Longitudinal assessment of repetitive blast mild traumatic brain injury in veterans using FDG-PET imaging

Objectives Repeat mild brain injuries (mTBI) lead to an increased risk of neurodegeneration and dementia. This research is to investigate if hypometabolic regions in brains of veterans that had previous repeat blast mTBIs are stable over time in longitudinal follow up imaging. We hypothesized that the downstream consequences of mTBI that may lead to neurodegenerative processes could be in part due to persistent hypometabolism.

Methods Positron emission tomography with [¹⁸F]-fluorodeoxyglucose (FDG-PET) imaging was used to assess brain metabolism in 13 veterans (mTBI) with mean age of 31.7 ± 8.0 (range 24-49) years. Mean time between scan 1 and scan 2 was 3.73 ± 0.8 (range 2.2-4.7) years. Age-matched community controls (n=9, 28.0 ± 8.1, range 20-45 years) were used for a normal database. Images were anatomically standardized to atlas, normalized to global values and the brain was segmented into 19 independent anatomical regions (NEUROSTAT).

Results Metabolism in 3 regions in mTBI subjects were more than 2 standard deviations below the mean of controls: (10/13 right frontal, 9/13 right medial frontal, and 6/13 left medial frontal and these regions resolved to within the normal range by scan 2 in 9/10, 7/9 and 5/6 subjects (78-89%). We also found one region, left frontal that was hypometabolic in 3 subjects and became further decreased by scan 2 in 2/3 (67%). In total 11/13 mTBI subjects had 1 or more hypometabolic frontal region that returned to normal values by scan 2. Interestingly, 7 of those 11 subjects had scattered hypometabolic regions in scan 2 that were not found in scan 1.

Conclusions These results indicate that hypometabolic frontal regions in veterans with multiple mTBIs may be normalizing with time and may be attributed to functional reorganization as indicated by scattered hypometabolic regions found only in scan 2 in most of the subjects. These findings might be due to compensatory mechanisms related to a functional metabolic reserve in the brains of mTBI subjects.

Research Support This study was supported by a VA Merit grant