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Measurement of Relaxation Times

In contrast to the proton nucleus (spin 1/2), the sodium nucleus has a spin of 3/2 and a quadrupole moment. It can have three nuclear transitions, each with a different relaxation rate. In solution the relaxation behaviour of sodium is mono exponential. In the presence of macro molecules the relaxation process becomes biexponential (due to the quadrupolar interaction). While intracellular tissue components relax biexponentially, extracellular tissue components mainly relax mono exponentially. In the future, the different relaxation rates may be used to separate intracellular from extracellular compartments, in order to get a deeper insight about underlying physiological and metabolic processes.

However, quantification of relaxation processes in vivo is difficult. First of all, the sodium concentration and NMR sensitivity is lower than in proton imaging. Compared to the proton nucleus, the acquired signal intensity is only on the order of 1/10000. Furthermore, the fast transverse relaxation leads to rapid signal decay after the excitation and therefore to low signal to noise ratio.

The aim of this project is to develop techniques to quantify T1 and T2 of the slow relaxing tissue components in the human brain in vivo.

Measurement of Relaxation TimesSpectroscopic T1- and T2 measurement of a sodium phantom (Sodium concentration c = 50mM, fitted T1: 55ms, T2: 53ms).