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This is the VOA Special English Health Report.

A concussion happens when the brain is shaken, often in a car crash or a fall or a strike to the head in sports. Concussions can be mild, but doctors may order a CT scan to look for a more serious injury. Computed tomography provides a detailed image of the brain. But a recent study warned that more children than necessary are being exposed to radiation this way.

A test for brain injury creates its own risks in children

A national team led by two doctors at the University of California, Davis, studied hospital records from thousands of children with head injuries. They found that in many cases, the risk of developing cancer from the radiation outweighed the risk of a serious brain injury.

The study found that one in five children over age two had a low risk of serious injury but received CT scans anyway. The same was true of almost one in four children under two years of age.

The researchers have developed rules to predict if a head injury is serious enough for a scan. For children under two, doctors are advised against it if there is:

Normal mental activity. No swelling in the back of the head. No feeling of a broken bone in the skull. And no loss of consciousness for more than five seconds. Doctors should also consider how the child was injured and whether the parents say the child is acting normally.

For patients from 2 to 18, the guidelines are similar -- except there should be no loss of consciousness, no vomiting and no severe headache.

The report appears in the Lancet medical journal.

Earlier this year, the British Journal of Sports Medicine published new guidelines for concussions in children and teens. International experts said they should not return to sports or school until fully recovered. The brain also needs a "cognitive rest," they say, by restricting activities like video games, texting and watching TV.

Young people often take longer than adults to recover from a concussion. The experts say individual progress and not a set time period should always guide a decision to return to play.

More than one million cases of concussion are reported each year in the United States alone.

A congressional committee has called a hearing Wednesday to discuss legal issues related to head injuries in football. Recently there have been concerns about players retired from the National Football League. But professionals are not the only ones getting concussions. There are high school players who have died from brain injuries.

And that's the VOA Special English Health Report, written by Caty Weaver. I'm Steve Ember.

tomography: any of several techniques for making detailed x-rays of a predetermined plane section of a solid object while blurring out the images of other planes X線體層照相術(shù)，X線斷層照相術(shù)