Fig. 3.

Minimally-invasive methods of human bioenergetic assessment. Near Infrared Spectroscopy (NIRS) operates on the principle that near-infrared light (700–900 nm) penetrates tissue with little scatter and is absorbed by heme-containing groups (e.g., hemoglobin, myoglobin, and the heme-containing prosthetic groups within the mitochondrial electron chain complexes) in an oxygen-dependent manner. Measurement of changes in absorbance can be used to reflect changes in tissue oxygenation and mitochondrial oxidative capacity. Magnetic Resonance Spectroscopy (MRS) utilizes magnetic resonant-visible isotopes (eg, ¹H, ³¹P and ¹³C), with each resonating at characteristic frequencies within a magnetic field. The distinct resonance of known isotopes creates a signature for identification of compounds within living tissues. MRS can distinguish products of glycolysis, creatine metabolism, choline metabolism, and amino acid metabolism. Positron Emission Tomography (PET) utilizes a radioactive isotope tracer such as glucose analogue 18-fluorodeoxyglucose ([¹⁸F]FDG) or other radio-labeled metabolites (acetate, choline, methionine or glutamine). The tracer is administered intravenously and becomes trapped within metabolically active cells. The nucleus of the isotope emits positrons as it decays. The positrons make contact with electrons to generate high-energy photons (gamma rays) that are detected by the PET camera and translated into an electrical signal to produce an image. The image that is produced is dependent on the metabolism of the cells/tissues that take up the isotope.