DEXA Bone Density
A bone density scan uses low dose X-rays to see how dense (or strong) your bones are. You may also hear it called a DEXA scan.
Bone density scans are often used to diagnose or assess your risk of osteoporosis, a health condition that weakens bones and makes them more likely to break.
As well as being quick and painless, a bone density scan is more effective than normal X-rays in identifying low bone density.
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+ Who needs to have bone density scan?
You may need to have a bone density scan if you're:
- For women who are post-menopausal.
- For men over the age of 70.
The results from a bone density scan are usually used alongside a fracture risk assessment to assess your chances of osteoporosis and breaking a bone.
Osteoporosis can affect anyone at any age, although older postmenopausal women are particularly at risk. This is because the level of oestrogen declines after the menopause, resulting in a decrease in bone density.
The more dense your bones, the stronger and less likely they are to break (fracture). Osteoporosis does not cause any symptoms until a bone is broken.
Measuring bone density
During a bone density scan, a special type of X-ray called dual energy X-ray absorptiometry is passed through your body. This is shortened to DEXA. Some radiation is absorbed by the bone and soft tissue, and some travels through your body. Special detectors in the DEXA machine measure how much radiation passes through your bones, and this information is sent to a computer.
Your bone density measurements will be compared with the bone density of a young healthy adult or an adult of your own age, gender and ethnicity.
+ How safe are bone density (DEXA) scans
Bone density scans are very safe. They use a much lower level of radiation than standard X-rays, which means that the radiographer (the technical specialist carrying out the scan) can stay in the scanning room with you during the scan.
The amount of radiation used during a bone density scan is very low and less than 2 days' exposure to natural background radiation (NBR). By comparison, a chest X-ray uses the equivalent of about 3 days' exposure to NBR, and a flight to North America is equivalent to approximately a week's exposure to NBR.
Despite being very safe, bone density scans and X-rays are not recommended for pregnant women, as X-rays can damage an unborn child.
+ When is it used
Bone density scans, also known as DEXA scans, help to work out your risk of breaking a bone.
They're often used to help diagnose bone-related health problems, such as osteoporosis, or to assess the risk of getting them. Total body bone density scans can also be used to measure the amount of bone, fat and muscle in the body.
This type of scan is routinely used in children, but is only used as part of a research study in adults.
+ Identifying bone problems
Unlike ordinary X-rays, DEXA scans can measure tiny reductions in bone density. This makes it possible to diagnose osteoporosis in its early stages, before you break a bone.
A DEXA scan also uses a low dose of radiation, which means the risks to health are much lower than with standard X-rays. Doctors use the results of bone density scans to help them decide whether treatment for low bone density is needed.
This may include making lifestyle changes to help improve your bone health, such as:
- eating a healthy, balanced diet that's high in calcium
- spending more time in the sun to help increase your levels of vitamin D
- regularly doing weight-bearing exercise, such as walking or running
+ When a bone density scan is recommended
A DEXA scan may be recommended if you have an increased risk of developing a bone problem like osteoporosis.
Your risk is increased if you:
- have had a broken bone after a minor fall or injury have a health condition, such as arthritis, that can lead to low bone density
- have been taking medicines called oral glucocorticoids for 3 months or more – glucocorticoids are used to treat inflammation, but can also cause weakened bones.
- are a woman who has had an early menopause, or you had your ovaries removed at a young age (before 45) and have not had hormone replacement therapy (HRT).
- are a postmenopausal woman and you smoke or drink heavily, have a family history of hip fractures, or a body mass index (BMI) of less than 21.
- are a woman and have large gaps between periods (more than a year).
A DEXA scan is not the only way of measuring bone strength. Other risk factors, such as family history and certain medicines, help to work out if you're at risk of breaking a bone.
All of the risk factors need to be considered before you have a bone density scan or start treatment. Some people need a bone density scan to confirm that their risk of breaking a bone is high enough to need treatment.
For others, particularly older people over the age of 75, the risk of breaking a bone may be so high that there's no need for them to have a bone density scan before treatment is prescribed.
Interpreting the results of a bone density scan can sometimes be difficult.
For example, it may not be easy to interpret the results of a scan of the spine when someone has a degenerative condition, such as osteoarthritis of the spine (spondylosis).
Sometimes spinal abnormalities or a previous spinal fracture can give a false result.
A bone density scan will not show whether low bone mineral density is caused by too little bone (osteoporosis) or too little calcium in the bone, usually because of a lack of vitamin D (osteomalacia).
+ How it is performed
A bone density scan is a quick and painless procedure that involves lying on your back on an X-ray table so an area of your body can be scanned. No special preparations are needed. You may be able to remain fully clothed, depending on the area of your body being scanned. But you'll need to remove any clothes that have metal fasteners, such as zips, hooks or buckles. In some cases, you may need to wear a gown.
When you have a DEXA scan, you will not need to go into a tunnel or have an injection.
Instead, you lie on your back on a flat, open X-ray table. You'll need to keep very still during the scan so the images are not blurred.
The scan will usually be carried out by a radiographer, a specialist in taking X-ray images. During the scan, a large scanning arm will be passed over your body to measure bone density in the centre of the skeleton.
As the scanning arm is moved slowly over your body, a narrow beam of low-dose X-rays will be passed through the part of your body being examined. This will usually be your hip and lower spine to check for weak bones (osteoporosis). But as bone density varies in different parts of the skeleton, more than one part of your body may be scanned. The forearm may be scanned for certain health problems, such as hyperparathyroidism, or if scans are not possible in the hip or spine. Some of the X-rays that are passed through your body will be absorbed by tissue, such as fat and bone. An X-ray detector inside the scanning arm measures the amount of X-rays that have passed through your body.
This information will be used to produce an image of the scanned area. The scan usually takes 10 to 20 minutes. You'll be able to go home after you have had it done.
+ Your results
A bone density scan compares your bone density with the bone density expected for a young healthy adult or a healthy adult of your own age, gender and ethnicity. The difference is calculated as a standard deviation (SD) score. This measures the difference between your bone density and the expected value.
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.
The World Health Organization (WHO) classifies T scores as follows:
- above -1 SD is normal
- between -1 and -2.5 SD is defined as mildly reduced bone mineral density (BMD) compared with peak bone mass (PBM)
- at or below -2.5 SD is defined as osteoporosis
If your Z score is below -2, your bone density is lower than it should be for someone of your age.
Although BMD results provide a good indication of bone strength, the results of a bone density scan will not necessarily predict whether you'll get a fracture.
For example, someone with low bone density may never break a bone, whereas someone with average bone density may have several fractures.
This is because other factors, such as age, sex or whether you have previously had a fall, also determine if you're likely to sustain a fracture.
Your doctor will consider all of your individual risk factors before deciding if treatment is necessary.