About a third of American troops who have suffered brain injury from bomb blasts show immediate evidence of stretched and damaged nerve fibers at both the front and the back of the brain, according to a new study.

Whether that observation will prove useful in identifying people at higher risk for depression, post-traumatic stress and thinking problems after a blast injury is not known.

Nevertheless, experts say the findings, reported Wednesday in the New England Journal of Medicine, shed some light on the unusually murky subject of traumatic brain injury (TBI).

“This is not a pregnancy test for TBI,” said Alicia Crowder, a neuroscientist at the Defense and Veterans Brain Injury Center, who was not involved in the research. “This is a foundation block on which to build a body of knowledge. It is a piece of a puzzle.”

TBI is common among troops in the Iraq and Afghanistan wars, where most wounds are caused by explosions, not bullets. Estimates of its prevalence vary widely. The Defense Department is uncertain how many troops have received TBI diagnoses either in the war zone or within a month of leaving it, but it believes the number is less than 50,000. A group of civilian researchers, however, says there may be as many as 320,000 sufferers.

Part of the reason for the uncertainty is TBI’s wide range of symptoms and their overlap with psychiatric disorders. Researchers are urgently searching for ways to more accurately diagnose TBI, predict its course and differentiate it from post-traumatic stress disorder, which often occurs simultaneously.

The new study involved a collaboration between researchers at Washington University School of Medicine in St. Louis and clinicians at Landstuhl Regional Medical Center, a huge military hospital in Germany where all evacuated combat casualties stop on their way back to the United States.

The researchers did magnetic resonance imaging (MRI) scans on 63 service members — all men, with an average age of 24 — who had diagnoses of mild TBI. Some had brief loss of consciousness and post-blast confusion, and lacked memory of the event.

Significantly, all suffered some other head trauma, such as hitting the roof of a vehicle or being struck by a flying object, at the time of the blast. Many troops exposed only to bomb blasts recuperate in the combat zone and are not injured enough to be evacuated to Landstuhl.

The brain scans of the TBI patients were compared with scans of troops with other wounds, mostly ones of the arms or legs.

The researchers used an MRI method that looked for damage to axons, the thin fibers that connect nerve cells and that often run inches in length. In 18 of the 63 TBI patients, there was distinct damage to axons in two areas of the brain, both soon after the blast and at least six months later, when the scans were repeated. When axons are damaged, communication between different regions of the brain suffers.

The two areas that the study found were most affected are near the opposite poles of the brain and near bony structures that could damage them in a blast. These areas are also involved in functions that change after TBI — emotional regulation in the case of a region called the orbitofrontal area, and balance and organization in the case of the other area, the cerebellar peduncles.

The researchers didn’t describe the symptoms of the patients with the abnormal scans. Consequently, it’s not possible to say whether there is a connection between the apparently damaged areas and behavioral effects, said Christine L. Mac Donald, the Washington University researcher who headed the study with a colleague, David L. Brody.

“We’re still investigating what the implications of these results are to functional outcome,” she said. A larger study, with more patients, is underway.

Ultimately, military physicians would like to have a combination of methods — including brain scans, blood tests and physical examination maneuvers — that identify bomb blast victims at highest risk for severe or permanent disability. Those patients then might get different or more intensive treatment than people whose brain trauma is less severe.