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 DEXA scan and what does it measure?
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. The scan usually takes between 10 and 20 minutes, depending on which part of your body is being examined and whether a central or peripheral scanner is being used. Peripheral scanners are available in some GP surgeries and can be used to check the bone mass density of the heel, wrist or finger.
By Nick Smith photography (ALSPAC web site), via Wikimedia Commons.
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.)
You do not need to do any special preparation prior to a DEXA scan.
Who should have a DEXA scan?
A DEXA scan may be advised if you are at increased risk of 'thinning' of the bones (osteoporosis). Osteoporosis usually causes no symptoms at first. However, if you have osteoporosis, you have an increased risk of breaking a bone. See separate leaflet called Osteoporosis for more details. If a DEXA scan shows that you have osteoporosis then you may be given advice and treatment to help strengthen your bones. Therefore, a DEXA scan may be advised if you have:
- A break in a bone (fracture) following a minor fall or injury.
- Loss of height due to fracture of a back bone (vertebra).
- Taken steroid tablets for three months or more.
- An early menopause (aged less than 45).
- A history of periods stopping (amenorrhoea) for more than one year before the menopause.
- Other disorders associated with osteoporosis such as rheumatoid arthritis or coeliac disease.
- A family history of hip fracture on your mother's side of your family.
- A body mass index of less than 19 (that is, if you are very underweight).
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 DEXA scan. This enables them to gain a clearer picture of your risk and then to decide whether you need any treatment.
Further help & information
Dr Rachel Hoad-Robson
Dr Mary Harding
Dr John Cox