CT scans of the sort used throughout the NHS can damage DNA and kill cells, a study has found.
Whether the harmful effects increase the risk of cancer remains unclear, but scientists stress they are "not benign".
Experts who examined the blood of heart patients undergoing CT (computed tomography) scans found evidence of DNA damage and cell death.
The activity of cell repair genes was also boosted. Although most of the damaged cells managed to repair themselves, a small percentage died.
Dr Patricia Nguyen, from Stanford University in the US, said: "We now know that even exposure to small amounts of radiation from computed tomography scanning is associated with cellular damage.
"Whether or not this causes cancer or any negative effect to the patient is still not clear, but these results should encourage physicians toward adhering to dose reduction strategies."
CT scans produce 3D X-ray images showing structures such as organs, blood vessels, bones and tumours in great detail.
But the radiation doses are much higher than those of a standard X-ray, leading to concerns about the possible risk of cancer. A single CT scan exposes a patient to at least 150 times the amount of radiation from a single chest X-ray, according to the study authors.
The scientists, whose findings are reported in the Journal of the American College of Cardiology: Cardiovascular Imaging, point out that little is known about the long-term effects of low dose radiation.
Most knowledge of what radiation does to the human body is derived from the atomic bomb attacks on Japan at the end of the Second World War, when people were subjected to extremely high doses.
In 2007, the US National Cancer Institute estimated that 29,000 future cancer cases could be attributed to the 72 million CT scans performed in the country that year.
Two years ago a patient record study of more than 10 million Australians, published in the British Medical Journal, found that having a CT scan increased overall cancer risk at least a year later by 24%.
A dose response was seen, with the risk increasing for each additional CT scan performed on a patient.
For the new study, tests were conducted on blood samples from 67 patients undergoing CT angiograms. The researchers used a variety of techniques to measure biomarkers of DNA damage and cell death before and after the procedure.
In their paper, the scientists said the findings raised the possibility that radiation exposure from CT heart scans "may cause DNA damage that can lead to mutations if damaged cells are not repaired or eliminated properly".
Harmful mutations in the DNA of cells that are not "culled" but allowed to proliferate are the root cause of cancer.
Dr Nguyen said: "We need to learn more because it's not a benign effect even at these low dosages. Our research supports the idea that maybe physicians shouldn't just use the best image quality in all cases.
"We shouldn't eliminate CT scans because they're obviously important, but you can make it safer by reducing the doses, by getting better machines and technology, and by giving patients something to protect them."
She added it was important to note that no DNA damage was detected in average weight patients receiving the lowest doses of radiation who had regular heart rates.
Professor Joseph Wu, director of the Stanford Cardiovascular Institute, said: "The use of medical imaging for heart disease has exploded in the past decade.
"These tests expose patients to a non-trivial amount of low-dose radiation. But nobody really knows exactly what this low-dose radiation does to the patient. We now have the technology that allows us to look at very subtle, cell-level changes."