A new study produced by professors at Washington University in St. Louis in collaboration with military researchers at the Landstuhl Regional Medical Center in Germany, to which service members injured in Iraq and Afghanistan are evacuated for treatment, may help explain why some military personnel exposed to blasts have symptoms of brain injury even though their CT and MRI scans look normal.

Using a highly sensitive type of magnetic resonance imaging, researchers studied 63 servicemen injured by explosions in Iraq or Afghanistan and found evidence of brain injuries in some that were too subtle to be detected by standard scans. All the men already had a diagnosis of mild traumatic brain injury (synonymous with concussion), based on symptoms like having lost consciousness in the blast, having no memory of it or feeling dazed immediately afterward.

About 320,000 American troops have sustained traumatic brain injuries in Iraq and Afghanistan, most of them mild, according to a 2008 report by the RAND Corp. The injuries are poorly understood, and sometimes produce lasting mental, physical and emotional problems.

"This sort of mild traumatic brain injury has been quite controversial," said Dr. David Brody, an author of the new study and an assistant professor of neurology at Washington University in St. Louis.

"Is it due to structural abnormalities in the brain, chemical dysregulation, psychological factors or all three? We show that at least in some there are structural abnormalities."

The pattern of the damage differed from that found in head injuries not caused by blasts, and matched computer simulations predicting how explosions would affect the brain, Brody said. If the new findings hold up, he added, they may eventually influence the design of helmets to provide more protection against blasts.

But Brody and other researchers cautioned that the study was only "a small first step." The study and an accompanying editorial were published online on Wednesday by The New England Journal of Medicine.

The special MRI technique, known as diffusion tensor imaging, is also being studied to help improve the diagnosis of concussions. It can be performed by most MRI machines and does not take longer or cost more than a standard MRI.

The test measures the movement of water in nerve fibers in the brain; abnormal flow may indicate injury. Changes can be detected in bundles of thousands of axons, the fibers that carry signals.

In 2008 and 2009, the researchers performed diffusion tensor imaging on 63 men who had recently sustained mild traumatic brain injuries from blasts; all but one had normal results on a standard MRI.

For comparison, 21 control subjects were also scanned — men exposed to blasts recently but with no symptoms of concussion.

Eighteen of the 63 men with traumatic brain injury had abnormalities consistent with nerve injury in two or more brain regions, areas not usually damaged by other types of mild head injury. The researchers said that only 2 of 63 healthy subjects would be expected to have such abnormalities. Twenty other men with traumatic brain injury had abnormalities in one area, and 25 had none.

"A negative scan, even with these advanced methods, does not rule out traumatic brain injury," Brody said.