Forty-six patients who underwent first ablation treatment for AF and completed the MRI scanning protocol at all 3 time points (baseline, IPA, and 3moPA) were eligible for inclusion in the study. The baseline scans were used to confirm normal pre-ablation LA wall and before including subjects for post-ablation analysis. Eight of the 46 patients were excluded because of inadequate LGE magnetic resonance images, 5 resulting from patient motion and 3 due to gating problems. An additional 1 patient was excluded after the discovery of a previous LA ablation procedure when LA wall hyperenhancement was seen on the baseline scan. Data from the remaining 37 patients were included in the study.
Each 3D LGE MRI scan resulted in approximately 20 to 30 axial slices to cover the entire left atrium depending on the size and shape of the chamber. For the 37-patient cohort, we segmented a total of 1,852 slices from the IPA and 3moPA scans (37 patients × 2 scans × 20 to 30 slices per patient). Two hundred sixty-two slices from the extreme superior and inferior surfaces of the left atrium were excluded from the computational analyses because of partial volume effects with minimal amount of LA tissue. Results from the remaining 1,590 slices, approximately one-half from each scan time point, were included in the quantitative data.
Lesion imaging on LGE MRI
In each subject (37 of 37 [100%]) who underwent RFA for AF, acute tissue injury IPA was seen as heterogeneous on LGE MRI. Ablated tissue demonstrated regions of differing contrast with bright HE as well as dark NE regions (). These imaging findings were seen <1 h after the completion of AF ablation, when the LGE sequence is performed approximately 15 min after contrast injection. When comparing early and late LGE imaging, we routinely found that regions of NE tissue IPA become enhanced 3moPA, suggesting that NE tissue represents ablation injury that results in necrosis and eventual scar ().
Acute Left Atrial Wall Injury and Scar Remodeling
Acute LGE injury and association with permanent scar
Quantification of the degree of LA wall injury IPA and 3moPA showed a significantly greater amount of tissue injury on the IPA scans. We analyzed data from 1,590 segmented slices from IPA and 3moPA scans, approximately one-half from each, and found similar amounts of HE, NE, and normal tissue (37.7 ± 13%, 34.3 ± 14%, and 28.0 ± 11%, respectively; p = 0.87, 0.64, and 0.76 using 3 paired t-test comparisons) (). A reasonable measure of the total extent of IPA injury was obtained by combining the NE and HE tissue volumes, which summed to 72.0% of the LA wall. However, the extent of permanent LA wall injury 3moPA was much smaller, with scarring in 33.9 ± 8% of the LA wall (). These data demonstrate poor correlation of the combined acute injuries with scar at 3 months.
Segmentation Process Used for Quantification of Left Atrial Wall Injury on IPA Scans
Comparison of Acute Injuries Immediately Post-Ablation to Scar 3 Months Post-Ablation
We next investigated the degree of scarring that resulted from either NE or HE tissue. To do this, we performed a detailed quantitative analysis of the LGE MRI injuries seen IPA and 3moPA using 3D nonrigid deformation processing methods described earlier. These processing methods were labor intensive and thus were performed on a subset of 10 patients with the best image quality MRI scans. A total of 498 segmented slices from IPA and 3moPA scans (approximately one-half from each) were included in the 3D analysis. Using these methods, we found that 59.0 ± 19% of scar resulted from NE tissue, 30.6 ± 15% from HE tissue, and 10.4 ± 5% from tissue identified as normal. Paired t-test comparisons were all statistically significant, including NE versus HE (p = 0.0016), NE versus normal (p < 0.0001), and HE versus normal (p = 0.0008) ( and ) Nearly 2-fold more scarring resulted from NE compared with HE injury (59.0% vs. 30.6%). These data reveal significant differences in the relative contribution of acute NE and HE injury to scar at 3 months.
Percentage Left Atrial Wall Scar 3 Months Post-Ablation That Results From the Acute NE or HE Lesions Immediately Post-Ablation
Late Gadolinium Enhancement No-Reflow Predicts Late Scarring
Tissue injury with “no-reflow” characteristics
Dark NE regions in the LA wall after ablation demonstrate findings compatible with the no-reflow phenomenon. These findings are best demonstrated on LGE when imaging is performed at different time intervals after the administration of gadolinium contrast (). On standard LGE imaging 15 min after contrast injection, we identified large regions of NE tissue within the LA wall along with HE regions (). When LGE imaging was performed at a longer time interval after the administration of gadolinium contrast, we saw progressive enhancement of the tissue initially seen as NE ().
Serial LGE Images Demonstrate No-Reflow
Two operators performed wall tracings, and a high degree of interobserver correlation for the LA endocardial and epicardial wall tracings was observed for both the IPA (r = 0.98; 95% confidence interval: 0.94 to 0.99; p < 0.001) and 3moPA (r = 0.99; 95% confidence interval: 0.94 to 0.99; p < 0.001) scans. The high degree of correlation reflects the good scan quality as well as the experience of the operators in our laboratory, who perform these tracings on a regular basis with the aid of computer processing tools in SCIRun.
LA wall scarring at 3 months and patient outcomes
One year post-ablation, 29 of 37 patients (78.4%) remained free of AF. When dividing patients into quartiles on the basis of percentage of LA wall scar on LGE 3moPA, a significant difference was seen between responders and nonresponders (). Patients in the fourth quartile, defined as moderate scar formation (LA wall scar >23%) had no AF recurrence at 1-year follow-up. In comparison, all 8 recurrences occurred in patients in the lower quartiles with mild scar formation (LA wall scar ≤23%) (p = 0.02). Because there were no recurrences in the first quartile, an odds ratio could not be calculated.
AF Ablation Procedural Success and Association With LA Wall Scar