In the medical field, imaging (MRI) is based on the measurement of the nuclear magnetism of substances that form biological tissues. This exploration technique comes from a physical phenomenon understood since 1946 (Bloch, Science 118(3068):425–430, 1953; Purcell, Science 118(3068):431–436, 1953), called nuclear magnetic resonance (NMR), which was first used in the fields of chemistry and biochemistry, through spectroscopy. Unlike other medical imaging methods, various physicochemical variables are involved in the formation of the magnetic resonance image, whereas, for example, in XCT, the only physical parameter involved in the formation of the image is the absorption coefficient of the X-ray beam. This feature is both an advantage and a disadvantage. The measurement of several parameters is richer because it makes it possible to obtain additional information but it complicates the interpretation of images. After presenting the physical principles of nuclear magnetic resonance, we will present techniques for measuring parameters that characterize the structure (white matter), the morphology (gray matter), and the cellular metabolism in the brain.
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