Imaging of White Matter Pathways - Structural Connectivity 

The white matter (Figure 1) posse about 60% of the human brain volume and is mostly composed of fibers connecting to the cortex (1). Study of the architecture and functionality of these fibers was limited to post mortem and lesion studies. In vivo virtual dissection of white matter fibers (tractography) based on magnetic resonance diffusion tensor imaging (DTI) was developed in the last years (2,3). Already this method became a leading tool for white matter specific imaging including the studying of normal brain connectivity (click on Figure 1 for more details) as well as pathological conditions involving the white matter (4).

Diffusion tensor imaging measures the diffusion in several directions in order to estimate the micron-scale diffusivity in 3D (the diffusion tensor) (3,5-10). This estimation of the 3D diffusivity became useful for characterization of white matter. In contrast to gray matter where the 3D diffusion is spherical, in white matter the 3D diffusion has a cylindrical shape (due to the geometrical arrangement of the fibers) (Figure 2). Intuitively, the diffusion along the fibers should be faster than that perpendicular to the fibers giving rise to a phenomenon called diffusion anisotropy (8,9,11). DTI enables quantification of the diffusion anisotropy of which the most popular index is the fractional anisotropy (FA) which approaches zero for a sphere and one for a cylinder (8) (Figure 2).

DTI provides for each image pixel the averaged 3D alignment of white matter fibers that lead several groups to develop algorithm that can estimate the 3D path of specific white matter fiber bundles (in the process termed tractrography, see Figure 1) (2,3,12-14). Fiber tracking or tractography provided with 3D maps of white matter tracts allowing the visual differentiation between distinct functional and anatomical systems (2,15). Example for that is given in Figure 3 where the superior longitudinal fascicle is connecting the major language related areas in the left hemisphere. It should be noted that DTI fails to reconstruct white matter orientations in areas where several fiber fascicles cross or insect.

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