Abstract Body (Do not enter title and authors here): Introduction/Background - Research Questions/Hypothesis:
There is a notable lack of robust evidence regarding imaging follow-up in Fontan patients from resource-limited settings, where access to advanced imaging modalities remains scarce. This study aims to describe the prevalence of cardiac magnetic resonance (CMR) use, key findings, and associated clinical outcomes in a Latin American cohort of Fontan patients.

Methods/Approach:
A retrospective, observational, single-center study included patients who underwent Fontan procedure completion and had an available postoperative cardiac magnetic resonance scan performed between 2006 and 2024. Data were analyzed descriptively using means ± SD, and frequencies (%).

Results
Of 174 Fontan surgeries performed in our center, we included 27 (12.7%) with a follow-up CMR, spanning all age groups. The cohort comprised 15 males (55.6%), with a mean age at the time of CMR of 23.4 years. The most prevalent congenital defect was tricuspid atresia (55.6%), and in 88.8% of patients, the left ventricle was the systemic ventricle. For the Fontan pathway the extracardiac conduit was the preferred surgical technique (92.5%) and fenestration was present in 48.1% of patients.
CMR was performed 19 ± 7.4 years after Fontan completion, revealing a mean indexed systemic ventricular end-diastolic volume of 101.7 ± 44.9 mL/m2 and a mean indexed end-systolic volume of 52.8 ± 30.8 mL/m2. The mean ventricular ejection fraction was 49.7% ± 7.2%, and the mean cardiac index was 4.25 ± 1.78 L/min/m2. Among those with a documented fenestration, 7.4% remained patent at time of CMR. Venovenous collaterals were present in 55.5% of the cohort. With 29.6% having more than moderate atrioventricular valve regurgitation. Venovenous collaterals were identified in 55.5% of patients, and 29.6% had more than moderate atrioventricular valve regurgitation. Findings consistent with Fontan-associated liver disease were present in 55.6% of patients, and 7.4% were diagnosed with hepatocellular carcinoma all of whom died, yielding an overall mortality rate of 7.4%.

Conclusion(s):
Despite limitations, our findings highlight that few Fontan patients undergo routine CMR follow-up. The high prevalence of complications suggests that CMR surveillance may be occurring too late to enable timely interventions. These results underscore the need for guideline-adherent imaging protocols to improve outcomes in this high-risk population.