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, ³⁹K, we compared ³⁹K 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 ₃₉K 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 K⁺ content by electron probe x-ray microanalysis (n=6). Three-dimensional ³⁹K MR images of the isolated hearts were acquired in 44 minutes with 3 x 3 x 3-mm resolution. ³⁹K 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 ³⁹K 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 ³⁹K 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 ³⁹K MR images of hearts subjected to infarction. In the pathophysiologies examined, potassium (³⁹K) MR image intensity primarily reflects regional intracellular K⁺ concentrations.