DEXA scan

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 DEXA scan?

DEXA stands for 'dual-energy X-ray absorptiometry'. DEXA (also sometimes known as DXA) 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 DEXA scanning devices:

• Central DEXA devices are large machines that can measure bone density in the centre of your skeleton, such as your hip and spine.

• Peripheral DEXA 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 DEXA machines.

How does a DEXA scan work?

A DEXA 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 DEXA 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.

Smaller peripheral scanners are available in some places and can be used to check the bone mass density of the heel, wrist or finger.

How long does a DEXA scan take?

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.

DXA scanner

By Nick Smith photography (ALSPAC website), via Wikimedia Commons

Are DEXA scans safe?

DEXA 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.)

Preparing for a DEXA scan

You do not need to do any special preparation prior to a DEXA 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.

Who should have a DEXA scan?

A DEXA 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.

If your doctor thinks you have risk factors for osteoporosis, they may use a risk calculator such as one called FRAX® or QFracture®. 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 DEXA scan. This enables them to gain a clearer picture of your risk and then to decide whether you need any treatment.

DEXA scans are also used to monitor whether treatment for osteoporosis is working.

DEXA scans are not advised for women who are pregnant. You should also not have a DEXA scan within two weeks of certain other types of scans - for example, those using contrast dye.

For further information, see the separate leaflet called Osteoporosis.