Abstract Body (Do not enter title and authors here): Background: Cardiovascular magnetic resonance (CMR) has an emerging role in graft surveillance for pediatric heart transplant recipients (PHTR). Transplanted grafts are susceptible to cardiac allograft vasculopathy, manifested as macrovascular narrowing on angiography, as well as micro-vessel disease. By CMR, myocardial perfusion abnormalities can be evaluated semi-quantitatively, by calculation of a myocardial perfusion reserve index (MPRI). However, normal MPRI values have not been well established in PHTR and prior investigation of associations between MPRI and graft pathology remain limited.

Research Aims: The goals of this study were to describe the MPRI findings in a large cohort of PHTR and to evaluate clinical associations with low MPRI.

Methods: We performed a retrospective chart review of consecutive, stress CMR studies at a single center from 2015-2024. Follow-up studies were excluded. Biventricular volume and function analyses were performed. A total dose of 0.15mg/kg gadobutrol was administered for rest and stress imaging. Regadenoson was the pharmacologic stressor at dose of 6-10mcg/kg, up to max 400mcg. Time signal intensity curves were obtained from perfusion datasets at stress and rest at the base, mid-ventricle, and apex. Segmental MPRI was calculated as a ratio of the maximal upslopes of the curves at stress versus rest. Global MPRI was computed as a mean of all segments.

Results: 128 PHTR were included. Mean age was 12.5±5.3y, with 5.9±3.8y since transplant. A clinical concern prompted CMR in 18% of studies; in 82% the indication was routine surveillance. History of CAV was present in 6% and moderate or severe rejection in 22%. In 11 studies, images were inadequate for MPRI analysis. In the 117 studies included for analysis, global MPRI was normally distributed with mean 1.38±5.3. Mean MPRI observed at the mid-ventricle (1.49±0.46) was higher than at the base (1.32±0.32) and apex (1.33±0.39), (p<0.001). PHTR with first tercile MPRI (<1.25), when compared to third tercile (>1.50), did not have significant differences in age, time since transplant, history of CAV, history of moderate or severe rejection, or LV or RV EF.

Conclusions: Mean global MPRI values in this cohort of relatively healthy PHTR are significantly lower than reference values for healthy adults and similar to previously reported values in adult heart transplant patients. Future study should evaluate associations between MPRI and subsequent clinical outcomes.