# Impact of LGE-MRI in Arrhythmia Ablation
## 4 Use of the Processed LGE-MRI for Ablation Procedures
### LGE-MRI for AF Ablation  
The use of LGE-MRI for the assessment of fibrosis in the atrium has not become routine clinical practice. This is because the wall thickness of the atrium is lower than 1 mm, which is near the limit of spatial resolution of MRI, and due to less extensive and more diffuse fibrosis in comparison with the fibrosis in the ventricle. Nevertheless, recent improvements have been developed in MRI acquisition, including 3D navigated inversion recovery sequences to enhance signal-to-noise ratios and resolution, allowing valid atrium characterization [30; 69]. These advances are now making LGE-MRI a widely accepted tool for assessing lesions, stratifying risks and selecting appropriate patients for AF ablation in many specialized centres [3].  
Many other studies since then have tried to identify LGE-MRI predictors of recurrence after PVI ablation, and the degree of atrial fibrosis was confirmed to be an important predictor. In a retrospective study including patients undergoing AF ablation, there was an observed 45% increased risk of recurrence for each 10% increase in atrial fibrosis at the 5-year follow-up [70]. These data were supported by findings from another cohort of 165 patients. Patients with an amount of LGE less than 35% had favourable ablation outcomes regardless of AF persistence at baseline, whereas those with LGE greater than35% had a higher rate of AF recurrence in the first year of follow-up after ablation [71]. In the DECAAF multicentre study [72], 260 patients who underwent PVI were included, and the extent of fibrosis in the LA was categorized as 1 (<10% of the atrial wall), 2 (>10% to <20%), 3 (>20% to <30%) and 4 (>30%). Recurrent arrhythmia during follow-up was direct and graded: from 15% in the stage I group to 51% in the stage IV group. In a subsequent randomized study, catheter ablation of left atrial fibrosis was proposed [73]. The benefit of ablation of these areas could not be demonstrated by this study. This result was supported by other randomized trials, where MRI fibrosis ablation plus PVI was not more effective than PVI alone [74].  
Another important potential use of LGE-MRI is to guide repeated ablation procedures, as most cases of AF recurrence after PVI ablation are associated with areas of incomplete ablation or gaps around the pulmonary veins [49; 75] (Figure 3). Some studies indicate that those gaps can be identified by LGE-MRI with high accuracy [6; 48; 49; 76]. [PERSON] et al. 2010 [75] demonstrated that LGE-MRI is able to confirm a PVI gap with very high predictive values. [PERSON] et al. 2014 [6] showed for the first time that the elimination of gaps detected by LGE MRI generates a reisolation of PPVV in the majority of cases. In this particular study, the LGE-MRI reconstruction was merged with the EAM, so the operator was blinded to electrical information, only isolating gaps localized in the MRI. Reisolation was acquired in 95.6% of the reconnected PVs. In addition to guiding redo procedures, gap detection by LGE-MRI has been considered a predictor of AF recurrence. [PERSON] et al. 2018 [69] found that the relative gap length calculated as the absolute gap length divided by total length of the ablation line measured during MRI at 3 months of follow-up was a marker predicting AF recurrence 1 year after PVI.  
Figure 3: Agreement between EAM voltage map and LGE-MRI gap localisation. From (**left**) to (**right**), voltage map with gaps localised during a repeated ablation intervention. Right, gaps localised at the same region by prior LGE-MRI 3 months post first ablation. Yellow arrows indicate detected EAM gaps and LGE-MRI discontinuities, respectively.