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Fig. 5C —Fourier transform (FT). MR spectroscopy. In contrast to MRI, which uses resonance frequencies and phase to encode an image, MR spectroscopy addresses a smaller region of interest (ROI) with a specific radiofrequency pulse bandwidth. Multiple neuronal metabolites (mI, myoinositol; Cho, choline; Cr, creatine; Glx, glutamate and glutamine; NAA, N-acetyl aspartate; Lac, lactate; Lip, lipid) resonate at characteristic frequencies on the basis of their unique chemical structure. The returning MR spectroscopy echo is a composite signal of many different echoes from metabolites in the ROI, which is resolved into individual resonance frequencies and their relative amplitudes (abundance) by the Fourier transform. The term "relative" is an important qualifier because the Fourier transform cannot measure the absolute nature of any frequency. The height of a peak in the MR spectroscopy Fourier spectrum makes sense only relative to another peak.