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Background: The diagnosis of acute rejection in cardiac transplant recipients requires invasive technique with endomyocardial biopsy (EMB) which has risks and limitations. Cardiovascular magnetic resonance imaging (CMR) with T2 and T1 mapping is a promising technique for characterizing myocardial tissue. The purpose of the study was to evaluate T2, T1 and extracellular volume fraction (ECV) quantification as novel tissue markers to diagnose acute rejection. Methods: CMR was prospectively performed in 20 heart transplant patients providing 31 comparisons EMB-CMR. CMR was performed close to EMB. Images were acquired on a 1.5 Tesla scanner including T2 mapping (T2 prepared balanced steady state free precession) and T1 mapping (modified Look-Locker inversion recovery sequences: MOLLI) at basal, mid and apical level in short axis view. Global and segmental T2 and T1 values were measured before and 15 min (for T1 mapping) after contrast administration. Results: Acute rejection was diagnosed in seven patients: six cellular rejections (4 grade IR, 2 grade 2R) and one antibody mediated rejection. Patients with acute rejection had significantly higher global T2 values at 3 levels: 58.5 ms [55.0–60.3] vs 51.3 ms [49.5–55.2] (p = 0.007) at basal; 55.7 ms [54.0–59.7] vs 51.8 ms [50.1–53.6] (p = 0.002) at median and 58.2 ms [54.0–63.7] vs 53.6 ms [50.8–57.4] (p = 0.026) at apical level. The area under the curve (AUC) for each level was 0.83, 0.79 and 0.78 respectively. Patients with acute rejection had significantly higher ECV at basal level: 34.2% [32.8–37.4] vs 27.4% [24.6–30.6] (p = 0.006). The AUC for basal level was 0.84. The sensitivity, specificity and diagnosis accuracy for basal T2 (cut off: 57.7 ms) were 71, 96 and 90% respectively; and for basal ECV: (cut off 32%) were 86, 85 and 85% respectively. Combining basal T2 and basal ECV allowed diagnosing all acute rejection and avoiding 63% of EMB. Conclusions: In heart transplant patients, a combined CMR approach using T2 mapping and ECV quantification provides a high diagnostic accuracy for acute rejection diagnosis and could potentially decrease the number of routine EMB. [ABSTRACT FROM AUTHOR]