T2 Mapping for Noninvasive Assessment of Interstitial Edema in Acute Cardiac Allograft Rejection in a Mouse Model of Heterotopic Heart Transplantation.

OBJECTIVES: Heart transplantation (HTX) in mice is used to characterize gene-deficient mice and to test new treatment strategies. The purpose was to establish noninvasive magnetic resonance imaging techniques in mice to monitor pathophysiological changes of the allograft during rejection.

MATERIALS AND METHODS: Magnetic resonance imaging was performed at baseline and days 1 and 6 after isogenic (n = 10, C57BL/6) and allogenic (n = 12, C57BL/6 to BALB/c) heterotopic HTX on a 7 T small animal scanner. Respiratory- and electrocardiogram-gated multislice multi-echo spin echo sequences were acquired, and parameter maps of T2 relaxation time were generated. T2 times in septal, anterior, lateral, and posterior myocardial segments as well as global T2 times were calculated and compared between groups. At day 7 animals were sacrificed and graft pathology was assessed by semiquantitative regional analysis and correlated with magnetic resonance imaging results.

RESULTS: Myocardial T2 relaxation time was significantly increased in allogenic (33.4 ± 0.1 ms) and isogenic cardiac grafts (31.8 ± 1.8 ms) on day 1 after HTX compared with healthy donor hearts at baseline (23.1 ± 0.3 ms, P < 0.001). Until day 6 after HTX, myocardial T2 further increased markedly in allografts but not in isografts (43.4 ± 1.9 vs 31.2 ± 1.1 ms, P < 0.001). Mean segmental T2 values as well as mean global T2 values in allogenic compared with isogenic cardiac grafts on day 6 were significantly higher (P < 0.01). Histologically, isogenic grafts were almost normal besides small focal leukocyte infiltrates and signs of interstitial edema, most likely due to ischemia reperfusion injury (histological sum score, 0.9 ± 0.4). In allogenic HTX, histology revealed severe inflammation and tissue edema representing allograft rejection with increased histological scores (5.3 ± 0.7, P < 0.001). Higher histological scores of rejection were significantly associated with increased T2 times on a segmental and a global level.

CONCLUSIONS: We could show that T2 mapping is a suitable noninvasive imaging method to monitor global and regional HTX pathologies in experimental heart transplantation in mice. Progressive prolongation of T2 time was significantly associated with pathological signs of rejection.