Exposure to ionising radiation from medical imaging: safety advice

Published 1 August 2014

1. What are the risks associated with medical imaging examinations using ionising radiation?

When properly justified and optimised, the risks from medical x-ray and nuclear medicine imaging examinations are much less than the risks associated with the medical conditions under investigation. The risks from imaging examinations using ionising radiation relate to a very small increase in the probability of cancer occurring many years or decades after the radiation exposure, This should be considered in context - unfortunately, we all have a 1 in 4 lifetime risk of dying from cancer.

For the most common imaging examinations, such as those of the arms, legs, chest or teeth, the radiation doses are very low and the increased risk from radiation is considered to be negligible. Higher-dose examinations are usually associated with more serious conditions, but even for those complex radiographic procedures involving many images, fluoroscopy, computed tomography scans (CT scans) or hybrid imaging, for example positron emission tomography and CT scans (PET CT) of a substantial part of the body, the risk is considered to be low.

One way of putting the doses of radiation from medical imaging into perspective is to compare them with the natural background radiation that we are all exposed to all of the time. This background radiation comes from naturally occurring radioactive material in the ground and from cosmic rays irradiating the earth from outer space. The lowest-dose x-ray examinations involve similar doses of radiation to those received in a period of just a few days from natural background radiation. Higher -dose procedures involve doses equivalent to a few years of natural background radiation.

As it will take many years or even decades for a cancer to develop after exposure to radiation, the risks are reduced even further for people who are elderly at the time of exposure. For many people over 65 years of age there will simply not be sufficient time in their remaining lifespan for a radiation-induced cancer to develop. Conversely, children undergoing x-ray examinations will have an increased lifetime risk, depending on age and gender.

2. What is a safe dose of radiation from medical exposures?

For medical imaging procedures, it’s not possible to define what a ‘safe’ dose of radiation is. As long as the procedure is justified and optimised, then the benefit of the examination will always outweigh any associated potential harm. It is possible to provide guidance on the approximate dose required for an examination such that the images produced will be suitable to allow an accurate diagnosis, but this will vary depending on a number of factors including the size of the patient and the condition being investigated.

If patients are concerned about the possible risks from having an x-ray examination they should seek reassurance from their doctor that the examination is indeed necessary and that it is likely to yield useful diagnostic information. Conversely, patients should not ask for imaging procedures if they are not needed.

3. Is any account taken of previous examinations when ordering a new imaging examination?

Clinical staff should be sure that the diagnostic information expected from the new imaging examination is not already available from any previous examinations, even if they were requested by another doctor or undertaken in another hospital. In every case, the need for an imaging examination should be determined on its own merits, in relation to the patient’s present medical condition. Imaging procedures should not be avoided if there is a clinical problem that requires a diagnosis.

4. What is the maximum number of imaging examinations that are advisable in a year?

There’s no limit on the number of examinations. The appropriate number depends entirely on that patient’s medical condition and presentation and how the investigation may help in the diagnosis and subsequent treatment of the patient.