Diffusion tensor imaging (DTI) allows a live look into the microstructure of white matter in the brain and is an important complement to volumetric studies of specific structures such as the amygdala.
DTI may be particularly informative for the study of autism because it has been speculated that white matter (the connections between neurons) defects may be even more pronounced than gray matter defects for affected individuals. Furthermore, in contrast to gray matter, white matter volume continues to increase across childhood and adolescence, thus allowing for analyses of growth curves and changes specific to the microstructure of axons. (See papers by Courchesne, et al., 2000; Durston, et al., 2001; Giedd, et al., 1999) for more information about the development of white matter across development.
What Do DTI Studies Tell us About Autism?
Given an abnormal brain growth trajectory in autism, what profile of white matter microstructure, as characterized by diffusivity and FA values, would be predicted? During the first phase of brain growth in autism, namely the overgrowth phase, there is no clear prediction. That is, FA values could theoretically be either increased, reflecting greater number of axons and increased myelination, or decreased, reflecting poorly organized fiber tracts within cortex particularly in the frontal lobes.
In the only study to use DTI to examine white matter during a time period that coincides with early brain overgrowth, Ben Bashat et al. (2007) examined FA and diffusivity indices (displacement) in seven toddlers with confirmed autism ranging in age from 1.8 to 3 years. Contrasting DTI values were also obtained from a sample of over 40 typically developing infants and children.
Although the autism sample size was small, results were remarkably consistent – 100% of the toddlers with autism had FA values that exceeded the normal mean in some brain regions such as the forceps minor region (the anterior portion of the corpus callosum that extends into the frontal lobes). Region of interest analyses revealed that the largest FA increases were in the genu of the corpus callosum, posterior limb of the internal capsule and corticospinal tracts.
The authors interpret their findings as evidence of accelerated maturation of white matter in autism. Although it is only one study, results from Ben Bashat et al . (2007) suggest that FA values in particular may be increased during the overgrowth phase of brain development in autism.
Advanced visualization of DTI data can be obtained via a method commonly referred to as “tractography” or “DTI fiber tracking.” Fiber tracking uses the diffusion tensor of each voxel to follow an axonal tract in three dimensions from voxel to voxel through the human brain.
Because DTI provides only microstructural information at relatively low spatial resolution, DTI fiber tracking is often combined with functional and/or higher resolution anatomic information to delineate specific pathways. Currently there are no published tractography studies in infants and young children with autism.
The UCSD Autism Center of Excellence has studies under way to examine white matter structure in infants at risk for autism using both DTI and tractography. Tractography may be an excellent mechanism to visualize aberrant white matter tract projections in infants at risk for autism.