69 ¹H-MR-SPECTROSCOPY OF THE BRAIN: IN-VTVO MEASUREMENT OF WATER COMPARTMENTS AND DETECTION OF WATER WITHIN MYELIN LAYERS

Abstract

AIM: Using differences of T₂-relaxation, ³H-MRS can measure intra- plus extracellular water within brain tissue (B in Tab. 1) and the water fraction of CSF and blood (C). In white matter (WM), a third water compartment (A) with a short T₂-relaxation is detected and considered to be enclosed between myelin layers. Measuring (A) is of potential pediatric interest because myelination during brain maturation might be observed. We present an in-vivo and noninvasive MRS-method to measure the three water compartments and the application in adult volunteers.

METH: 1.5 T-whole body system, adapted short-echo STEAM, voxel 2 cm³. Measuring T₂-decay: 22 acq. (TE: 7-10, 12, 15, 20, 25, 30-6O, 80, 100, 120, 160, 200, 280, 36O, 50O, 800, 1600ms), triexponential least-square fit. Measuring T₁-saturation (TR: 0.4, 0.55, 0.7, 0.9, 1.2, 2, 3s). 10 volunteers (29±3 y), WM: dorsc-lat. of post, ventr., n=10; grey matter (GM): occipital cortex, n=3.

RES: 1). Tests: a) H₂O phantom (0.15mM MnCl₂, 0.14 M NaCl): a single compartment is found (T₂=0.13s, T₁=0.97s). b) Water suppressed brain spectra do not show lipid resonances, comp. A reflects water. 2) Tab.1 shows mean±SD for T₂- and T₁-relaxations and for the fraction of the water compartments in WM. The triexponential fit (3 compartments) approximates best as compared to the mono- or biexponential fits (1 or 2 compartments).

3) Compartment A (water within myelin) is not observed in grey matter.

DISC: Using the presented spectroscopic MR-sequence. absolute sizes of water compartments and their T₂- and T₁-times may be precisely measured in white and grey matter. Assessment of the time course of compartment sizes and relaxation times is of interest to characterize brain development or white matter diseases.

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