Each year in the United States, well over 1 million people sustain a traumatic brain injury (TBI) and seek treatment at an emergency room in the United States, approximately 80% of which are “mild”[1] TBIs (mTBI). A mTBI involves a traumatically induced physiological disruption of brain function, as manifested by at least one of the following:
(1) any period of loss of consciousness;
(2) any loss of memory for events immediately before or after the event;
(3) any alteration in mental state at the time of the injury (ie, feeling dazed, disoriented or confused); and
(4) focal neurological deficits that may or may not be transient; but where the severity of the injury does not exceed
(a) loss of consciousness of approximately 30 minutes or less;
(b) after 30 minutes at Glasgow Coma Scale of 13/15; and
(c) post traumatic amnesia not greater than 24 hours.[2]
mTBIs are not diagnosed well with conventional computed tomography (CT) and magnetic resonance imaging (MRI) scans as those scans are not sensitive enough to detect diffuse axonal injuries (DAI), also called traumatic axonal injuries (TAI), which make up most brain injuries of a mTBI. For those unfortunate victims of mTBI whose cognitive, physical and/or behavioral symptoms do not resolve without radiological evidence to confirm a brain injury, has caused some to opine that the injury is not real and is more psychological in nature. However, applying neuroimaging tools that are actually sensitive enough to DAI/TAI, such as diffusion tensor imaging (DTI), can assist in confirming the diagnoses of mTBI, will likely lead to future research in evaluating and treating brain abnormalities in mTBI, and respond to the claim that the results of a mTBI can not cause functional impairments or ongoing residuals.
What is D.T.I.?
DTI is a MRI-based, neuroimaging technique which makes it possible to visualize the location, orientation and anisotropy of the brain’s white matter fiber tracts. By applying the appropriate magnetic field gradients, MR imaging may be sensitized to the random, thermally driven motion (diffusion) of water molecules in the direction of the field gradient. Diffusion is anisotropic (directionally dependent) in white matter fiber tracts, as axonal membranes and myelin sheaths present barriers to the motion of water molecules in directions not parallel to their own orientation. DTI uses the rate at which water diffuses between cells to gather information about the internal structures of the body. The diffusion rate varies around barriers between different structures in the body, and this trait can be used to create a complex and detailed map of internal structures with the assistance of DTI.
The science behind diffusion tensor imaging is based in physics, but put in the simplest of terms, this type of medical imaging involves agitating molecules with electromagnetic fields and recording the release of that energy. This data is weighted with known information about the rates of diffusion between different types of tissue to create a map of structures like nerves, muscles, and so forth. Essentially, a diffusion tensor image creates a “wiring map” of the body, allowing for the clear visualization of various pathways.[3]
Researchers take advantage of the fact that a diffusion tensor image reveals more information about the wiring of the brain than other types of scans. By looking at the network of connections at the core of the brain, a neuroradiologist and other medical specialists can identify areas of difference between subjects, potentially using this data to explain cognitive problems, behavioral-impulse difficulties, physical impairments, degenerative diseases, and other medical topics of interest.
Application
A recent study using DTI on the white matter skeleton in individuals with sports-related concussion detected structural changes with this type of injury. The study assessed white matter fiber tract integrity in varsity level college athletes with sports-related concussion without loss of consciousness, who experienced protracted symptoms for at least 1 month post injury (these athletes tested neuropsychologically in the abnormal range immediately after injury, and continued to present with symptoms for at least 1 month). Researchers evaluated the fractional anisotropy (FA) and mean diffusivity (MD) of the white matter skeleton using tract-based spatial statistics and found a large cluster of significantly increased MD for concussed subjects in several white matter fiber tracts in the left hemisphere, including parts of the interior/superior longitudinal and front-occipital fasciculi, the retrolenticular part of the internal capsule, and posterior thalamic and acoustic radiations. Researchers determined that qualitative comparison of average FA and MD suggested that, with the increased level of injury severity (ranging from sports-related concussion to severe TBI), MD might be more sensitive at detecting mild injury, whereas FA captures more severe injuries. The study concluded that DTI is sensitive enough to identify disruptions of white matter fiber tract integrity in individuals who sustained a sport-related concussion without loss of consciousness.[4]
Implications
TBI survivors include athletes, victims of assault workers injured on the job, occupants of motor vehicle collisions, individuals who have suffered a fall, pedestrians struck in a crosswalk, and veterans subject to blast injuries in Iraq and Afghanistan and many others. More importantly, these survivors are our parents, our daughters and sons, our neighbors, our colleagues. Tools such as DTI can not only assist in detecting brain injuries in mTBI cases to assist survivors and their family members and care givers access to necessary future care and rehabilitation, but this advanced imaging can reassure all that the injury is real and that future research in the areas of treating brain abnormalities in mTBIs is at hand.
[1] A label of “mild” suggests that the injury is minor or trivial. However, the word “mild” describes only the initial insult relative to the degree of neurological severity. The term “mild” usually does not relate to the degree of short or long term functional difficulties or disability.
[2] Mild Traumatic Brain Injury Committee of the American Congress of Rehabilitation Medicine.
[3] “Diffusion Tensor Imaging-Based Research on Human White Matter Anatomy,” Scientific World Journal, Volume 2012 (2012), doi: 10.1100/2012/530432
[4] “A Diffusion Tensor Imaging Study on the White Matter Skeleton in Individuals with Sports-Related Concussion,” Cubon, et al, February, 2011, J. Neurotrauma, doi: 10.1089/neu.2010.1430.
