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National Center for Image Guided Therapy

Diffusion Tensor Imaging and Tractography

Fiber laterality indices and fiber laterality histograms in example subjects from each handedness group. Top row: individual subject fibers painted by their LIs, where blue and cyan are left-lateralized, red and yellow are right-lateralized, and green is not lateralized, i.e. it is symmetric. The inconsistent-handed subject, center, has relatively few asymmetric fibers as shown by the prevalence of green color. Bottom row: Fiber laterality histograms showing the distribution of LIs over all the fibers in the brain of each subject from the top row.

The Neurosurgical arm of the NCIGT is making strides in delineating the tracts of the human brain through tractography generated through diffusion tensor imaging (DTI) techniques. Through tractography, we are able to align anatomical understanding of brain tracts with an understanding of their functional activation through knowledge from fMRI.

The NCIGT's tractography efforts to date are now being translated into its surgical navigation software platform, 3D Slicer, to enable multiple types of displays of these views of the brain during operations. In order for surgeons to have optimal intraoperative information concerning white matter anatomy, we developed a platform that allows the intra-operative real-time querying of tractography datasets during frameless stereotactic neuronavigation. The seeded white matter tracts close to the lesion and other critical structures, as defined by the functional and structural images, were interactively visualized during the intervention to determine their spatial relationships relative to the lesion and critical cortical areas. Latency between tracking and visualization of tracts was less than a second for fiducial radius size of 4-5mm.

Our recently-developed novel interactive visualization tools for tractography allow placement of dynamic seedpoints for local selection of fibers, as well as fiber display around a segmented structure such as a tumor- both with tunable parameters. Interactive tractography successfully enabled inspection of white matter structures that were in proximity to lesions, critical structures, and functional cortical areas, allowing the surgeon to explore the relationships between them. Efforts with international collaborators are underway to extract and make viewable the full extent of the cortical spinal tract by using diffusion imaging data to perform two-tensor tractography, a method developed within the NCIGT.

The quantification of brain asymmetries may provide biomarkers for presurgical localization of language function and can improve our understanding of neural structure-function relationships in health and disease. Recent work has analyzed the relationship between white matter (structural) lateralization and gray matter (functional) lateralization of language. A new computational method was developed to assess whole-brain lateralization of the white matter fiber tracts and its relationship to handedness.

A recent milestone met by the Core was the development of a quantitative model of the biological source of the DTI signal. The more that is understood about the source of the DTI signal captured through MRI, the more that can be determined about neural water diffusion and tissue characteristics to improve our 3D understanding of brain anatomy.

New DTI methods also include estimating, from MRI scans of the brain, the restricted water in brain tissue from high b-value data so as to make decisions before surgery about what tissue is healthy, tumor or edematous. Used already, prior to surgery, with MRI scans of patients with brain tumors, a fast, high b-value, clinical DTI scan protocol yielded the restricted water fraction. NCIGT researchers theorize that in areas of suspected edema around tumors, if a population of restricted water still exists, some brain tissue may still be viable. An abstract of this work shows images of the scans following analysis.

Recently an automatic method was introduced called tract-based morphometry, or TBM, for the measurement and analysis of diffusion MRI data along white matter fiber tracts.

Tract-Based Morphometry for White Matter Group Analysis. Using subject-specific tractography bundle segmentations, we generate an arc length parameterization of the bundle with point correspondences across all fibers and all subjects, allowing tract-based measurement and analysis. The TBM approach brings analysis of DTI data into the clinically and neuroanatomically relevant framework of tract anatomy.





















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