Researchers are already aware of the ravaging effect repeated concussions can have on the human brain, but a new report suggests less powerful strikes to the skull may pose significant risks as well, especially among children.

Scientists studied the brain activity of 25 boys between the ages of 8 and 13 before and after a single season of tackle football, and published their findings in Monday’s issue of the academic journal Radiology.

While none of the children concurred a concussion during the season, a comparison of their before and after brain scans revealed changes the researchers consider to be statistically significant.

Each one of the athletes underwent an MRI so researchers could map out the microstructural changes to their brains’ “white matter” — bundles of nerve fibers tasked with transmitting signals across the body’s central nervous system. A special type of imaging procedure was used to measure each athlete’s fractional anisotropy (FA), or the movement of water molecules in the brain.

Under normal circumstances, water movement is relatively uniform and given a high FA level, while lower levels have been tied to various abnormalities, including traumatic brain injuries (TBI), according to the researchers.

“Most investigators believe that concussions are bad for the brain, but what about the hundreds of head impacts during a season of football that don’t lead to a clinically diagnosed concussion? We wanted to see if cumulative sub-concussive head impacts have any effects on the developing brain,” said the study’s lead author, Christopher T. Whitlow, associate professor and chief of neuroradiology at Wake Forest School of Medicine in Winston-Salem, N.C.

“We found that these young players who experienced more cumulative head impact exposure had more changes in brain white matter, specifically decreased FA, in specific parts of the brain,” Mr. Whitlow said.

Similar changes have been linked to cases of mild TBI, the researcher noted, raising concerns about the health risks young athletes face even when they manage to avoid heavy hits to the head. Absent further studying, however, the scientists aren’t certain what the changes could mean.

“We’re seeing changes in the brain related to exposure, but if you talk to these players, if you look at their clinical imaging, there’s nothing you can identify that’s abnormal about them,” Mr. Whitlow said. “So the question becomes, what do these changes mean? And that we don’t have an answer to, yet.”

“Do these changes persist over time or do they just simply go away?” he told NBC News. “Do you get more changes with more seasons of play? And most importantly, do these changes result in any kind of long-term change in function like memory or attention or anything that would be important in your ability to function day to day?”

About 3 million youngsters play annually in organized youth tackle leagues in the U.S. — or about 2,000 athletes for every professional player in the NFL, according to the researchers.

“We do not know if there are important functional changes related to these findings, or if these effects will be associated with any negative long-term outcomes,” Dr. Whitlow said. “Football is a physical sport, and players may have many physical changes after a season of play that completely resolve. These changes in the brain may also simply resolve with little consequence. However, more research is needed to understand the meaning of these changes to the long-term health of our youngest athletes.

“There is more we don’t know about these changes than we do know,” he said.