Fast ²³Na magnetic resonance imaging of acute reperfused myocardial infarction: Potential to assess myocardial viability

Background: The ability of the myocyte to maintain an ionic concentration gradient is perhaps the best indication of myocardial viability. We studied the relationship of ²³Na MRI intensity to viability and explored the potential of fast-imaging techniques to reduce ²³Na imaging times in rabbits and dogs. Methods and Results: Eighteen rabbits underwent in situ coronary artery occlusion and reperfusion. The hearts were then either imaged following isolation and perfusion with cardioplegic solution (n=6), imaged in vivo (n=6), or analyzed for ²³Na content and relaxation times (n= 12). Normal rabbits (n= 6) and dogs (n=4) were imaged to examine the effect of animal size on ²³Na image quality. ²³Na imaging times were 7, 11, and 4 minutes for isolated rabbits, in vivo rabbits, and in vivo dogs, respectively. Infarcted, reperfused regions, identified by triphenyltetrazolium chloride staining, showed a significant elevation in ²³Na image intensity compared with viable regions (isolated, 42±5%. P<.02; in vivo, 95±6%, P<.001), consistent with increased tissue sodium content. Similarly, ²³Na MR spectroscopy showed that [Na⁺] was higher in nonviable than viable myocardium (isolated, 99±4 versus 61±2 mmol/L; in vivo, 91±2 versus 38±1 mmol/L; P<.001 for both). Image signal-to-noise ratios were higher in dogs than rabbits despite shorter imaging times, primarily due to larger voxels. Conclusions: Following acute infarction with reperfusion, a regional increase in ²³Na MR image intensity is associated with nonviable myocardium. Fast gradient-echo imaging techniques reduce ²³Na imaging times to a few minutes, suggesting that ²³Na MR imaging has the potential to become a useful experimental and clinical tool.