FAST NA-23 MAGNETIC-RESONANCE-IMAGING OF ACUTE REPERFUSED MYOCARDIAL-INFARCTION - POTENTIAL TO ASSESS MYOCARDIAL VIABILITY

Background The ability of the myocyte to maintain an ionic concentrati on gradient is perhaps the best indication of myocardial viability. We studied the relationship of Na-23 MRI intensity to viability and expl ored the potential of fast-imaging techniques to reduce Na-23 imaging times in rabbits and dogs. Methods and Results Eighteen rabbits underw ent in situ coronary artery occlusion and reperfusion. The hearts were then either imaged following isolation and perfusion with cardioplegi c solution (n=6), imaged in vivo (n=6), or analyzed for Na-23 content and relaxation times (n=12). Normal rabbits (n=6) and dogs (n=4) were imaged to examine the effect of animal size on Na-23 image quality. Na -23 imaging times were 7, 11, and 4 minutes for isolated rabbits, in v ivo rabbits, and in vivo dogs, respectively. Infarcted, reperfused reg ions, identified by triphenyltetrazolium chloride staining, showed a s ignificant elevation in Na-23 image intensity compared with viable reg ions (isolated, 42+/-5%, P<.02; in vivo, 95+/-6%, P<.001), consistent with increased tissue sodium content. Similarly, Na-23 MR spectroscopy showed that [Na+] was higher in nonviable than viable myocardium (iso lated, 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 limes, primarily due to larger v oxels. Conclusions Following acute infarction with reperfusion, a regi onal increase in Na-23 MR image intensity is associated with nonviable myocardium. Fast gradient-echo imaging techniques reduce Na-23 imagin g times to a few minutes, suggesting that Na-23 MR imaging has the pot ential to become a useful experimental and clinical tool.