Millions of children suit up every year to play youth football, but a new study raised concerns about the impact on their brains after playing a single season.

Researchers at Wake Forest Baptist Medical Center analyzed the number of hits to the head that a player likely received over the course of a season. They suited up 25 players with helmets with sensors inside to measure the frequency and severity of the impacts.

The researchers recorded every hit at every practice and in every game, monitoring to make sure they were contact hits and not players dropping a helmet.

Researchers analyzed brain images before and after the season.

Dr. Chris Whitlow, an associate professor and chief of neuroradiology at Wake Forest School of Medicine in Winston-Salem, North Carolina and the study's lead author, said there were changes in the brain’s white matter, which he defined as "the different wires that connect the different parts of the brain for function to take place."

The more hits the head took, the more change doctors said they saw.

"These are not changes you would be able to see with your naked eye," Whitlow said. "These are very, very subtle changes."

Whitlow said there are still many unanswered questions and parents shouldn't be alarmed.

"There's a lot of things we don't know," Whitlow said. "We don't know if they persist. We don't know if they go away after the season. We don't know if there are even more changes if people play multiple seasons."

Those answers could be years away. Researchers say more information is needed to understand whether the changes could lead to negative long-term health issues.