Metabolic imaging of the human heart: clinical application of magnetic resonance spectroscopy

Cardiovascular MRI has earned its place in the field of clinical cardiac imaging. Regularly used techniques include anatomical imaging, functional imaging, perfusion, and delayed enhancement (DE). Cardiac magnetic resonance spectroscopy (MRS) uses the same hardware, measuring the abundance of metabolites in the myocardium in vivo non-invasively without the use of radiation or external tracers. Its main application is currently scientific to gain insight into metabolic changes in cardiac pathologies. The heart is a metabolically active organ using on average 6 kg of adenosine triphosphate (ATP) each day. As energy is crucial for both systole and diastole, derangements in energy metabolism may be the first step in failure of the heart.1 By using the gyromagnetic properties of 1H, 31P, 13C, and 23Na, MRS is a powerful tool to relate energy metabolism to (dys)function of the heart. The aim of this article is to provide an overview of the current use, opportunities and limitations of MRS in relation to common cardiac diseases: ischaemic heart disease, heart failure, inherited cardiomyopathy, the metabolic syndrome, valvular heart disease, and heart transplantation. A simplified schematic representation of cardiac energy metabolism and opportunities to assess components of metabolism with MRS is depicted in figure 1. On average, the heart cycles 10 tons of blood each day in 100 000 heart beats. To meet the enormous ATP requirement, cardiomyocytes fuel themselves with free fatty acids (FFA) and glucose as the primary source of chemical energy. FFA and glucose contribute to ATP synthesis in terms of supply of chemical energy in a ratio of 3:1 in normal situations. Derangements in substrate utilisation are associated with a wide variety of diseases which will be discussed below. The uptake of FFA by the fatty acid transporter is an energy consuming process. Fatty acids enter the mitochondrion where …