Physiological Basis for Potassium (K) 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, we compared K 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 (n526) or to 24 hours of permanent occlusion (n54). The hearts were then isolated and imaged by K MRI (n510), or tissue samples were analyzed for regional K content by MR spectroscopy (n59), K and Na concentrations by atomic emission spectroscopy (inductively coupled plasma atomic emission spectroscopy; n55), or intracellular K content by electron probe x-ray microanalysis (n56). Three-dimensional K MR images of the isolated hearts were acquired in 44 minutes with 33333–mm resolution. K MR image intensity was reduced in infarcted regions (51.764.8% of remote; P,0.001). The circumferential extent and location of regions of reduced K image intensity were correlated with those of infarcted regions defined histologically (r50.97 and r50.98, respectively). Compared with remote regions, tissue analysis revealed that infarcted regions had reduced K concentration (by MR spectroscopy, 40.569.3% of remote; P,0.001), reduced potassium-to-sodium ratio (by inductively coupled plasma atomic emission spectroscopy, 20.762.1% of remote; P,0.01), and reduced intracellular potassium (by electron probe x-ray microanalysis, K peak-to-background ratio 0.9560.32 versus 2.8661.10, respectively; P,0.01). We acquired the first K MR images of hearts subjected to infarction. In the pathophysiologies examined, potassium (K) MR image intensity primarily reflects regional intracellular K concentrations. (Circ Res. 1999;84:913-920.)