NMR Spectroscopy of the Human Heart

The heart is the largest consumer of energy per gram of tissue, and disruptions in energy metabolism, including energy supply and demand, are thought to play a central role in many common diseases affecting the heart. Therefore, it is perhaps inevitable that the first human cardiac spectroscopy focused on those energy metabolites that are NMR-detectable.1,2 Phosphorus (31P) NMR spectroscopy (MRS) can detect and measure adenosine triphosphate (ATP), the fundamental energy currency of the body,1 – 3 phosphocreatine (PCr), a cellular energy reserve, as well as the metabolic by-product, inorganic phosphate (Pi) in the heart. The chemical shift of Pi relative to PCr is pH-dependent, permitting a measure of intracellular pH.4,5 Unfortunately, the signal-to-noise ratio (SNR) of these metabolites is about 2 × 10−6 to 20 × 10−6 that of the proton (1H) NMR signal from tissue water from the same volume at the same magnetic field strength (B0), primarily due to the low metabolite concentrations, which typically fall in the range 1–20μmol g−1 wet weight of tissue (Table 1). PCr and ATP therefore cannot be imaged with the same spatial resolution, SNR and scan time as tissue water protons at the same B0. Nevertheless, with compromises, primarily to spatial resolution and to a lesser extent scan time, PCr and ATP can be measured in volume elements (voxels) as small as about 8 ml in the anterior myocardium.6,7 This, with SNR levels that are comparable to the SNR seen in the sub-microliter voxels routinely acquired by 1H MRI (Table 1).