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Introduction: Atrial cardiomyopathy without atrial fibrillation is suspected to lead to embolic stroke of unknown source (ESUS). Late gadolinium enhancement cardiac MRI (LGE-MRI) is a non-invasive technique used to assess atrial fibrosis and cardiomyopathy.Hypothesis: We hypothesized that patients with ESUS have a higher percent of atrial fibrosis, quantified using LGE-MRI, compared to other stroke subtypes.Methods: Cardiac LGE-MRIs were obtained in ischemic stroke patients treated at two comprehensive stroke centers. Acquisition of images took approximately 30 minutes and was well tolerated by all of the patients. Each patient had a complete standard of care evaluation. Stroke neurologists blinded to LGE-MRI results characterized the stroke subtypes according to TOAST methods modified by the addition of ESUS to the cryptogenic category. Atrial enhancement was defined on LGE-MRI by an intensity threshold that was determined by expert inspection with initial visualization via a volume-rendering tool and expressed as a percentage. We used a t-test to compare mean percent atrial fibrosis between ESUS and non-ESUS patients.Results: We enrolled 17 patients: 3 (17%) female; mean age 53 years (SD 16); and 10 (59%) with ESUS. Age and sex did not differ between ESUS and non-ESUS patients. Atrial fibrosis estimation by LGE-MRI was successfully quantified in all patients. Mean left atrial fibrosis for the entire sample was 9.8% (SD 7.9) and was significantly higher in ESUS (14.1%, SD 6.6%) versus non-ESUS patients (3.6%, SD 5.2%), p=0.003. See Figure.Conclusions: Compared to non-ESUS stroke patients, ESUS patients had significantly higher percentages of atrial fibrosis, supporting atrial cardiomyopathy as one possible mechanism for ESUS. Larger sample sizes are needed to confirm that LGE-MRI is a useful tool to reduce uncertainty about the cause of stroke in the ESUS population.