Introduction Manganese is a potent MRI contrast agent and can enter myocytes through l-type Calcium channels, making it an interesting imaging probe for Ca2 +channel activity.^{1 2} Mn2 +accumulates in the normal myocardium, reducing the longitudinal (T1) relaxation rate and increasing the relaxivity (R1) of the tissue. Hence, we hypothesised that Mn2 +mediated changes in T1 could reflect intracellular Ca2 +changes in the myocardium acutely after myocardial infarction (MI). By preloaded the heart with Mn2 +prior to coronary occlusion, we aimed to assess the efficacy of T1 mapping-Manganese-enhanced MRI (MEMRI) in quantifying intracellular Ca2 +response to ischaemic injury in a mouse model of MI.

Materials/methods In the MI group, adult male C57B1/6 mice received intraperitoneal injections of 0.10 mmol/kg MnCl2 40 min before left anterior descending (LAD) coronary artery occlusion. T1-mapping was used to monitor dynamic alterations of calcium influx at 1, 2 and 3 hours after LAD occlusion and at 2 days, with Mn2 +injected 100 min prior to MRI (Mn2 +loading time similar to the 1 hour post-MI group). Imaging was performed in the mid-short axis view as described^{3 4} using a Look-locker inversion recovery sequence: TE/TR=0.99/3 ms, effective TR=~3 s, TI=~100 ms intervals, FOV=25.6 mm, matrix=128, slice thickness=1.0 mm using a 9.4T MRI Agilent system. T1 maps were generated using MATLAB.⁵ For analysis R1 values (1/T1=the relaxivity of the tissue) were analysed from 3 tissue types: Infarcted hearts were classified into area at risk segments (AAR-MI, n=12) and viable segments (Viable-MI, n=12): These were compared to naïve control heart data (Viable-Naïve, n=12). R1 values of the LV blood pool were also measured.

Results As soon as 1 hour after LAD occlusion R1 values were significantly higher in the Viable-MI tissue compared with AAR-MI tissue (p<0.0001), allowing early delineation of the infarct region (figure 1, 2 and table 1). R1 in the Viable-MI tissue continued to rise at 2 and 3 hours post-MI (p=0.02 and p=0.01), but remained constant in the AAR-MI. When compared to naïve control animals, R1 values were slightly, but significantly increased in the AAR-MI tissue at 1 and 2 hours after occlusion (p=0.006 and p=0.03), while R1 was elevated to an even greater degree in the Viable-MI tissue (figure 1, 2 and table 1). The R1 values of LV blood pool in MI and Naïve groups did not change over the 3 hours. When the same animals were imaged at 2 days post-MI, R1 values were still significantly higher in the Viable-MI tissue compared with AAR-MI tissue (p=0.03). However at this point Viable-MI tissue had a similar R1 to naïve hearts, while R1 in the infarcted AAR-MI was lower than controls (p=0.03) (figure 1, 2 and table 1).

Discussion Acutely after ischaemic injury a large increase in R1 (reflecting increased Mn2 +uptake) occurred in the Viable-MI myocytes, while a small Mn2 +increase was also present in the AAR-MI. Increased R1 in the viable-MI myocytes is likely due to elevated catecholamine levels acutely post-MI leading to increased cardiac work and thus Ca2+/Mn2 +uptake. The increased R1 at 1 and 2 hours post-MI in the AAR-MI segments following ischaemic injury may be a result of calcium overload in the ischaemic tissue, which reduces over time as cell integrity is lost. By 2 days the catecholamine storm has passed and R1 levels in the surviving myocardium had normalised, while Mn2 +uptake in the dead infarct region was reduced due to lack of functional myocytes.

Conclusion The present study shows that T1-Mapping MEMRI allows sensitive in vivo detection of subtle changes of Mn2+in viable myocytes in the early phase after myocardial injury. Preloading of the myocardium with Mn2+prior to infarction makes this a sensitive approach which could be used to monitor imbalance in Ca2+homeostasis.

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