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.
CT scanners have made exponential jumps in speed over the years, allowing them to freeze the subtle motions of the gut and the heaving of the lungs. Modern scanners are fast enough to capture a snapshot of a beating heart.
Today, CT scans of nearly every body part are increasing swiftly as doctors have embraced the technology to conduct virtual colonoscopies, lung cancer screenings, blood vessel inspections and a host of other procedures.
"Talking about reducing the number of scans is like trying to stop the future," said Dr. Daniel Rosenthal, a professor of radiology at Harvard Medical School. "The equipment and images are so much better that it is pointless to try and stop it."
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Cancer risk
Every so often, scientists believe, a CT scan unleashes the following chain of events:
Radiation knocks loose an electron from an atom, creating an ion that damages a cell's DNA.
Although the damage is not big enough to kill the cell, it is too big to repair. Over the next two or three decades, the cell divides and multiplies, 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% lifetime chance of getting cancer.
Still, even the small amount of radiation from a CT can compound over time as the number of scans adds up.
Scientists measure effective radiation doses using millisieverts, which represent 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 U.S. 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 thus unavoidable.
But although individual doctors may believe ordering a scan is justified, they often have no idea how many scans a patient has already had. The numbers can quickly slip out of control, repeatedly exposing patients to added radiation.
