Tractography

Tractography is a set of neuroimaging techniques that infer the pathways of white matter tracts in the living human brain from diffusion-weighted magnetic resonance imaging (dMRI). By modeling the diffusion of water, which tends to follow axonal bundles, tractography creates three-dimensional representations of putative fiber pathways, often visualized as streamlines or probabilistic connectivity maps.

Two broad families of tractography exist: deterministic and probabilistic. Deterministic tractography follows the principal diffusion direction voxel by voxel to produce continuous fiber pathways, while probabilistic approaches estimate a distribution of possible directions at each voxel and generate multiple plausible pathways to reflect uncertainty. More advanced methods, such as constrained spherical deconvolution or other higher-order models, aim to resolve crossing fibers and complex configurations.

Data acquisition involves diffusion-weighted MRI with multiple gradient directions and b-values; preprocessing includes motion and eddy current correction, susceptibility distortion correction, and brain extraction. The resulting models may be based on diffusion tensor imaging (DTI) or higher-order representations, influencing accuracy and interpretability.

Limitations include that tractography infers connectivity rather than providing direct evidence of synaptic connections or directionality. It is sensitive to noise, crossing fibers, partial volume effects, and model assumptions, which can yield false positives or negatives. Tractography should be interpreted as a probabilistic map of white matter organization rather than a definitive wiring diagram.

Applications include study of brain networks and connectivity in health and disease, and pre-surgical planning to identify critical tracts to avoid during tumor resection or other interventions. It complements but does not replace histological tract-tracing in animals, and results should be validated with additional evidence when possible.