While recent advances in neuroimaging techniques have improved surgical planning in cortical areas, they have been unable to map white matter. As a result, tumor resection in subcortical regions presents a serious risk. It has been shown that injury to white matter tracts results in significant neurological deficits that correspond to injuries involving he corresponding cortex. As a result, understanding the relationship of white matter tracts to intraparenchymal brain lesions could provide useful information in surgical brain mapping. Diffusion tensor imaging (DTI) is capable of resolving individual pathways in the brainstem and individual nuclei in the thalamus (not surgically relevant). Thus, this technology can be used to locate white matter tracts, which separates it from other brain imaging techniques.
DTI uses diffusion vectors to measure the directionality of white matter tracts. Since the diffusion of water is limited by the presence of myelin and axonal membranes, the directionality of white matter tracts can be inferred from studies of water diffusion patterns. Magnetic field gradients are applied in multiple directions. A mathematically modeled matrix (diffusion tensor) is then used to determine the direction of greatest diffusion. The results are analyzed and used to create a principal eigenvector map, which parallels the local fiber direction. This map is visualized as a vector field and can be superimposed onto a structural image, such as a high resolution weighted MRI. Many surgically relevant white matter tracts have been anatomically defined. For example, the corticospinal and corticobulbar tracts (motor), arcuate fasciculus (language), optic radiations (visual) can be modeled by diffusion tensor imaging.
There are limitations to DTI technology. Unlike other brain imaging techniques, DTI cannot determine functionality. It also has a poor signal to noise ratio, cannot resolve crossing tracts, and cannot be used in areas close to air spaces.
One potential use of DTI in neurosurgery is to distinguish between infiltration and edema in the peritumoral region. Two quantities derived from the tensor, the FA and the MD, have been measured in this region. Fractional anisotropy, or FA, quantifies the anisotropy of water diffusion. The FA indirectly measures tissue organization (high FA corresponds to fibrous tissue with one orientation, and low FA is caused either by tissue disorganization such as in infiltration, by crossing fibers, or by high water content). Mean diffusivity (MD) quantifies the total amount of diffusion, which is mainly determined by the water content. MD is approximately constant in white and gray matter but increases with edema. A drop in FA relative to normal tissue can be caused by infiltration of tumor or by increased water content, hence the relationship between FA and MD in the peritumoral region is of interest.