Diffusion tensor imaging (DTI)
Diffusion properties of water molecules in brain tissue measured by MRI can give distinct information about fiber integrity and connectivity. Use of diffusion sensitive gradients in many directions makes it possible to quantify degree of diffusion anisotropy in a given tissue, usually described in terms of the fractional anisotropy (FA). FA is assumed to be related to the integrity of fiber myelinization. Multi-directional diffusion analysis is referred to as DTI, reflecting the fact that the tissue diffusion properties can be described both in terms of magnitude and direction in 3D space (as a tensor) (see figure).
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Left picture shows the FA map, right shows the eigenvector map, indicating directionality of diffusion in each voxel in a plane. Green indicates diffusion along the anterior-posterior, red along the lateral-medial, and blue along the dorsal-ventral axis. Data from a young person participating in a project in the Oslo-lab. |
DTI yields an in vivo metric related to the microstructure of the brain’s white matter (WM), thus making it possible to quantify characteristics of specific fiber tracts connecting brain structures. This is highly relevant, given that even simple cognitive tasks involve a complex interplay between multiple brain areas, and that the integrity of the connections between the areas involved thus may be related to cognitive function. Other scanning techniques (structural MRI, functional MRI, event-related potentials) yield information about brain areas that correlate with cognitive performance. However, it is reason to believe that the connections between the different areas are as important for cognitive function as the areas themselves (Cardenas et al., 2005). Disconnection of cortical circuits by decreased white matter integrity has been proposed as a general mechanism of age-related decline in cognition (Bartzokis et al., 2004), a hypothesis that can only be tested in vivo with DTI.
The organization of brain functions in complex neural networks warrants a study of the integrity of the connections between brain structures. DTI offers the possibility of looking into WM micro-structure, and mapping neuroanatomical pathways based on diffusion correlations represents a promising, novel tool for cognitive and clinical neuroscience. Studies reporting robust correlations between DTI data and cognitive function are now emerging, but the method has too rarely been applied to study cognition. The paradigm relating diffusion and cognitive function will enable investigators to identify the specific WM pathways which regulate individual differences in performance on specific tasks, also in clinical condition. Salat et al., 2005ab have demonstrated reduction in FA in specific brain areas with age, and Tuch, Salat et al. (2005) have correlations between FA and reaction time. Preliminary data from a development project in our lab in Oslo (n = 25, age 8-13 yrs) indicate that FA in specific regions correlate with neuropsychological measures (see fig below). DTI is also sensitive to clinical conditions, e.g have four reported studies found DTI differences between MCIs and controls (Kantarci et al., 2001; Muller et al, 2005; Fellgiebel et al. 2004,2005).
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Scatterplots showing the relationship between FA in a specific brain region (left) and cognitive function
