Bone densitometry scan

Page last reviewed: 13/07/2011

A DXA scan is used to measure bone mineral density (BMD) of the spine and hips which helps to assess the risk of bone fractures. This measurement is often referred to as bone mineral density (BMD).

DXA scans utilise a relatively low radiation dose which is similar to ambient daily exposure. DXA scans are most commonly used for diagnosing osteoporosis (weakened bones that may fracture easily) and assessing the risk of osteoporosis developing. They can also be used to detect other bone disorders and conditions, such as osteopenia, and to measure the relative amounts of body fat and muscle.

A DXA scan is a quick and painless way of measuring BMD. It is more sensitive than a normal X-ray when identifying low bone density. It uses a much lower level of radiation, equivalent to less than one day's exposure to natural background radiation.

You may be offered a DXA scan if you are considered at high risk of having or developing osteoporosis. The scans are also used to monitor the effectiveness of treatment for conditions such as osteoporosis.

DXA scans are sometimes called bone densitometry scans, QDR scans or BMD measurement.

How bone density is measured

During a DXA scan, X-rays are passed through the body. Some radiation will be absorbed in bone and soft tissue, and some will travel through the body. Special detectors in the DXA scanner measure how much radiation passes through the bones. This information is sent to a computer. 

Measurements are then compared to the normal range of bone mineral density in healthy people of the same gender and ethnic background.

Types of scan

There are two different types of DXA scan:

  • Axial or central DXA scan: a large scanning arm that passes over the body to measure bone density in the centre of your skeleton, such as your hip and lower spine.
  • Peripheral DXA scan (pDXA): a large scanning arm or a smaller portable device used to measure bone mineral density in peripheral parts of your skeleton, such as your wrist, heel or hand.

Central DXA devices are more commonly used to diagnose osteoporosis. However, peripheral devices have the advantage of being small and portable.

Spine
The spine supports the skeleton and surrounds and protects the delicate spinal cord and nerves. It is made up of 33 bones called the vertebrae.
X-ray
An X-ray is a painless way of producing pictures of inside the body using radiation.

Why measure bone density?

Generally, the denser your bones are, the stronger and less likely they are to break (fracture). Osteoporosis causes no symptoms until a bone is broken.

In the past, it was difficult to measure the amount of calcium in the skeleton and identify people at risk of osteoporosis until a fracture had occurred. Now, with bone densitometry (measurement of bone density) techniques, such as DXA scans, it is possible to measure bone mineral density before fractures occur.

Page last reviewed: 13/07/2011

A DXA scan is used to measure bone mineral density (BMD) and assess the risk of bone fractures.

It can also be used to measure body composition (the amount of bone, fat and muscle in the body). A DXA scan is sensitive to small (2.8% or more) changes in bone mineral density, and can diagnose osteoporosis in its early stages before fractures occur.

Recent developments in DXA enable images of the whole spine to be obtained (using a lower radiation dose than conventional X-rays). This makes it possible to diagnose spine (vertebral) fractures, which can occur in osteoporosis.

DXA scans can also identify other conditions that affect bone density. Such conditions include osteomalacia (vitamin D deficiency) and osteopenia, when your bone is less dense than average but is not low enough to be classed as osteoporosis. However, it cannot distinguish between the different causes of low bone mineral density.

A scan is often used to help a doctor decide whether treatment for low bone density is needed. For example, if you are diagnosed with osteopenia, lifestyle changes to improve bone health will usually be recommended, such as eating a balanced diet rich in calcium, having adequate exposure to sunlight for vitamin D and doing regular weight-bearing exercise.

Who can benefit from a DXA scan?

Your doctor may recommend that you have a DXA scan if you are considered to be at high risk of developing osteoporosis. For example:

  • if you have had a fracture after a minor fall or injury
  • women who have an early menopause or have had their ovaries removed and do not have hormone replacement therapy (HRT)
  • post-menopausal women who smoke or drink heavily, have a family history of hip fractures, or have a body mass index (BMI) of less than 21
  • men or women who have a disease that leads to low bone density, such as rheumatoid arthritis
  • women who have large gaps (more than a year) between periods
  • men or women taking oral glucocorticoids (a type of steroid that treats inflammation) for three months or more, as these medicines can contribute to weakening bones
  • Family history of osteoporosis
  • Maternal history of hip fracture
  • X-ray evidence of osteopenia

Limitations

A DXA scan is not the perfect measure of bone strength or fragility. Doctors need to take into account other risk factors, such as family history and steroid use, to determine who is at risk of bone fractures.

These risk factors will be considered before you are offered a DXA scan or any treatment is started. Some people need a DXA scan to confirm that their risk of bone fractures is high enough to need treatment. In other cases, particularly in the elderly, the risk of fracture may be so high that there is no need for a DXA scan before treatment is prescribed.

It can become more difficult to interpret the results of a DXA scan, especially of the spine, when degenerative diseases such as osteoarthritis are present. Such spinal abnormalities or a previous spinal fracture in the site can give a false result.

Contraindications to DXA Scanning

  • Pregnant women are advised against bone density testing. X-rays are not considered safe during pregnancy because they can damage an unborn child.
  • Patients who have had an investigation using contrast materials may not be suitable and should leave one week between tests. (NOTE: Contrast may take longer than one week to leave the body in elderly patients)
  • Patients with metal implants (e.g. hip replacements) should inform the operator prior to the test as it may lead false results.

 

Corticosteroids
Corticosteroid is a naturally occurring hormone that is produced by the adrenal gland or a synthetic hormone with similar properties. It is used to reduce inflammation, which reduces swelling and pain.
Deficiency
If you have a deficiency, you lack a particular substance needed by the body.

Page last reviewed: 13/07/2011

You do not usually need to prepare for a DXA scan. Depending on which part of your body is being scanned, you may be able to remain fully dressed if you are wearing clothes without any metal fastenings. Occasionally, it is necessary to remove clothing and wear a gown.

You will be asked to lie on your back on an X-ray table and keep still while the scan is being taken. The scan usually takes one to five minutes, depending on which part of your body is examined and whether a central or peripheral scanner is being used.

A central scan is usually performed by a radiographer (a specialist trained in taking X-ray images).

The scanning arm will slowly pass over your body and direct radiation through the relevant part of your skeleton. This is usually your hip and lower spine to check for osteoporosis. Because bone density varies in different parts of the skeleton, more than one part of your body will be scanned.

A peripheral scan may be performed using a small portable device in which either your forearm, heel or hand is scanned.

A DXA scan does not involve being in a tunnel, like a magnetic resonance imaging (MRI) or computerised tomography (CT) scan, or having an injection. It is a painless procedure and you can go straight home afterwards.

Your results

A DXA scan compares your bone density result to the bone density of either a young healthy adult or an adult of your own age, gender and ethnicity. The difference is then calculated as a standard deviation (SD). The difference between your measurement and that of a young healthy adult is known as a T score. The difference between your measurement and that of someone of the same age is known as a Z score.

According to the World Health Organization (WHO), T scores are classified as follows:

  • Above -1 SD is normal.
  • Between -1 and -2.5 SD is classed as osteopenia.
  • Below -2.5 SD is classed as osteoporosis.

If your Z score is below -2, your bone density is lower than it should be for someone of your age.

Although bone mineral density predicts 60-70% of bone strength, the results from a DXA scan will not necessarily predict whether a fracture is likely to occur. Some people who have low bone density will never break a bone, while some people with average bone density may have several fractures. This is because other factors, such as age or suffering a fall, also determine whether a fracture occurs.

Your doctor will always consider other risk factors in your individual case, such as family history and steroid use, before deciding whether treatment is necessary.

Dose
Dose is a measured quantity of a medicine to be taken at any one time.
Tissue
Body tissue is made up of groups of cells that perform a specific job, such as protecting the body against infection, producing movement or storing fat.  
X-ray
An X-ray is a painless way of producing pictures of inside the body using radiation.

Page last reviewed: 13/07/2011

DXA scans are commonly performed and considered safe. During the scan, you are exposed to a very low amount of radiation. The amount of radiation is less than one tenth of the amount used during a normal chest X-ray and equivalent to one day of exposure to natural background radiation.

The amount of radiation used during a DXA scan is considered safe for adults but can cause damage to unborn babies. If you are pregnant or think you might be, inform your doctor before your appointment. Pregnant women are advised not to have DXA scans unless it is essential.

 

Content provided by NHS Choices www.nhs.uk and adapted for Ireland by the Health A-Z.

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