Myocardial magnetic resonance imaging contrast agent concentrations after reversible and irreversible ischemic injury

Background - Discrepant reports have been published recently regarding the relationship of contrast-enhanced magnetic resonance image intensities to reversible and irreversible ischemic injury. Unlike image intensities, contrast agent concentrations provide data independent of the MRI technique. We used electron probe x-ray microanalysis (EPXMA) to simultaneously examine concentrations of Gd, Na, P, S, Cl, K, and Ca over a range of myocardial injuries. Methods and Results - Reversible and irreversible injury were studied in 38 rabbits divided into 4 groups defined by occlusion and reperfusion time, as well as time the animals were euthanized. Gd-DTPA was administered, and the hearts were excised and rapidly frozen, cryosectioned, freeze-dried, and examined by EPXMA in up to 3 regions: remote, infarcted, and at risk but not infarcted. Infarcted regions were defined by anti-myoglobin antibody or triphenyltetrazolium chloride staining. Regions at risk were defined by fluorescent microparticles administered during occlusion. Compared with remote regions, in acutely infarcted regions, Gd was increased (235±24%, P<0.005) in the same 50×100-μm areas in which Na was increased (154±5%, P<0.001) and K was decreased (52±8%, P<0.001). Similarly, in chronically infarcted regions, Gd was increased (472±78%, P<0.001) in areas in which Na was increased (332±28%, P<0.001) and K was decreased (47±5%, P<0.001). Also compared with remote regions, however, concentrations of Gd, Na, and K were not elevated after reperfusion in regions that were at risk but not infarcted (P=NS). Conclusions - Regional elevations in myocardial MRI contrast agent concentrations are exclusively associated with irreversible ischemic injury defined histologically and by regional electrolyte concentrations.