There is a lot of unjustified fear and alarm about radiation from medical procedures. These notes should put the subject into perspective.
What is radiation dose? We see images on an x-ray film because your body absorbs some of the x-ray beam, producing shadows. "Dose" is the amount of energy absorbed from radiation, adjusted for the particular area of the body. It is measured in sieverts (Sv) or millisieverts (mSv).
What does it do? X-rays can trigger chemical reactions. We use this property to make an image on a film, or to destroy tumours in the body. But such reactions can also harm healthy tissue, so we are careful to ensure that the benefit of medical x-rays outweighs any harm that they might do.
Background dose is energy that we absorb from cosmic rays, and radioactive elements in the earth's crust, the atmosphere, and our food. The average dose is about 2.6 mSv per year in the UK, more in Scotland and the West, less in South and East England. Over 99% of background radiation is natural.
What is the risk associated with radiation dose? Background radiation has been fairly constant during the evolution of life, so our bodies naturally repair damage caused by small doses. There is no evidence of any health risk associated with normal background doses in the UK. But we know (mostly from studying industrial accidents and radiotherapy treatments) that a sudden dose of 1000 mSv can overwhelm our capacity for repair, so we keep medical doses to the minimum needed for accurate diagnosis and treatment.
What dose will I receive from an x-ray examination? This table shows some useful comparisons with exposure to natural background radiation
||typical dose (mSv)
equivalent period of natural background
| limbs & joints
|| < 1.5 days|
| teeth (bitewing)
|| <1.5 days|
| chest (1 film)
|| 3 days|
|| 2 weeks |
|| 7 weeks |
thoracic spine / abdomen / pelvis
|| 4 months|
| lumbar spine
|| 7 months|
|| 14 months |
| barium meal
|| 16 months |
| barium enema
|| 3.2 years|
| head scan (CT)
|| 1 year|
| chest scan (CT)
|| 3.6 years |
| CT abdomen / pelvis
|| 4.5 years|
Note that these figures are based on the UK population average, and most people live in the south-east of England, where the natural background is lowest. A dose of 6 mSv would be equivalent to living in Cornwall or the Scottish Highlands for a year - not a prospect that would worry a Londoner!
So why do the staff use lead screens and aprons? Some radiation is scattered from the patient towards the operators. The amount from one patient is tiny, but we need to protect ourselves from the doses accumulated from the hundreds or thousands of patients that we see each year.
Will I be radioactive after my examination? Not if you are having an x-ray, bone density or CT examination. If you are having a radionuclide test ("bone scan", some blood tests, etc) you will be given particular information and instructions. MRI and ultrasound examinations do not use ionising radiation at all.
What if I'm pregnant? You must inform your doctor or radiographer before the examination. A fetus is more sensitive to radiation at some stages of its development, particularly between 2 and 8 weeks after conception, so we need to be certain of the urgency of some tests (particularly from "IVU" downwards in the Table) if you have missed a period, and to take special care to minimse the dose to your abdomen.
Who looks after my safety? We are governed by regulations that ensure patient safety. Your referring physician must provide enough information for us to evaluate the risk and benefit of a particular test. The practitioner (radiologist) reviews the request and considers how to best to obtain the required information. The operator (radiographer) selects the method that uses the least practicable dose. Chiropractors, podiatrists and dentists, who take their own x-rays, have to follow the same rules as if they were referrers, practitioners and operators separately. We are advised and monitored by nationally recognised experts who review our equipment and procedures regularly.
How do you know the correct dose to use? In complex examinations, the dose is set and recorded automatically. For simple x-ray films we use standard tables to calculate dose according to the size of your body.
I've had previous x-rays. Is this taken into account? Yes. In deciding whether an examination is necessary, we look at recent, relevant films. However there is no limit on the number of x-rays that you should have in a year: the appropriate number depends entirely on your medical condition. But we must be sure that every x-ray examination provides useful information so that the benefit outweighs the very small radiation risk for each examination requested.