*Purpose: We hypothesize that acute cellular rejection (ACR) episodes during the first year post heart transplant (HTx) may lead to structural and functional alterations of the myocardium, which can be non-invasively assessed by cardiac magnetic resonance imaging (CMR).

*Methods: HTx patients (50±13 years, 14 male) underwent 49 CMR scans during year one post-HTx (1-4 scans per patient). CMR included left ventricular (LV) T1 mapping for calculation of native T1 time, which is a measure for structural changes in the myocardium and is increased in the presence of inflammation or interstitial expansion. Biventricular cardiac motion was assessed using tissue phase mapping (TPM), a CMR modality that measures myocardial velocities in all three principal motion directions (long-axis, radial, circumferential) of left and right ventricle (LV/RV) over the course of the cardiac cycle. Positive values represent radial contraction and long-axis shortening. In addition, LV and RV ejection fractions (EF) were calculated by standard global function analysis using cine short-axis images. Rejection episodes were diagnosed by endomyocardial biopsy (EMB) and the number of ACR episodes preceding each CMR scan was recorded (2R counted twice).

*Results: Patients were diagnosed with a total of 115 mild (1R, 4.8±2.7 per patient) and 9 moderate (2R, 0.4±0.6 per patient) ACR episodes. Increasing number of ACR episodes was significantly associated with elevated LV T1 time (r=0.36,p=0.01) and with a reduction of biventricular peak velocities during systolic contraction, for the LV in long-axis (peak v_z, r=-0.37,p=0.01) and for the RV in radial (peak v_r, r=-0.32,p=0.02) direction. No significant correlations were found between biventricular EF and number of ACR episodes.

*Conclusions: Multiple ACR episodes occurring during the first year post-HTx show an impact on structure and systolic velocities of graft myocardium, whereas standard global function parameters are not affected. CMR is a valid tool for graft monitoring post-HTx and has the potential to valuably complement histological information provided by EMB in patients undergoing repetitive ACR episodes.

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