after the ablation procedure. After 8 weeks, another set of RF ablations was performed and the MRI study was repeated. Volume and depth of enhancement in native T1w MRI and LGEMRI acquired after the initial ablation procedure were correlated with chronic lesion volume and depth in histology. Results: 33 lesions were analyzed. Native T1w MRI visualized the acute lesions but not the chronic lesions. LGE-MRI showed both acute and chronic lesions. Acute native T1w MRI volume (average of 102.1±48.5mm3) and depth (4.9±1.2 mm) correlated well with chronic histological volume (105.9±51.8mm3) and depth (4.8±1.3mm) with R2 of 0.881, p<0.001 and 0.874, p<0.001 respectively. Acute LGE MRI had significantly higher volume of enhancement of 499.7±214.4mm3 (p< 0.001) and depth of 7.5±1.8mm (p<0.001) compared to chronic histological lesion volume and depth. Conclusion: Native T1w MRI acquired acutely after RF ablation is a good predictor of chronic lesion size. Acute LGE-MRI significantly overestimates chronic lesion size.

Background: Atrial fibrillation (AF) is related to numerous electrophysiological changes; however, the extent of structural and electrophysiological remodeling with long-term AF is not well characterized.Methods: Dogs (n = 6) were implanted with a neurostimulator in the right atrium (AF group). No implantation was done in the Control group (n = 3). Electroanatomical mapping was done prior to and following more than 6 months of AF. Magnetic resonance imaging was also done to assess structural remodeling. Animals were euthanized and tissue samples were acquired for histological analysis.Results: A significant increase was seen in the left atrial (LA) volume among all AF animals (22.25 ± 12.60 cm 3 vs 34.00 ± 12.23 cm 3 , P = .01). Also, mean bipolar amplitude in the LA significantly decreased from 5.96 ± 2.17 mV at baseline to 3.23 ± 1.51 mV (P < .01) after chronic AF.Those significant changes occurred in each anterior, lateral, posterior, septal, and roof regions as well. Additionally, the dominant frequency (DF) in the LA increased from 7.02 ± 0.37 Hz to 10.12 ± 0.28 Hz at chronic AF (P < .01). Moreover, the percentage of fibrosis in chronic AF animals was significantly larger than that of control animals in each location (P < .01). Conclusions:Canine chronic AF is accompanied by a significant decrease in intracardiac bipolar amplitudes. These decreased electrogram amplitude values are still higher than traditional cut-off values used for diseased myocardial tissue. Despite these "normal" bipolar amplitudes, there is a significant increase in DF and tissue fibrosis. K E Y W O R D S animal model, atrial fibrillation, magnetic resonance imaging, mapping 1 930

With increasing contact force, there is a marginal increase in lesion size but accompanied with a significantly larger edema. The reversible edema that is much larger than the chronic lesion volume may explain some of the chronic procedure failures.

Radiofrequency (RF) ablation results in creation of acute edema which can lead to temporary disruption of electrical propagation. The goal of this study was to find the effective contact force (CF) to minimize edema formation in comparison to the lesion size. Ventricular RF lesions (n = 49) were created by a CF-sensing catheter in a canine model (n = 10) with varying force for 30 seconds. Animals underwent T2-weighted (T2w) and late gadolinium enhancement MRI (LGE-MRI) immediately after ablation and at 12 weeks. Acute LGE lesion volume, acute edema, and chronic LGE lesion volume were measured. Acute edema/acute LGE lesion volume ratio was used to divide the lesions into two groups. Mean edema/lesion volume ratio was 5.0 ± 2.8. The lesions were divided into greater edema group (n = 8) and smaller edema group (n = 41) based on a cutoff edema/lesion volume ratio. When comparing the two groups, the CF and force time integral (FTI) were significantly lower in the greater edema group. There was no difference in catheter power setting, tip temperature change, impedance drop, and bipolar electrogram voltage change. Acute LGE volume and chronic lesion depth were significantly smaller in the greater edema group. Moreover, receiver-operator characteristic curve for the smaller edema lesion group showed that the most discriminant cutoff values for CF and FTI were 12.4 g and 584 gs, respectively. To minimize edema size while still forming permanent lesions, ablation should be performed with FTI > 584 gs or CF > 12.4 g.