Dual energy X-ray absorptiometry (DEXA scan)

About DXA scan

A DXA scan uses X-ray equipment and a computer to measure the density (the degree of compactness) of your bones.

A DXA scan projects X-rays onto your bones. Some of this radiation will travel straight through your bones and a certain amount will be absorbed by them – how much depends on how dense your bones are. A detector will measure how much radiation passes through your bones and it will send the information to a computer. This will work out an average bone density score and will compare it to normal measurements for someone of your age and sex. A low score indicates that your bones are fragile and are therefore more likely to fracture.

Doctors use DXA scans to assess your risk of developing osteoporosis, which is a condition where your bones lose their density and become weak, brittle and more likely to fracture. Your doctor may ask you to have a DXA scan if you fracture a bone after only a minor bump or fall, if you have an eating disorder such as anorexia nervosa, or if you're already at high risk of osteoporosis. Doctors can also use DXA scans to monitor how well you’re responding to a treatment for osteoporosis.

DXA scans are usually done as a hospital outpatient procedure in the imaging or radiology department. The scan is quick and painless and both adults and children can have it.

What are the alternatives to a DXA scan?

Alternative imaging tests, such as an ultrasound and CT scan, can be used to assess your risk of fracture. However, these tests aren't usually used in these circumstances because they aren't as reliable as DXA scans. Also, CT scans use larger doses of radiation than DXA scans. Although a normal X-ray, also called a plain X-ray, can identify a fracture, they aren't helpful for diagnosing osteoporosis.

Some types of blood and urine test can check the levels of certain chemicals that are produced when your body breaks down and forms new bone. These can provide information about factors that might affect your bone strength, but they can’t diagnose osteoporosis or bone strength directly.

Preparing for a DXA scan

Your radiographer (a health professional trained to perform imaging procedures) will explain how to prepare for your scan.

If you usually take medication, continue to take this as usual unless your doctor specifically tells you not to. You shouldn’t take calcium supplements for at least 24 hours before your scan.

It's best to wear light clothing that doesn't have metal fasteners at the hips or along your spine (trouser zips are fine). You may be asked to remove any metal jewellery because this can interfere with the scan.

Your radiographer will discuss with you what will happen before, during and after your scan. This is your opportunity to understand what will happen, and you can help yourself by preparing questions to ask about the risks, benefits and any alternatives to the procedure. This will help you to be informed, so you can give your consent for the procedure to go ahead, which you may be asked to do by signing a consent form.

What happens during a DXA scan?

The scan usually takes about 10 to 20 minutes. You may be asked to put on a gown.

You will be taken to the X-ray room and asked to lie down on a padded table. Your radiographer may position your legs to get an image of your hip and raise your knees to get an image of your back. He or she will operate the scanner and will be able to see, hear and speak to you during the procedure.

                                                                 DXA scan

The X-ray machine will slowly pass over your body and send X-rays through the bones being tested. A detector will measure how much X-ray radiation passes through your bones and will send this information to a computer. It’s important to lie very still as each bone is being scanned – you may be asked to hold your breath for a few seconds while the X-ray picture is being taken to prevent a blurred image.

What to expect afterwards

You will usually be able to go home when you feel ready and you can drive if you wish.

Your scan will then be sent to your doctor. Usually, a report will also be sent out by a radiologist (a doctor who specialises in using imaging methods to diagnose medical conditions). This can take several days. Before you go home, ask your radiographer when you can expect to get your results.

The results of the scan are given as a 'T-score'. This is a measure of how your bone density compares to the normal average for young, healthy adults whose bone density is at its peak. A T-score of 0 means your bones are the same density as an average young, healthy person of the same sex.

If your T-score is:

  • between 0 and -1 (for example, -0.5), your bone density is normal
  • between -1 and -2.5, this is classed as osteopenia – this means your bone density is lower than normal, but you don't yet have osteoporosis
  • below -2.5, this is classed as osteoporosis

If your T-score shows that you have osteopenia your doctor may suggest you make some lifestyle changes to help slow down the possible progression to osteoporosis. These can include changing your diet or doing some weight-bearing exercise, for example. However, this T-score can be normal for an older person.

If your T-score indicates that you have osteoporosis, your doctor may give you advice about starting treatment to prevent possible fractures.

Even though the results of a DXA scan provide a T-score, this information can be limited. It's therefore important that you and your doctor consider other risk factors when deciding whether to have treatment to help prevent fractures.

What are the risks?

As with every procedure, there are some risks associated with having a DXA scan. We have not included the chance of these happening as they are specific to you and differ for every person. Ask your doctor or radiographer to explain how these risks apply to you.

During the scan you will be exposed to some X-ray radiation. However, the dose is very low and roughly a tenth of that used for a chest X-ray – one of the lowest doses of radiation used in medical procedures.

If you’re pregnant

If you’re pregnant, you will usually be advised not to have a DXA scan as there is a risk that the radiation could harm your unborn baby. If you’re pregnant, or think you might be, let your doctor or radiographer know.

I'm a 79-year-old woman and broke my wrist in a recent fall. Do I need to have a DXA scan?

Answer

No, you probably won’t need to have a DXA scan. But you may need to speak to your GP about starting treatment to help prevent any further breaks (fractures).

Explanation

DXA scans can help doctors to spot people who are at high risk of developing osteoporosis and who could benefit from treatment to prevent fractures.

If you're over 75, you're already at increased risk of getting osteoporosis and having a wrist fracture may suggest that you have fragile bones.

Speak to your GP for advice and information about starting treatment to help prevent fractures.

What will happen if I'm classed as having osteopenia?

Answer

If you have had a DXA scan and been diagnosed with osteopenia, your GP will probably advise you to make some lifestyle changes.

Explanation

Having osteopenia means you have a lower bone density than normal and could be at risk of developing osteoporosis. If you're given a diagnosis of osteopenia after your DXA scan, your doctor will probably suggest you make some lifestyle changes. These can help to slow down the possible progression to osteoporosis and therefore reduce your risk of fracturing a bone. Your doctor may recommend the following lifestyle changes.

  • Include more calcium in your diet. Foods that are high in calcium include milk, hard cheese, yoghurt, sardines and white bread. Your doctor may advise you to take vitamin D supplements as they help calcium to be absorbed into your body.
  • Carry out regular, low-impact, weight-bearing exercise, such as walking.
  • Give up smoking if you smoke.
  • Drink alcohol in moderation – stick to the recommended daily limits set by the Department of Health. This is no more than two to three units a day for women and three to four units a day for men.

You won't usually be prescribed medicine if your bone density isn't low enough for you to be diagnosed with osteoporosis. However, you may be advised to have another DXA scan two to five years later to check on your progress.

Will I have a repeat DXA scan to check if my treatment for osteoporosis is working?

Answer

You will usually only need to have another scan if you fracture a bone, but only after a minor bump or fall.

Explanation

If you haven't broken any bones when you have been taking treatment for osteoporosis, this is thought to be enough evidence that your treatment is working well. It's not usually necessary to have another DXA scan just to check up on your progress.

However, if you do break a bone after you have been taking a treatment for osteoporosis (for at least two years), you may be advised to have a repeat scan. You won't usually be offered a scan if you have been on treatment for less than this time, as it can take up to two years for the treatment to have a beneficial effect on your bones. If a repeat scan shows that your bone density is still below normal, your doctor may suggest that you try another type of treatment.

Will a scan on my heel show whether I have osteoporosis?

Answer

No, a DXA scan of your heel can't diagnose osteoporosis. This is because the bone density in your heel isn't always a good indication of the density in your other bones.

Explanation

A scan of your hip and spine is currently considered to be the most reliable method (the 'gold standard') for diagnosing osteoporosis and predicting the risk of fractures. This is partly because international statistics that enable your risk of fractures to be worked out are based on scans of only these bones. Also, a fractured hip is one of the most serious consequences of osteoporosis and the best way to predict it is to scan the hip rather than any other bone.

Alternative scanners are available that can measure the bone density of your heel and other bones including those in your forearm, finger and hand. These are known as peripheral scans and are sometimes used as screening to see whether you may need further investigations.

Although peripheral scans might give an accurate measure of bone density at the part of your body that is tested, researchers aren't sure how good they are at predicting your risk of fractures. If you have a peripheral scan that shows you have a low bone density, you're likely to need a hip and spine DXA to confirm the findings.

The National Institute for Health and Clinical Excellence (NICE) is working on guidance to find out the best way to identify people who are likely to be at risk of osteoporosis. Until this is published, follow your GP's advice and have your DXA scan at a clinic that carries out the conventional spine and hip measurements.