Magnetic Resonance Spectroscopy of Skeletal Muscle
The interaction between nuclear magnetic moments and an external magnetic field gives rise to the phenomenon of nuclear magnetic resonance (NMR), which was discovered in 1946 by Bloch and Purcell The Larmor frequency (ν = γB eff ) depends on the gyromagnetic ratio (γ) of a particular nucleus and on the effective field strength (B eff ) at the spin site. As the electron cloud causes diamagnetic shielding of the external field, the resonance frequency is influenced by the chemical structure. In this way, many compounds of the same element may be identified by their— usually very small—chemical shift relative to a reference substance. In magnetic resonance imaging (MRI) the chemical shift between protons in water and in fatty tissue results in image artifacts at the tissue boundaries but can also be used for chemically selective excitation. While tissue characterization by MRI is based upon differences in the relaxation times T1 and T2 and in the spin density, magnetic resonance spectroscopy (MRS) investigates the distribution and the dynamic changes of biochemically important metabolites by analyzing the resonance lines in the frequency spectrum. Extremely high homogeneity of the magnetic field is needed, however, to obtain sufficient separation of the spectral components. As the frequency shift is directly proportional to the field strength, spectral resolution is improved by an increase of the magnetic field. This is especially important in 1H-MRS, where the chemical shifts are in the order of only a few parts per million in most cases.
KeywordsMagnetic Resonance Spectroscopy Nuclear Magnetic Resonance Spectroscopy Nicotinamide Adenine Dinucleotide Calf Muscle Fatty Degeneration
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