Normal white matter composition.

NORMAL WHITE MATTER

      The white matter of the brain is located in the central and subcortical regions of the cerebral and cerebellar hemispheres and accounts for about 60 % of the total brain volume. The white matter includes the major commissural tracts, the cortical association fibers, and all the cortical afferent and efferent fibers. Histologically, the white matter contains nerve fibers, supporting cells, interstitial space, and vascular structures. White matter consists mostly of axons with their envelope of myelin, along with two types of neuroglia: oligo-dendrocytes and astrocytes. Axons are extensions of neurons that reside within the gray matter of the brain, spinal cord, and ganglia. The myelin is produced and maintained by oligodendrocytes. Myelin functions as an insulator of the axons, and its structure facilitates rapid transmission of impulses. 

      Myelin has relatively short T₂ and T₁ relaxation times, primarily owing to its lipid content. As a result, normal myelin is hypointense to gray matter on T₂-weighted images and hyperintense on T₁-weighted images. If a disease process reduces the myelin content, the white matter becomes less hydrophobic and takes on more water. Less myelin and more water protons prolong the relaxation times of both T₁ and T₂, resulting in more signal on T₂-weighted and less signal on T₁-weighted images.