Relationship of Elevated 23 Na Magnetic Resonance Image Intensity to Infarct Size After Acute Reperfused Myocardial Infarction

BACKGROUND: Elevated 23Na MR image intensity after acute myocardial infarction has previously been shown to correspond to high tissue Na+ and loss of myocardial viability. In this study, we explored the potential of in vivo 23Na MRI to assess infarct size and investigated possible mechanisms for elevated 23Na image intensity. METHODS AND RESULTS: Thirteen dogs and 8 rabbits underwent in situ coronary artery occlusion and reperfusion and were imaged by 23Na MRI. For anatomically matched left ventricular short-axis cross sections (n=46), infarct size measured by in vivo 23Na MRI correlated well with triphenyltetrazolium chloride staining (r=0.87, y=0.92x+3.37, P<0.001). Elevated 23Na image intensity was observed in infarcted myocardium (206+/-37% of remote in dogs, P<0.001; 215+/-58% in rabbits, P<0.002) but was not observed after severe but reversible ischemic injury (101+/-11% of baseline, P=NS). High-resolution ex vivo imaging revealed that regions of elevated 23Na image intensity appeared to be identical to those of infarcted regions (r=0.97, y=0.92x+1.52, P<0.001). In infarcted regions, total tissue Na+ was elevated (89+/-12 versus 37+/-9 mmol/L in control tissue, 156+/-60% increase, P<0.001) and was associated with increased intracellular sodium (254+/-68% of control, P<0.005) and an increased intracellular sodium/potassium ratio (868+/-512% of control, P<0.002). Morphometric analysis demonstrated only a minor increase in extracellular volume (17+/-8% versus 14+/-5%, P<0.05) in the infarcted territory. CONCLUSIONS: Elevated 23Na MR image intensity in vivo measures infarct size after reperfused infarction in both a large and a small animal model. The mechanism of elevated 23Na image intensity is probably intracellular sodium accumulation secondary to loss of myocyte ionic homeostasis.