Physiological basis for potassium (39K) magnetic resonance imaging of the heart

The potassium cation (K+) is fundamentally involved in myocyte metabolism. To explore the potential utility of direct MRI of the most abundant natural isotope of potassium, K-39, We compared K-39 magnetic resonance (MR) image intensity with regional myocardial K+ concentrations after irreversible injury. Rabbits were subjected either to 40 minutes of in situ coronary artery occlusion and 1 hour of reperfusion (n=26) or to 24 hours of permanent occlusion (n=4). The hearts were then isolated and imaged by K-39 MRI (n=10), or tissue samples were analyzed for regional 39K content by MR spectroscopy (n=9), K+ and Na+ concentrations by atomic emission spectroscopy (inductively coupled plasma atomic emission spectroscopy; n=5), or intracellular KC content by electron probe x-ray microanalysis (n=6). Three-dimensional K-39 MR images of the isolated hearts were acquired in 44 minutes with 3 X 3 X 3-mm resolution. K-39 MR image intensity was reduced in infarcted regions (51.7+/-4.8% of remote; P<0.001). The circumferential extent and location of regions of reduced K-39 image intensity were correlated with those of infarcted regions defined histologically (r=0.97 and r= 0.98, respectively). Compared with remote regions, tissue analysis revealed that infarcted regions had reduced K-39 concentration (by MR spectroscopy, 40.5+/-9.3% of remote; P<0.001), reduced potassium-to-sodium ratio (by inductively coupled plasma atomic emission spectroscopy, 20.7+/-2.1% of remote; P<0.01), and reduced intracellular potassium (by electron probe x-ray microanalysis, K+ peak-to-background ratio 0.95+/-0.32 versus 2.86+/-1.10, respectively; P<0.01). We acquired the first K-39 MR images of hearts subjected to infarction. In the pathophysiologies examined, potassium (K-39) MR image intensity primarily reflects regional intracellular K+ concentrations.