High school football players can undergo significant brain changes after only a single season— even if they don’t get a concussion, Wake Forest University researchers have found.

Scientists said the ongoing study of 24 students, ages 16 to 18, from a Winston-Salem, N.C. high school, is the largest and most comprehensive research of its kind.

“There’s been a lot of interest in NFL (National Football League) football and head impacts, and it’s gotten a lot of press,” study author Christopher T. Whitlow, associate professor of radiology at Wake Forest School of Medicine and radiologist at Wake Forest Baptist Medical Center, told FoxNews.com.

“But for every one NFL player, there [are] 2,000 high school players. Seventy percent of people playing football are adolescents, and it’s a really understudied population,” he said.

Researchers used Head Impact Telemetry System (HITs) helmet-mounted accelerometers to measure the frequency and severity of the players’ helmet impacts during practices and games. They then used the HITs data— which included the number of hits, as well as each hit’s magnitude and direction— to calculate each athlete’s risk-weighted exposure. Each player’s risk-weighted exposure was the product of the number of hits multiplied by the risk of concussion.

“The system is like an accelerometer because [the helmets] measure g-forces and acceleration, and go in the crown of the helmet and in the gap in the padding,” study author Joel Stitzel, professor and chair of biomedical engineering at Wake Forest University, told FoxNews.com.

Based on cumulative data from the HITs, study authors divided the students into two groups: heavy hitters and light hitters. Among the risk-weighted exposure data, the heavy hitters were among the top 25 percentile, and the light hitters were among the lower 25 percentile.

Whitlow said the heaviest hitters exhibited the most brain changes, while the light hitters had fewer brain changes at the end of the season.

“The important thing to note is although we’re seeing these changes, none of these players are experiencing any common symptoms of concussion,” Whitlow said.

To quantify how these hits affected the brain, researchers analyzed players using an advanced MRI technique called diffusion tensor imaging (DTI) pre- and post-season. DTI identifies microstructural changes in the brain’s white matter— an area comprised of millions of nerve fibers called axons that connect various regions of the organ.

DTI measures fractional anisotropy (FA), or the movement of water molecules along these axons. A uniform direction of the water molecules indicates healthy white matter, while an arbitrary movement indicates decreased FA— or microstructural abnormalities. Decreased FA was most prevalent among the heavy-hitter group, the study authors noted.

“The kinds of changes we’re seeing aren’t visible to the naked eye, so there’s no clinical correlation we can make,” said Whitlow, who cautioned that these findings are preliminary. He said further study would explore the potential long-term consequences of repeated high-impact exposure in contact sports.

“We don’t know if there’s an association between these changes and cognitive function or behavior,” Whitlow said, “but those are all the kinds of things we’re going to be pursuing.”

According to the Centers for Disease Control and Prevention (CDC), each year, U.S. emergency departments treat an estimated 173,285 sports- and recreation-related traumatic brain injuries, or TBIs— including concussions— in youth and adolescents up to age 19. Emergency visits for TBIs have increased by 60 percent over the past decade, and more than 248,000 of these cases were reported in 2009. These cases include those among other contact sports, not just football.

“When you look at other youth sports— lacrosse, hockey, basketball, cheerleading for girls— there’s a lot of work that needs to be done. The methodology of this study is transferrable,” Robert Gfeller, executive director of the Childress Institute for Pediatric Trauma, told FoxNews.com. The institute helps fund the data collection portion of the study.

Gfeller’s son Matthew, 15, died two days after getting hit and knocked out during his first varsity high school football game as a linebacker. Doctors said he suffered from significant subdural brain bleeding. Following his death in August 2008, the Gfeller family established the Matthew Alan Gfeller Foundation in Matthew’s memory.

In light of concussion-related injuries and deaths like Matthew’s, football leagues have already implemented some changes to reduce players’ exposure to high-impact hits.

Because this level of exposure is risky during kickoffs, the kickoff line from youth sports leagues to the NFL has been reduced by 5 yards. In youth sports, some blocking and tackling drills have been ruled out, and the number of days that players can have direct contact has been adjusted.

In a separate portion of the research on youth football players, the study authors observed that two-thirds of hits among this age group occurred during practices, not games. For high school football players, about 56 percent of hits occurred in practices. The researchers haven’t yet analyzed whether more time spent on the field at practice is a primary factor for these differences, Stitzel said.

But one cause may be that there’s no hard-and-fast safety helmet rating system for youth players, while there is for adult helmets, which high school football players use, Gfeller said. The National Operating Committee on Standards for Athletic Equipment (NOCSAE) uses a pass-fail safety system to certify adult football helmets.

A separate system, developed in 2011 by researchers at Wake Forest and Virginia Polytechnic Institute and State University, goes a step further. Called the STAR evaluation system, the methodology of the Virginia Tech Helmet Ratings ™, it uses a five-star rating to indicate the ability of different helmets to reduce accelerations of normal, multi-directional impacts in football.

“All over the country there have been lots of school districts that have replaced lower-star ratings with higher-rated helmets,” Stitzel said.

Another element the researchers plan to explore is how the presence of trained medical personnel at practices and games may impact the treatment of concussions when they do occur.

While many high school and NFL teams have athletic trainers and sports medicine physicians on the sidelines during games, most youth sports leagues have neither, Stitzel said. These personnel help maintain protocol in the event of an injury, and they identify concussion symptoms and determine when an athlete can safely resume play.

“At high schools across the country, some have athletic trainers at games but not practices,” Stitzel said. “In youth sports leagues, virtually no one has trained medical personnel on the sidelines to look for concussions in kids.”

According to the CDC, Washington was the first U.S. state to pass a concussion in sports law in 2009. Since then, all 50 states and as the District of Columbia have passed concussions in sports laws, often called Return to Play laws.

However, while these rules are often defining at the high-school level, they are less consistent among youth leagues, Gfeller said.

While the study is ongoing with its term ending in 2019, the researchers’ HITs findings will be presented Monday at the annual meeting of the Radiological Society of North America (RSNA).

“Our objective hasn’t been to say that football is bad— it’s to look at ways to increase the safety of the game,” Stitzel said. “We know we can do that through equipment, but we can also do that through rule changes, and better diagnosis and treatment.”