Abstract-The potassium cation (K ϩ ) is fundamentally involved in myocyte metabolism. To explore the potential utility of direct MRI of the most abundant natural isotope of potassium, 39 K, we compared 39 K magnetic resonance (MR) image intensity with regional myocardial K ϩ concentrations after irreversible injury. Rabbits were subjected either to 40 minutes of in situ coronary artery occlusion and 1 hour of reperfusion (nϭ26) or to 24 hours of permanent occlusion (nϭ4). The hearts were then isolated and imaged by 39 K MRI (nϭ10), or tissue samples were analyzed for regional 39 K content by MR spectroscopy (nϭ9), K ϩ and Na ϩ concentrations by atomic emission spectroscopy (inductively coupled plasma atomic emission spectroscopy; nϭ5), or intracellular K ϩ content by electron probe x-ray microanalysis (nϭ6