But Cynthia McCollough, a medical physicist who oversees the Mayo Clinic's 23 CT scanners, said data on the long-term effects of low-level radiation are still too limited.
No one is certain if there is a minimum threshold of radiation exposure that must be reached before it becomes dangerous, she said.
At less than a long-term cumulative dose of 100 millisieverts, "you get more into religion than science," she said.
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New questions
As long as a scan is medically necessary, the benefit "in practically all cases outweighs the small risk in the future," said John Boice, a radiation expert who is the scientific director of the International Epidemiology Institute in Rockville, Md.
But studies over the last few years have begun to question whether all the scans are actually needed.
In 2000, when the number of scans was half of today's total, Highmark Blue Cross Blue Shield of Pennsylvania reviewed 162,000 claims for CT scans and other imaging procedures and deemed at least 30% either inappropriate or not contributing any useful information.
Even some routine uses have been called into question. For example, one study found that a radiation-free ultrasound may be just as good as a CT scan at diagnosing appendicitis in children. Other research found that in children with chronic headaches, a normal neurological examination made scanning unnecessary.
Doctors concede that the realities of modern medicine -- a complex mix of business, law and expediency -- have exerted a subtle pressure that pushes up the number of scans.
In a 2003 survey of 824 doctors published in the Journal of the American Medical Assn., more than half of the emergency room doctors, orthopedic surgeons and neurosurgeons said they ordered CT scans and other imaging tests just to protect themselves from lawsuits.
"Patients come in with a headache," said Dr. Scott Lederhaus, a neurosurgeon in Pomona. "If they don't get scanned and something gets missed, they can sue."
"If they are persistent about it, I will just order a scan," he said. "I don't care. It's just not worth the aggravation."
As medicine has become less profitable, CT scanners have also become a potential source of income, allowing doctors to offer additional services they can bill for instead of referring patients elsewhere.
