A new study may help explain why some military personnel exposed to blasts have symptoms of brain injury even though their CT and M.R.I. scans look normal. Using a highly sensitive type of magnetic resonance imaging, researchers studied 63 servicemen wounded 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 Corporation. The injuries are poorly understood, and sometimes produce lasting mental, physical and emotional problems.