Assessment of relaxation times, MT effects and diffusion characteristics

ASSESSMENT OF RELAXATION TIMES, MT EFFECTS AND DIFFUSION CHARACTERISTICS 

Relaxation times, MT ratios, and diffusion properties allow insight into the microstructure of various tissues. Determination of these parameters is possible by recording and analysing of a series of volume selective spectra, even for metabolites with relatively low concentrations in vivo. For recording series of spectra usually one parameter is changeable (e.g., inversion time TI for Ti measurements, echo time TE for T2 measurements, MT preparation for assessment of spin transfer and chemical reaction rates, or diffusion sensitizing gradients for assessment of apparent diffusion coefficients or even diffusion 

longitudinal and transverse relaxation of protons in tissue depend on the microstructure and on the chemical composition of several microscopic compartments. Relaxation properties are not necessarily constant for the diiferent compartments inside the cells (cytosol and cavities in cell organella) and in the extracellular space (interstitium and vessels). However, water exchange processes between the compartments are often fast enough to generate one effective relaxation time, which can be assessed by monoexponential fitting of the relaxation dependent data. 

Mainly the water resonance can often be used as an indicator for inflammations or tumours, because effective relaxation times tend to be prolonged in tissue with increasing interstitial or intracellular water content. As reported in Section 3.2, lipids (mainly consisting of triglycerides) reveal more stable relaxation characteristics in all tissues and no significant alterations have been reported so far. 

Spectroscopic relaxometry performed in human muscles of the lower leg is described in the following sections. 

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Assessment of Relaxation Times, MT Effects and Diffusion Characteristics 35... 

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