Will getting two or three CT scans of the abdomen expose a person to the same amount of radiation as people who lived not far from the epicentre of the 1945 Hiroshima atomic bombing and survived?
Will they increase a person's lifetime cancer risk?
If you answered yes to both questions, you're right - and also more informed than many patients at US inner-city emergency departments, according to a survey by Cooper University Hospital in Camden, New Jersey.
Researchers there asked more than 1 100 patients to rate statements similar to the above questions. Half said they had very little faith in the comparison between Hiroshima survivors and patients who had CT scans, rating their agreement at 13 on a scale from 0 to a perfect 100.
The majority also tended to disagree that the scans would increase their cancer risk, while three-quarters underestimated the x-ray radiation from a CT scan compared with traditional chest x-rays, which are at least 100 times weaker.
"The point of the paper was not to create mass hysteria," said Brigitte Baumann, an emergency physician at Cooper, whose findings appeared online in the Annals of Emergency Medicine.
"The concern is patients who keep coming back to their physicians and get a lot of scans," she said.
The number of Americans having computed tomography scans has soared in recent decades to 72 million in 2007, leading some doctors to worry that they may be overused.
"We have people who have gotten as many as 57 scans. That is a huge number," Baumann said, noting that checks of the medical records of their patients by her and her colleagues found that half had gotten scans at the hospital in the previous five years.
Patients told researchers that tests such as blood work and CT scans would boost their confidence in their medical check.
"When they go to the emergency department, they're not really happy if all you do is speak to them. They want more," she said.
While Baumann said that some patients obviously needed a CT scan, such as those coming in with chest pain and coughing up blood, in general the whole topic is "just a big shade of gray".
According to one government study, CT scans done in 2007 alone will cause about 29 000 cancers and kill nearly 15 000 Americans.
But it would take 1 000 "average" scans to produce one extra case of cancer in 50-year-olds, the National Cancer Institute's Amy Berrington said in November - perhaps a small risk to pay for better treatment, given that about one in three Americans will develop some type of cancer.
The radiation dosage from one scan typically ranges from a few millisieverts - comparable to the annual background radiation from natural sources - to tens of millisieverts.
Hiroshima survivors living a couple of kilometres from the blast often received between five to 100 millisieverts, said David Brenner, who heads the Centre for Radiological Research at Columbia University in New York and was not involved in the study.
He said there is little difference between the x-rays beamed at patients in hospitals and the mix of x-rays and gamma radiation produced by a nuclear explosion.
"The biggest difference is that the atomic bomb survivors got whole-body radiation, whereas CT is very directed exposure."
The US Food and Drug Administration has recently started pushing doctors to curb unneeded radiation from medical imaging.
Baumann said she now takes extra pains to explain the risks and benefits of CT scans to patients, and that it seems to be having an effect. Recently, one of her patients who was determined to get a scan for kidney stones chose to wait and see if the pain would disappear on its own. - (Reuters, January 2011)