The issue of head injuries has been a topic of debate in sports such as hockey, football and boxing. Now, researchers are raising concerns about the brain health of players in another sport: soccer.

Researchers in the U.S. recently completed brains scans on soccer players who regularly "head" the ball and say they found subtle signs of brain damage in many of them -- raising concerns about whether the practice is safe, especially in chldren.

Using a form of brain imaging called diffusion tensor imaging, researchers from the Albert Einstein College of Medicine at Yeshiva University in New York studied 38 amateur soccer players with an average age of 31, all of whom had played soccer since childhood.

They found that those players who said they head the ball frequently had changes in their brains that were similar to those seen in traumatic brain injury patients.

Dr. Michael Lipton and his team were looking for something called fractional anisotropy, which is the movement of water molecules along brain axons. In healthy white matter brain tissue, the direction of water movement is fairly uniform and measures high in FA. When water movement is more random, FA values decrease.

Players who headed most frequently had significantly lower FA in brain regions that are responsible for attention, memory, executive functioning.

In a related study, Dr. Lipton and colleague Molly Zimmerman gave the same 38 players memory tests and found that the players with the highest annual heading frequency performed worse on tests of verbal memory and psychomotor speed, which are activities that require mind-body coordination, like throwing a ball, relative to their peers.

Lipton says the findings are worrying but further study is needed to determine what the long-term consequences of excessive head hits might be.

"At this point in time, we clearly have evidence that heading may be related to changes similar to brain injury, but it is not such hard and clear information that we can make a clear recommendation about heading," Lipton said.

When players hit the ball with their heads, it's not an impact that breaks nerve fibres in the brain. But repetitive heading could set off responses that lead to brain cell degeneration.

The results of the study were presented at the annual meeting of the Radiological Society of North America and have not been peer-reviewed.

The findings are concerning given that soccer is such a popular sport, especially among children.

Three years ago, brain injury specialists with the non-profit group ThinkFirst identified heading as a possible safety issue, and drew up guidelines on how to headhit the ball safely. Their number one rule was not to teach the skill too early to children.

"So anybody out there who's listening to this who has a child less than 10, do not teach that child to head the ball. Then if they are of the appropriate age, which is 10 to 12, then they can start learning how to head the ball with the appropriate sized ball," ThinkFirst founder Dr. Charles Tator tells CTV News.

Since heading is an integral part of soccer and unlikely to be eliminated from the game, Lipton and his colleagues want to determine if they can find a safe threshold of heading hits.

So far, they've found that players who said they headhit the ball more than 1,000 to 1,500 head per year were among those who had the most significant signs of brain injury. While 1,000 times a year may seem high, it amounts to only a few times a day for a regular player.

With a report from CTV medical specialist Avis Favaro and producer Elizabeth St. Philip