Almost all parts of the body are anatomically well understood, but the human brain is a notable exception.  The complexity of the brain alone cannot account for this paucity of information because the brain essentially consists of only about 400 “neural organs” and approximately 1000 major pathways.  The real obstacle to understanding the brain’s anatomy is the fact that the structures in the brain overlap in three-dimensional space.  Because of this overlap, traditional techniques such as dissection and microscopy will never be able to untangle the brain’s anatomy.  To understand the brain’s pathways, one must have a way to image the brain in more than three dimensions.  One technique capable of accomplishing this “de-overlapping” is diffusion spectrum MRI (DSI). 

DSI builds on traditional MRI imaging by incorporating additional mathematics.  It replaces ordinary space with position-oriented space.  Like diffusion tensor imaging (DTI), DSI measures the diffusion of water molecules in the brain.  Each point, or voxel, is described in six dimensions, three spatial dimensions and three dimensions for diffusion.  Using this information, one can perform tractography, which involves connecting voxels in the direction of maximal diffusion.  The images produced provide a fairly accurate map of the brain’s the neural pathways. 

Researchers in the laboratory of Van Wedeen, MD, of Harvard Medical School are investigating the potential usefulness of DSI.  They have imaged ex vivo brains from a number of species, and in these images, they have been able to identify a number of known neuronal structures, such as the optic nerve.  In vivo imaging is also possible, but it is trickier and provides less resolution than ex vivo imaging.

Theoretically, DSI should be able to map all the major connections within the brain, and the results obtained by Dr. Wedeen’s team have led to a number of intriguing insights.  Certain pathways in the brain seem to reflect the brain’s developmental history, and with modifications, DSI can be used to delineate different parts of the cortex.  In addition, it seems that brain is composed of a number of hubs, including one region in the medial parietal area thought to be a “default” region that is active when one is thinking about nothing in particular.  Although DSI is in its infancy as a technology and a science, it is hoped that the technique will prove useful as a tool for basic scientists and for clinical diagnosticians.  

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