The Role of Diffusion Tensor Imaging in Traumatic Brain Injury

Over 2.5 million Americans live with disabilities related to traumatic brain injury (TBI).  For these patients, neurobehavioral deficits are the most common cause of disability.  The neuropathology of TBI often involves diffuse axonal injury, or damage to the white matter of the brain that occurs when the head is rapidly accelerated or decelerated.  Diffuse axonal injury is difficult to detect using traditional imaging techniques, but in some cases of TBI, it is the only significant pathology.  Diffusion tensor imaging (DTI) is a version of magnetic resonance imaging (MRI) that is being explored as a means of imaging the white matter of the brain.  Researchers hope that DTI will someday become part of a more standardized methodology for evaluating and treating patients who have experienced TBI.

DTI takes advantage of the fact that water molecules in the brain diffuse at different rates in different tissues.  In more structured tissues, water molecules diffuse more slowly.  By measuring the rate of diffusion in many directions from a single point, one can calculate a tensor for that point, and by calculating tensors for many different points, one can create a three-dimensional image of the brain’s structure.  An important value for any given point in the brain is the fractional anisotrophy of that point.  Fractional anisotrphy values range from zero to one, and the higher the value, the more ordered the tissue.  From this information, it is possible to mathematically recreate much of the brain’s neuroanatomy.

To study the usefulness of DTI, researchers led by Marilyn Kraus, MD, of the University of Chicago studied 18 healthy volunteers and 37 people with varying degrees of TBI.  They analyzed 13 areas of the brain, and they found that the fractional anisotrophy value for each area was lower for patients with moderate to severe TBI than for healthy volunteers.  This difference seems to reflect structural damage in the brains of the patients with TBI.  The researchers also found the white matter load, a gross measure of white matter damage, increased with the severity of TBI.  Finally, they noticed differences in radial and axonal diffusivity that may someday help scientists differentiate between myelin-related problems and direct axonal injury.  Taken together, these results suggest that DTI is a useful technique for evaluating the severity of TBI.           

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