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.