He recalled the case of a 19-year-old hockey player whom he was asked to clear for return to play. Gradient echo imaging was normal, as were the athlete’s neurologic exam and impact test, but SWI revealed multiple areas of TAI.
Magnetic Resonance Spectroscopy
Sometimes referred to as a virtual biopsy, MRS detects changes in metabolites in the brain. Most notably, the technique identifies changes in N-acetylaspartate (NAA), a marker of neuronal and axonal health that declines in TBI.
In a study of professional soccer players with a baseline NAA:creatine ratio greater than 2, participants’ postconcussion ratio was less than 2. Although the athletes reported complete symptom resolution on day 3, brain imaging did not return to normal for one month or longer. Because of the lag time between symptom resolution and metabolic recovery, Dr. Dodick said, “I’m cautious, with children and youth in particular, about sending them back to play.”
Diffusion Tensor Imaging
DTI, another element of the Mayo Clinic’s concussion protocol, is based on the diffusivity of water, which is variably restricted in different tissues. In CSF, for example, diffusion of water is isotropic. In white matter, diffusion is highly anisotropic, and a reduction of fractional anisotropy (FA) correlates with the loss or destruction of white matter.
Multiple DTI studies have been conducted in the past 30 years. Despite various limitations, the findings provide striking evidence of small and subtle brain abnormalities in most athletes, said Dr. Dodick. A study comparing patients with mild TBI vs patients with moderate-to-severe TBI, for example, found a graded alteration in white matter integrity. Patients with mild TBI also had abnormalities.
Notably, evidence of abnormalities has been found even in the absence of symptomatic concussion. “You can alter brain structure and function without clinical symptomatology,” said Dr. Dodick.
Patients may ask whether these brain changes are reversible. Dr. Dodick described a patient who underwent DTI immediately after concussion and again 16 months later. At the time of the second imaging, the abnormal reduction of FA showed a loss of integrity. “That [injury] probably does not repair itself,” he said. At least in some cases, the changes can be persistent.
Current Imaging Can Guide Clinical Practice
“There is great potential for advanced brain imaging of concussion,” said Dr. Dodick, “but you can detect and characterize brain abnormalities that will guide your clinical decision making” here and now. Even if testing is limited to SWI, he added, it will identify many abnormalities that otherwise would be missed.
If in vivo testing for CTE is proven to be effective, “we’ll be able to premortem early detect those at risk for neurodegenerative disease,” said Dr. Dodick. “The holy grail, of course, is that this would be used in future research to evaluate interventions and new treatments.”