Anisotropy of spin-echo T 2 relaxation by magnetic resonance imaging in the human brain in vivo

Abstract

BACKGROUND: The use of $T_{2}$ relaxation contrast, as measured by MRI, is particularly commonplace in non-invasive assessment of the brain. However, the mechanisms and uses of $T_{2}$ relaxation in the brain are still not fully understood. The hypothesis that $T_{2}$ relaxation may show anisotropy in the human brain was studied at 3 T. $T_{2}$ anisotropy refers to the variation of $T_{2}$ in ordered structures with respect to the direction of the applied magnetic field.

METHODS: Using a 3 T clinical MRI scanner, we made quantitative multi-contrast spin-echo $T_{2}$ and diffusion tensor imaging (DTI) measurements in healthy volunteers, repeating the measurements with the subject’s head oriented differently relative to the applied field, for the measurement of possible spin-echo $T_{2}$ anisotropy.

RESULTS: We report $T_{2}$ relaxation anisotropy measurements and present a means for visualising it according to the principal orientation of ordered structures in the brain parenchyma. We introduce a parameter for the model-free description of $T_{2}$ anisotropy, namely the $T_{2}$ “fractional anisotropy”, similar to that used to describe anisotropy of translational diffusion. This parameterisation enables the overall level of anisotropy in $T_{2}$ across a chosen region or tissue to be calculated. Anisotropic $T_{2}$ relaxation was observed in both gray and white matter, though to a greater extent in the latter, with a strong relationship with the anisotropy of translational diffusion. This is evidenced by making repeat measurements with the subject’s head tilted to different angles relative to the applied magnetic field, by which means we observed the $T_{2}$ at the same anatomical site to change.

CONCLUSIONS: Relaxation anisotropy has a significant effect on $T_{2}$ in the brain parenchyma. It has the potential to offer non-invasive access to tissue microstructure not available by other imaging modalities, and may be sensitive to pathology or noxious factors not detected by other means.

Keywords

MRI brain microstructure anisotropy

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