Tracking neuronal fiber pathways in the living human brain

Functional imaging with positron emission tomography and functional MRI has revolutionized studies of the human brain. Understanding the organization of brain systems, especially those used for cognition, remains limited, how ever, because no methods currently exist for noninvasive tracking of neuron al connections between functional regions [:Crick, F & Jones, E, (1993) Nat ure (London) 361, 109-110]. Detailed connectivities have been studied in an imals through invasive tracer techniques, but these invasive studies cannot be done in humans, and animal results cannot always be extrapolated to hum an systems. We have developed noninvasive neuronal fiber tracking for use i n living humans, utilizing the unique ability of MRI to characterize water diffusion, We reconstructed fiber trajectories throughout the brain by trac king the direction of fastest diffusion (the fiber direction) from a grid o f seed points, and then selected tracks that join anatomically or functiona lly (functional MRI) defined regions. We demonstrate diffusion tracking of fiber bundles in a variety of white matter classes with examples in the cor pus callosum, geniculo-calcarine, and subcortical association pathways, Tra cks covered long distances, navigated through divergences and tight curves, and manifested topological separations in the geniculo-calcarine tract con sistent with tracer studies in animals and retinotopy studies in humans. Ad ditionally, previously undescribed topologies were revealed in the other pa thways. This approach enhances the power of modern imaging by enabling stud y of fiber connections among anatomically and functionally defined brain re gions in individual human subjects.