Noninvasive quantification of total sodium concentrations in acute reperfused myocardial infarction using ²³Na MRI

The transport of sodium and potassium between the intra- and extracellular pools and the maintenance of the transmembrane concentration gradients are important to cell function and integrity. The early disruption of the sodium pump in myocardial infarction in response to the exhaustion of energy reserves following ischemia and reperfusion results in increased intracellular (and thus total) sodium levels. In this study a method for noninvasively quantifying myocardial sodium levels directly from sodium (²³Na) MRI is presented. It was used to measure total myocardial sodium on a clinical 1.5T system in six normal dogs and five dogs with experimentally-induced myocardial infarction (MI). The technique was validated by comparing total sodium content measured by ²³Na MRI with that measured by atomic absorption spectrophotometry (AAS) in biopsied tissue. Total sodium measured by ²³Na MRI was significantly elevated in regions of infarction (81.3 ± 14.3 mmol/kg wet wt, mean ± SD) compared to noninfarcted myocardial tissue from both infarcted dogs (36.2 ± 1.1, P < 0.001) and from normal controls (34.4 ± 2.8, P < 0.0001). Myocardial tissue sodium content as measured by ²³Na MRI did not vary regionally in the lateral, anterior, or inferior regions in normal hearts (ANOVA, P = NS). Sodium content measured by ²³Na MRI agreed with the mean AAS estimates of 31.3 ± 5.6 mmol/kg wet wt (P = NS) in normal hearts, and did not differ significantly from AAS measurements in MI (P = NS). Thus, local tissue sodium levels can be accurately quantified noninvasively using ²³Na MRI in normal and acutely reperfused MI. The detection of regional myocardial sodium elevations may help differentiate viable from nonviable, infarcted tissue.