MR spectroscopy of the human heart: the status and the challenges

Noninvasive measurements of high-energy phosphate metabolism in the anterior myocardium of heart patients are now possible with image-guided, localized nuclear magnetic resonance (MR) spectroscopy. The results, reviewed herein, are largely consistent with those of prior animal studies. Quantification with phosphorus-31 MR yields normal phosphocreatine (PCr) and adenosine triphosphate (ATP) concentrations of about 11 and 6 mumol per gram wet weight, respectively, with a PCr/ATP ratio of around 1.8. Studies of patients with hypertrophic and dilated cardiomyopathy, left ventricular hypertrophy, valve disease, transplanted hearts, myocardial infarction, or reversible ischemia reveal abnormalities in the PCr/ATP ratio and/or the metabolite concentrations. Differences in reported findings for cardiomyopathies might be attributable to statistical sensitivity and the presence of heart failure. The technique might find use in the clinic for identifying failure when other factors complicate diagnosis. The PCr/ATP ratio is often reduced in transplanted hearts but is not a reliable predictor of histologic rejection involving myocyte necrosis. In myocardial infarction, metabolite levels may be reduced while the remaining PCr and ATP signals likely reflect surrounding surviving tissue. Stress-test studies of anterior myocardial ischemia produce transient reductions in the PCr/ATP ratio, which appear to be specific for ischemic disease. This may lead to a new way of assessing ischemia, particularly if the technology can gain access to a larger portion of the heart. Cardiac spectroscopy with nuclei other than P-31 shows promise.