More teenagers are being diagnosed with kidney stones now than in years past, a study from one U.S. state suggests.

The research, which followed Minnesota children from 1984 to 2008, found that the rate of kidney stones climbed six percent each year among teenagers.

Between 1984 and 1990, the annual rate was 13 cases for every 100,000 12- to 17-year-olds. That figure nearly tripled, to 36 per 100,000, between 2003 and 2008.

Researchers say they are not sure of the reasons -- or even whether teenagers are actually suffering the stones at a higher rate. It may be that more cases are being diagnosed since doctors started using highly sensitive CT scans that can detect smaller kidney stones.

"There has been much speculation about how the incidence of kidney stones might be changing over time in the pediatric population," lead researcher Dr. Moira E. Dwyer, of the Mayo Clinic in Rochester, said in an email.

"But until now," she added, "there has not been a sound epidemiologic study to confirm or refute suspicions."

Dwyer and her colleagues report their findings in the Journal of Urology.

Kidney stones develop when the urine contains more crystal-forming substances -- like calcium, uric acid and a compound called oxalate -- than can be diluted by the available fluid.

Small stones usually pass through the urine without any special treatment, though kids may need pain medication and plenty of fluids to help the stones along.

Large stones that block the urinary tract, however, may require hospital treatment.

It's not always clear why some kids are "stone formers." There might be lifestyle factors, like getting too little fluid and too much salt. And some researchers have suspected that the rising obesity rate might be leading to a higher incidence of kidney stones in adults and kids alike.

For the current study, Dwyer's team combed through a database with medical records from all healthcare providers in Olmstead County, Minnesota. They focused on the 25 years between 1984 and 2008.

Overall, 84 children and teenagers were diagnosed with kidney stones over the years, with 12- to 17-year-olds accounting for most of the cases. And they were the only age group to show a rise in kidney stone rates over time.

There was no evidence, however, that obesity was to blame.

On average, teens who developed kidney stones were normal-weight -- whether in the 1980s or in more recent years.

It's possible, according to Dwyer, that more kids are simply being diagnosed with the problem now.

CT scans, which take X-rays at multiple angles to create three-dimensional images, are being increasingly used in medicine. And Dwyer's team found that up to the mid-1990s, only 10 percent of kidney stones in their study group were spotted on CT scans. That compared with 82 percent for the years 1997 to 2008.

CT scans catch more stones than other forms of imaging, like traditional X-rays or ultrasound. Ultrasound, for example, picks up about 60 percent of the stones that CT does.

"CT scans are undoubtedly the most sensitive imaging study currently utilized for the detection of stones in children," Dwyer said.

But, she added, that has to be weighed against the downside of radiation exposure. CT involves much higher radiation doses than traditional X-rays do.

And a study published Wednesday found that children who have had multiple CT scans over time might have a higher-than-normal risk of leukemia or brain cancer.

Any one child has a very low risk of those, or any, cancers. So even if that baseline risk is increased, it is still quite small. And experts who responded to the study said that parents should not refuse CT scans that could diagnose serious problems in their kids -- like serious chest infections or injuries to the head or spine.

In other situations, though, they said parents can ask whether CT is necessary, and what other non-radiation options there might be.

Kidney stones are usually diagnosed through a combination of symptoms, urine and blood tests and imaging tests. The symptoms can include painful urination, blood in the urine and pain in the lower abdomen or back.