T MR Spectroscopy6

M. Tosetti, T. ScHiRmeR, V. d'Alesio, A. Di CostAnzo, T. ScARAbino

Magnetic resonance spectroscopy (MRS) is a non-invasive technique that can be used to measure the concentrations of different low-molecular weight chemicals. The technique is based on the same physical principles as magnetic resonance imaging (MRI), i.e. the detection of energy exchanges between external magnetic fields and specific nuclei within atoms. The principal differences between MRI and MRS lie in the different use of frequency, phase and signal amplitude as carriers of information:

• In MRI, frequency and phase are used to encode the spatial coordinates, while the signal amplitude of the signal is translated into a grey value of the resulting image (Fig. 6.1a).

• In MRS, phase and frequency are used to identify spectral patterns unique to specific metabolites, while the amplitude is used as a scale for the concentration of these metabolites (Fig. 6.1b).

The information obtained during MRS experiments, usually acquired as a series of FIDs, spin echoes or stimulated echoes in the time domain, is displayed graphically as a spectrum in the frequency domain with individual peaks representing the various chemical compounds. The diagnostic ability of MRS can be increased by improving the spectral quality through changes in hardware and software, and/or by improving the analytical approach aiming for objective absolute concentration measurements.

MRS should be performed as an adjunct to MRI gain additional information for a reliable clinical diagnosis: while conventional MRI provides anatomical images of the brain, MRS provides functional information related to its underlying dynamic physiology.

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