MRI Innovation Allows Detection of Subtle Traumatic Brain Injuries

DALLAS, May 12 -- Tracking progress and predicting outcomes after severe traumatic brain injury may be possible with an innovative MRI-based technique, researchers here said.

Called diffusion tensor tractography, the method offers an improved way to measure multifocal nerve damage within the brain after severe head trauma, according to Ramon Diaz-Arrastia, M.D., Ph.D., of the University of Texas Southwestern Medical Center, and colleagues in the May issue of Archives of Neurology.

In a case-control study involving 12 patients with head injuries and 12 healthy volunteers, the technique showed clear differences in axonal integrity within key brain regions, the researchers reported.
Action Points

* Explain to interested patients that the small study found that a novel MRI technique could measure an otherwise hard-to-detect form of brain injury that correlates with functional outcome.

* Explain that the case-control study involved only two dozen patients. The findings need to be confirmed in a larger study.

* Point out that the method is investigational and is not commonly available.

The scans also highlighted changes within the brain that correlated with patients' clinical condition for up to 11 months after injury, which had not previously been possible with imaging technologies.

In an interview, Dr. Diaz-Arrastia said the goal is to find a good measure for diffuse axonal injury, an "overlooked and understudied" phenomenon that increasingly appears to be an important factor in head-injured patients.

He said it used to be considered a rare form of injury, but mainly because it was hard to see with technologies such as CT imaging.

Recent studies suggest it plays a role in at least half of fatal head injuries," he added. Diffuse axonal injury may also explain puzzling cases in which people with apparently mild head injuries suffer lingering disability.

Diffusion tensor tractography measures water movement within tissues on the basis of MRI data. When nerve cell axons are damaged, as they frequently are in severe head trauma, they swell, absorbing water from surrounding tissues. That reduces movement of extracellular water.

On the other hand, when axons later die, they release water, thereby increasing extracellular water flows.

The method allows individual neuronal fibers to be measured and counted.

Dr. Diaz-Arrastia and colleagues used serial MRI scans to detect changes in fiber lengths, volumes, counts, and other parameters in the corpus callosum, the fornix, and the peduncular projections of the study participants.

Patients were adults with closed-head injuries and no history of neurological disorders or previous brain injury. The controls were matched for age and gender.

Clinical condition was assessed with the Glasgow Outcomes Scale-Extended by raters who were blind to the imaging data.

Significant differences between patients and controls in many fiber parameters were found immediately after injury.

"At least one fiber variable of the whole corpus callosum, all subareas of the corpus callosum and peduncular projections, and the fornix body were significantly different between the patients and controls, with patients showing worse measures (P< 0.005)," the researchers wrote.

Changes in fiber measures were also strongly correlated with outcome, with a Spearman rank coefficient of 0.76 (P<0.005) for at least one fiber variable of the whole corpus callosum and subareas 2 and 4 with Glasgow scores.

Overall, said Dr. Diaz-Arrastia, "we could explain 60% to 70% of the variance" in Glasgow scores with diffusion tensor tractography.

The researchers also analyzed the MRI data with another method that has been used to study diffuse axonal injury, called fluid attenuation and inversion recovery imaging. These findings did not correlate significantly with clinical outcomes.

The researchers pointed out that diffusion tensor tractography still has wrinkles to be ironed out. For example, the fiber assignment algorithm has limitations when dealing with crossing fibers.

They also noted that brain regions other than those included in this study may have diffuse axonal injury. The small sample size is a limitation as well.

Dr. Diaz-Arrastia said that, in addition to helping with prognosis, a reliable system for measuring diffuse axonal injury will be valuable in developing new treatments.

"When we develop therapies for this mechanism of injury, it will allow us to measure efficacy," he said.

The study was funded by the National Institute on Disability and Rehabilitation Research and the National Institutes of Health.

No potential conflicts of interest were reported.

Primary source: Archives of Neurology
Source reference:
Wang J, et al "Diffusion tensor tractography of traumatic diffuse axonal injury" Archives of Neurology 2008; 65: 619-26.

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