Background. Delayed enhancement MRI (DE-MRI) allows visualization of post-procedure scarring in atrial fibrillation (AF) patients following pulmonary vein isolation (PVI) by radiofrequency ablation. We report on a method for quantification of atrial scarring in DE-MRI using dynamic thresholding.

Methods. Patients undergoing PVI were imaged 3 months post-procedure using 3D inversion recovery, navigated, gradient echo DE-MRI. Left atrial (LA) endocardial and epicardial contours were traced using custom software written in MATLAB. To differentiate between normal and enhanced regions we assumed Gaussian distributions of pixel intensities. A histogram of all LA pixel intensities was used to determine the mean and standard deviation (SD) of normal intensities for each slice of MRI data. Any pixel intensity greater than 3 SD’s above the mean was labeled as hyper-enhanced and the set of all such pixels served to localize and quantify regions of post-procedure scar throughout the entire LA.

Results. The left-hand images below show a single slice of MRI data with atrial contours (upper panel) and LA hyper-enhancement (predicted scarring) marked with black pixels (lower panel). The right-hand images show two projections of the LA data with predicted scarred regions colored in blue. The hyper-enhanced regions were cohesive and consistently located at the pulmonary vein ostia and posterior wall.

Conclusion. This form of dynamic thresholding provides a reproducible means for quantifying hyper-enhancement and thus scar formation in the LA of AF patients post PVI.