Asked by: Pamela Braithwaite, by email

It depends on the type of scan. For instance, X-ray scans expose the body to radiation. While that might seem alarming, we’re all exposed to natural X-ray radiation in the environment anyway. The average chest X-ray yields the equivalent of just a few days of natural radiation. It’s way too low to cause harmful effects like radiation sickness. The increased cancer risk is minuscule too – about one in a million.

CT (computerised tomography) scans involve multiple X-rays and therefore have a slightly higher risk, but this is still virtually negligible, especially given the diagnostic benefits.

Positron Emission Tomography (PET) also involves radiation. Here, radioactive tracers are injected into patients, but the dose is tiny and is therefore pretty much risk-free.

Magnetic Resonance Imaging (MRI) doesn’t use ionising radiation at all and is therefore almost 100 per cent safe. But given the strong magnetic fields involved, MRI can be unsuitable in people with certain metallic implants. And in 2008, research suggested that, under some circumstances, the scans can disrupt pacemakers.

Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.