Too many young children are getting CT scans after a minor bump on the head, and a new set of guidelines may help doctors sort out which children need them and which do not, US researchers said.

CT or computed tomography scans are a souped-up form of X-ray that can spot internal injuries, but CT scans expose children to radiation, which can increase the risk of cancer.

"Now we have much better evidence to assist with making decisions regarding CT use," Nathan Kuppermann of the University of California Davis Children's Hospital and lead author of a study said. The study appears in the journal Lancet.

Kuppermann helped lead a team of researchers who analyzed data on more than 42,000 children with head injuries.

CT scans were done in nearly 15,000 of those patients. Serious brain injuries were diagnosed in 376 children, and 60 children underwent brain surgery.

Of these, the team found one in five children over age 2, or about 20 percent, got a CT scan that was not needed because they were at low risk of having a serious brain injury.

In children under age two, nearly one in four or 25 percent got a head CT that was not needed.

Based on their analysis, the team developed a set of rules for spotting mild injuries that do not need CT scans. In children under two, low-risk injuries involved no scalp swelling, no significant loss of consciousness, no palpable skull fracture. These children acted normally and had an injury that was sustained in a non-severe way, such as a minor fall.

Severe accidents, which excluded children from the low-risk group, included car crashes in which the patient was ejected from the vehicle.

In children older than two, those at low risk had normal mental status, no loss of consciousness, no vomiting, no signs of fracture of the base of skull and no severe headache. Their injuries occurred in a non-severe way.

The team applied these criteria to data from a second group of more than 8,000 children and found they were 99.9 percent accurate at predicting low-risk injuries.

A study in the New England Journal of Medicine last month cited CT scans as one of the two biggest contributors to radiation exposure. In that three-year study of nearly 1 million Americans aged 18 to 64, the researchers found as many as 4 million Americans a year are exposed to what they viewed as high doses of radiation. - Reuters