What she did not realize was that the perfection of the images was a result of a radiation dose equivalent to more than a dozen standard abdominal X-rays.

"I never thought twice about it," says the 38-year-old mother. Since learning of the radiation, she has been worried that the scans may have played a role in two miscarriages. "I knew there was radiation, but I didn't realize how strong it was."

Scanlan is part of an explosion in the use of one of the most revolutionary medical technologies of the last half century.Introduced in the 1970s, computed tomography scans have become a standard procedure for common problems as kidney stones, headaches and appendicitis.

Generating tens of billions of dollars each year, CT scanning has become an economic engine for hospitals and doctors. "It's gotten into the culture of doctors," says Geoffrey Rubin, a Stanford University radiologist.

But with the boom has a come a rising concern that the abundant use of radiation is beginning to have a subtle effect on health. Although the risk of a single CT scan is minuscule, even a tiny increase in radiation exposure spread over a large population can eventually add up to tens of thousands of cancer deaths a year.

A recent study published in the New England Journal of Medicine estimated that CT scans administered today could cause up to 2 percent of cancer deaths in two or three decades.

The doctors who have embraced the technology in increasing numbers say the small increased risk is a minor price to pay.

Some researchers estimate that up to a third of scans could have been avoided or replaced by safer technologies, such as ultrasound or magnetic resonance imaging. Scans can cost from a few hundred dollars for a single organ to a few thousand for a full-body image.

Today, scanner manufacturers, including Siemens and General Electric, tout the ease of making money with the devices. Just two scans a day can pay for a machine and its operation over five years.Ten scans a day can bring in more than US$400,000 (HK$3.11 million) a year in profit.

For diagnosis, CT can offer huge advantages over its main competitor, MRI, which avoids radiation but costs more and requires the patient to lie in a clanging cylinder for half an hour or longer.

Every so often, scientists believe, a CT scan unleashes the following chain of events: Radiation knocks loose an electron, spreading the faulty genetic instructions. The result is cancer.

The increased risk varies with age but, at most, adds about a tenth of a percent to a person's 42 percent lifetime chance of getting cancer. Still, even the small amount of radiation from a CT can compound over time. Scanner manufacturers have responded to rising concerns by improving their machines, allowing operators to select the lowest dosage necessary for a useful image.

Medical tests are now the biggest source of radiation exposure, recently surpassing background radiation, according to the National Council on Radiation Protection & Measurements in the United States. Of particular concern is the rising use of CT scans for children and pregnant women.

For example, an abdominal scan in a five-year-old carries a 0.1 percent risk of triggering a fatal cancer, nearly 10 times the risk in adults older than 35.

The National Academy of Sciences weighed in on the issue in a 2006 report, saying that there is no safe level of radiation exposure and that even small doses pose some health risks.

LOS ANGELES TIMES

RADIATION EXPOSURE

Scientists measure effective radiation doses using millisieverts, which represents the amount and type of radiation a person receives as well as the sensitivity of various organs.

Most CT scans deliver an effective dose of 5 to 25 millisieverts. That is below the US occupational limit of 50 millisieverts a year but well above the exposure limit for the public of 1 millisievert per year.

Both figures purposely exclude background radiation from natural sources as well as medical radiation, which is deemed necessary and, therefore, unavoidable.*Overset by 920.8*