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(Circulation Research. 1999;84:913-920.)
© 1999 American Heart Association, Inc.

Original Contribution

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

David S. Fieno, Raymond J. Kim, Wolfgang G. Rehwald, Robert M. Judd

From the Feinberg Cardiovascular Research Institute and the Departments of Biomedical Engineering and Medicine, Northwestern University Medical School, Chicago, Ill.

Correspondence to Robert M. Judd, Feinberg Cardiovascular Research Institute, Northwestern University Medical School, 303 East Chicago Ave, Tarry 12-723 Chicago, IL 60611-3008. E-mail rjudd{at}nwu.edu

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, 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 3x3x3–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.

Key Words: K⁺ • MRI • infarction • viability • myocyte

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