Diffusion Tensor Imaging

Diffusion tensor imaging (DTI) is a magnetic resonance technique that is sensitive to the diffusion of water in brain tissue, thus revealing the anisotropy and orientation of white matter tracts in the brain. Myelin or protein fibers likely account for diffusion anisotropy. Measurement of ADCs along six independent axes of the tensor provides the shape of the diffusion ellipsoid, that is, fully characterizes diffusion in three dimensions. Mean diffusivity (MD) and fractional anisotropy (FA) can be calculated for each pixel, representing the magnitude and directionality of water diffusion within that pixel. Once fiber direction is known in each pixel, a three-dimensional (3-D) map can be generated that depicts patterns of connectivity throughout the brain. Display of the effects of mass lesions on large nerve fiber tracts by DTI can be used for preoperative planning. Specifically, fiber mapping enables visualization of subcortical fiber tracts that are important in motor function.

Although many studies have demonstrated the usefulness of diffusion tensor imaging in the delineation of tumor infiltration, vascularity, and cellularity55-57 (Figure 27.9), in the differentiation between peritumoral edema and tumor infiltration,58 and in the differentiation between metastasis and primary brain tumors,59 more studies and well-organized clinical trials need to be performed before fiber tracking can be integrated into a neurosurgical planning system. Newer techniques decreasing the time of acquisition and minimizing image distortion are being investigated60 and could help in making diffusion tensor imaging applicable for routine clinical practice.

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