DXA scans (also called DEXA scans) are used to check the density of bones. This test uses X-rays to show how strong bones are.
Note: the information below is a general guide only. The arrangements, and the way tests are performed, may vary between different hospitals. Always follow the instructions given by your doctor or local hospital.
What is a DXA scan?
DXA stands for 'dual-energy X-ray absorptiometry'. DXA (also sometimes known as DEXA) is a test that measures the density of bones. Density means how much of something there is in a certain amount of space. The denser the tissue, the less X-rays pass through. Air and water are less dense than solid things such as bone. This is because the particles which make air and water are not held closely together. In general, the more dense the bone, the stronger it is, and the less likely it is to break (fracture).
There are two different types of DXA scanning devices:
- Central DXA devices are large machines that can measure bone density in the centre of your skeleton, such as your hip and spine.
- Peripheral DXA devices are smaller, portable machines that are used to measure bone density on the periphery of your skeleton, such as your wrist, heel or finger. These are mainly to get an idea about whether further tests are needed, as they are not as accurate as the larger DXA machines.
How does a DXA scan work?
A DXA scan uses low-energy X-rays. A machine sends X-rays from two different sources through the bone being tested. Bone blocks a certain amount of the X-rays. The more dense the bone is, the less X-rays get through to the detector. By using two different X-ray sources rather than one it greatly improves the accuracy in measuring the bone density.
The amount of X-rays that comes through the bone from each of the two X-ray sources is measured by a detector. This information is sent to a computer which calculates a score of the average density of the bone. A low score indicates that the bone is less dense than it should be, some material of the bone has been lost and it is more prone to fracture.
How is a DXA scan done?
You lie on your back on a couch and are asked to keep still while an X-ray detector (the 'scanner') comes over the area to be tested. An X-ray machine fires X-rays towards the detector. The bones commonly scanned are the bones of the back (the vertebrae), hip and wrist. The scan usually takes between 5 and 20 minutes, depending on which part of your body is being examined and whether a central or peripheral scanner is being used. There is no 'tunnel' to pass through as there is in other types of scans such as an MRI or CT scan, so it should not affect people who do not like being in enclosed spaces.
By Nick Smith photography (ALSPAC website), via Wikimedia Commons.
DXA scans use a very low level of X-ray radiation. This means it is safe for the technician doing the scan to stay in the room with you. (In standard X-ray tests, the technician has to stay behind a protective screen.)
You do not need to do any special preparation prior to a DXA scan. You can normally remain fully clothed, although you will need to avoid or remove clothes with metal in them (for example, zips, belts, buttons). You may also be asked to remove jewellery for the scan. In some places, you may be given a gown to wear.
Smaller peripheral scanners are available in some places and can be used to check the bone mass density of the heel, wrist or finger.
Who should have a DXA scan?
A DXA scan may be advised if you have had a fracture of a bone after a minor injury. It may also be advised if you are considered at increased risk of 'thinning' of the bones (osteoporosis) and therefore at increased risk of having a fracture in future. To read about what might make you at increased risk, see the separate leaflet called Osteoporosis.
If your doctor thinks you have risk factors for osteoporosis, they may use a risk calculator such as one called FRAX®. This gives an idea of how likely you are to fracture your bones after a minor knock. If your risk is at a medium level, your doctor would then arrange a DXA scan. This enables them to gain a clearer picture of your risk and then to decide whether you need any treatment.
DXA scans are also used to monitor whether treatment for osteoporosis is working.
DXA scans are not advised for women who are pregnant. You should also not have a DXA scan within two weeks of certain other types of scans - for example, those using contrast dye.
Further reading and references
Osteoporosis: assessing the risk of fragility fracture; NICE Clinical Guideline (August 2012, updated February 2017)
Management of osteoporosis and the prevention of fragility fractures - A national clinical guideline; Scottish Intercollegiate Guidelines Network - SIGN, (March 2015)
Clinical guideline for the prevention and treatment of osteoporosis; National Osteoporosis Guideline group (NOGG) 2017