The trend is raising concern because children may have an increased cancer risk from repeated radiation exposure.
The study, the first large review of X-rays, CT scans and other imaging tests in children, found 42 percent had one radiation test and one-quarter had two or more from 2005 to 2007, according to reports.
More troublesome was the finding that nearly 8 percent of kids got one CT scan and 3.5 percent got two or more CT scans, which offer very detailed images but involve much more radiation than X-rays.
"That's particularly concerning," the study's lead author, Dr. Adam Dorfman of the University of Michigan Medical School in Ann Arbor, told The Associated Press. "Today's children are undoubtedly getting many more of these studies than previous generations."
About 85 percent of the imaging tests in the study were X-rays, and the AP said most were of the chest, hand and foot. The most common CT scans were of the head and stomach, the AP reported, noting that hospitals use them to check for head injuries and appendicitis.
The study, published Monday in the Archives of Pediatrics and Adolescent Medicine, was based on health insurance records of 355,000 children in Arizona, Florida, Texas and Wisconsin.
The dangers of radiation may be greater in children than in adults, Dorfman said.
"We know that exposure to radiation is additive over a lifetime," he told WebMD. "And we know that kids are more susceptible to the adverse effects of a given amount of radiation than adults."
There isn't a generally accepted amount of radiation for children, the AP noted. Radiation in children hasn't been studied much, Dorfman said, adding that more research is needed.
"It really seemed like there was a big hole in the data in kids," Dorfman told Reuters.
The study excluded dental X-rays and did not determine whether all of the tests were justified. Still, Dorfman said it's meaningful because it gives a benchmark for how many tests children are getting.
"While I can't say any given procedure was appropriate or inappropriate, I think as a whole we have to make sure when these studies are being used, they are used only when they are absolutely necessary," he told Reuters.
One group, Alliance for Radiation Safety in Pediatric Imaging, has been working to make sure tests with radiation are used at the lowest dose possible and only when necessary.
"We want to be as cautious as we can and protect children as much as possible," the chairwoman of the group, Dr. Marilyn Goske, told the AP.
The group also tries to educate parents and urges them to keep track of what tests are performed.
"Having a record of what test was done, and when and where it was done, can result in fewer repeat procedures," Goske told WebMD.