During surgery for brain lesions, injury to critical white matter tracts must be avoided as this is actually more likely to leave the patient with permanent neurological deficits than even injury to the cortex (Naeser et al. 1989). However, demonstrating critical white matter tracts has been particularly difficult as white matter tracts are not visualizable on conventional imaging and there is no reliable way to test for their presence, even using invasive intra-operative testing Diffusion Tensor Imaging (DTI), an MRI-based technique based on measurement of water diffusion, is able to demonstrate WM anatomy including location and trajectory of white matter tracts, information that cannot be obtained with any other technique. In DTI the orientation of a fiber tract is modeled with a 3x3 symmetric tensor whose major eigenvector gives the principal diffusion direction. DTI tractography follows the major eigenvectors to estimate the trajectories of the fiber tracts and has been widely applied for neurosurgery. We have developed DTI methods to define white matter anatomy in healthy subjects and in patients with brain tumors. However due to factors including crossing fibers, tract displacement, and edema, the full anatomical extent of many tracts is underestimated. Localization of tracts is usually performed using regions of interest (ROIs) drawn perpendicular to the tract, however this method is only reproducible in a subset of tracts and requires additional ROIs in surgical subjects.