Windows on the working brain: magnetic resonance spectroscopy

Nuclear magnetic resonance (NMR) spectroscopy is an observational technique based on detection of signals from magnetic atomic nuclei such as 1 H, 31 P, 13 C, 15 N, and 17 O. It is most familiar to physicians and the public as magnetic resonance imaging (MRI), which uses the strong signal from water protons to make the most highly detailed pictures of living tissue available from any non-invasive method. In consequence, MRI, including its special forms magnetic resonance angiography, diffusion-weighted imaging, and magnetization transfer imaging – quickly became a major tool for medical diagnosis and research on living creatures. Its applications to neurological disease are described in several other chapters of this book. Magnetic resonance spectroscopy (MRS) is the designation used in the biomedical world for measurement of NMR signals from non-water protons and other magnetic nuclei. The usage is not accurate, MRI is the MRS of water, but it is convenient. MRS signals detectable in living brain are thousands of times weaker than the water proton signal; hence observing them requires extra time and special procedures. The reward for the effort is an abundance of chemically specific information which can be acquired as often as necessary, since the measurement process is non-invasive. In the living human brain, 1 H signals can be obtained from N -acetyl aspartate, creatine, choline moieties, glutamate, glutamine, lactate, and several other small molecules. Phosphocreatine, adenosine triphosphate, and inorganic phosphate can be measured directly by their 31 P signals, and intracellular pH calculated from its effect on these signals. Information from the 31 P spectrum allows calculation of the rate of the creatine kinase reaction. The spectra of 13 C, 15 N, 17 O, and other magnetic nuclei contain many more small signals from a variety of molecules which will become detectable as technology advances. This unprecedented measurement capability provides an opportunity for characterization of human neurological diseases along several axes of chemical variation throughout their natural histories. The data are obtained without hazard to the patient, are free from artefacts of tissue preparation, and can be compared in as much detail as necessary to identically acquired information from normal subjects. As MRS matures technically over the first decades of the twenty-first century, it can be expected to take a place among the principal technologies contributing to illumination of disease processes and evaluation of new treatments.

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