Diffuse elevations of T1 and T2 are common in newly transplanted pediatric cardiac grafts without rejection and...

Pediatric heart transplantation carries a high risk of subclinical myocardial injury and rejection. While cardiovascular magnetic resonance (CMR) parametric map elevations detect subtle myocardial changes with rejection, the typical temporal evolution of T1 and T2 values and their correlation with diastolic function in new grafts remains poorly defined.

This prospective study evaluated 14 pediatric heart transplant recipients without acute rejection who underwent 22 non-contrast 1.5T CMR with their routine catheterization. New grafts (≤6 months; n=7) were compared to age-, sex- and body surface area-matched controls with established, established grafts (≥12 months; n=15). Comprehensive T1 and T2 mapping was performed and systolic/diastolic function were quantified with left atrial (LA) strain, volumes, and ejection fraction. Data were analyzed using linear mixed-effects models (LMM) and Hedge's g effect sizes.

Compared to established grafts, new grafts demonstrated significantly elevated Mean T1 (Δ 27 ms; p=0.005, g=1.25), Peak T1 (Δ 43 ms; p=0.02, g=1.03), and Mean T2 (Δ 2.6 ms; p=0.05, g=1.18). Longitudinal follow-up in three new grafts revealed temporal normalization of T1. Peak T2 (Δ2.5 ms; p=0.2, g=0.62) did not reach statistical significance. Functionally, new grafts exhibited trends of early diastolic impairment, including lower LA strain and higher indexed LA volumes. While these functional differences did not reach statistical significance, they demonstrated large, standardized effect sizes (g ≥ 0.8), suggesting directional trends that warrant larger investigation.

During the first 6 months post-transplant, pediatric grafts exhibit diffuse and focal T1 and T2 elevations accompanied by transient diastolic impairment. In this pilot study, these directional trends likely reflect expected ischemia-reperfusion injury, edema, and new graft remodelling rather than acute rejection. Utilizing graft-age-specific normative values is essential to avoid false-positive rejection diagnoses during early post-transplant CMR surveillance.