Introduction: MRI/MRS as Metabolic Imaging Tools
Metabolic imaging using magnetic resonance has its roots in a relatively obscure technology, used by physicists in the late 1940s to study the nuclear magnetic moments of nuclei. This technique was nuclear magnetic resonance (NMR), based on the original observations of Bloch and Purcell in 1946 that nuclei with a given spin could be studied by applying an oscillating radiofrequency field, at a frequency corresponding to energy difference between nuclear orientations. When this so-called “resonance” frequency is applied to matter, the resulting emitted signal is the basis for NMR and magnetic resonance imaging (MRI). The discovery most relevant to metabolic imaging was that of chemical shift, described in several publications in 1949–1950, and the consequence of the subtle changes in local magnetic field, resulting from electric shell interactions. This remarkable finding is the fundamental principle of magnetic resonance spectroscopy (MRS), whereby nuclei can be identified reliably depending on their chemical structure, independent of magnetic field strength. Today in the clinic, several key metabolites present in the brain are easily identified using MRS techniques. Changes in the frequency-specific metabolic map or “spectrum” are used to diagnose disease and monitor the effects of medical treatments, in cancer and other illnesses.
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