SUMMARY: Atrial fibrillation (AF) affects approximately 3 million patients and its prevalence is expected to rise as the population ages. Catheter ablation, the preferred treatment strategy for symptomatic patients, creates local regions of tissue necrosis in the atria called lesions that are meant to abolish abnormal electrical activity that causes AF. However, AF recurrence is significant in these patients mostly due to inadequate lesions formed by ablation or by recovery of abnormal electrical activity. Currently, standard-of-care technology to guide lesion formation relies on surrogates for lesion formation that only marginally improve outcomes. Additionally, in some patients with AF the abnormal activity responsible cannot be localized, which then requires adjunctive lesions targeting alternative structural substrates for AF such as fibrosis and scarring. However, alternative AF substrates are currently identified indirectly based substrate proxies rather than the substrate itself, which has yet to be proven effective. Therefore, the main barriers to the successful treatment of AF continue to be: a) creating lesions that completely and permanently block abnormal electrical activity, and b) accurately identifying substrates that are known to cause AF. Optical coherence tomography (OCT) is an imaging method that can monitor lesion formation in real-time, and near-infrared spectroscopy (NIRS) can quantify optical properties from measured diffuse tissue reflectance during RF ablation. When combined, OCT/NIRS can provide direct information on lesion formation, depth, and tissue composition. Based on our previous work and preliminary data we hypothesize that compared to standard-of-care methods, combined OCT/NIRS can be safe and more effective at creating lesions that are complete and durable and for mapping AF structural substrates. To test this hypothesis, we aim to: 1. Develop and validate real-time optically guided atrial ablation lesion formation, and 2. Develop and validate real-time optical mapping of AF structural substrates. For both of these aims, development includes innovative OCT/NIRS software and hardware methods along with validation in vivo and ex vivo in atria from normal pigs and pigs with AF as well as in atrial tissue from humans with or without AF. Then, based on these developments we aim to: 3. Compare real-time optically guided lesion formation and mapping of AF structural substrates against standard-of-care methods. Outcomes will be determined acutely in vivo and verified ex vivo immediately after the procedure and chronically to determine durability of lesions. To achieve these aims, we have assembled a uniquely qualified multi-disciplinary multi-PI team with expertise in OCT (Drs. Rollins and Hendon), NIRS (Dr. Hendon), and basic cardiac electrophysiology (Dr. Laurita), along with Co-I expertise in clinical AF ablation (Dr. Ziv) and cardiovascular pathology (Dr. Marboe). Our team's long-term goal is to develop innovative technologies to improve AF ablation outcomes.