Physiological Basis for Potassium ( 39 K) Magnetic Resonance Imaging of the Heart

Abstract

Abstract —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, 39 K, we compared 39 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 (n=26) or to 24 hours of permanent occlusion (n=4). The hearts were then isolated and imaged by 39 K MRI (n=10), or tissue samples were analyzed for regional 39 K content by MR spectroscopy (n=9), K + and Na + concentrations by atomic emission spectroscopy (inductively coupled plasma atomic emission spectroscopy; n=5), or intracellular K + content by electron probe x-ray microanalysis (n=6). Three-dimensional 39 K MR images of the isolated hearts were acquired in 44 minutes with 3×3×3–mm resolution. 39 K 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 39 K 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 39 K 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 39 K MR images of hearts subjected to infarction. In the pathophysiologies examined, potassium ( 39 K) MR image intensity primarily reflects regional intracellular K + concentrations.