Bone densitometry scan
A DEXA scan is a special type of X-ray that measures bone mineral density (BMD).
DEXA stands for “dual energy X-ray absorptiometry”. This type of scan is also often known as DXA, or “dual X-ray absorptiometry”. It’s also sometimes referred to as a bone density scan or a bone densitometry scan.
DEXA scans are often used to diagnose osteoporosis (when the bones become weak and fragile, and are more likely to break).
They can also be used to assess the risk of osteoporosis developing in women aged over 50 and in men over 60.
As well as being quick and painless, a DEXA scan is more effective than normal X-rays in identifying low bone mineral density.
If you’re over 50 years of age, you may need to have a DEXA scan if you’re at risk of developing osteoporosis. If you’re under 50, you may need a DEXA scan if there are other risk factors at play, such as smoking or if you have had a previous fracture.
Such risk factors are used in the World Health Organization (WHO) 10-year Fracture Risk Assessment Tool, which applies to both men and women between 40 and 90 years of age.
The tool can be used to assess if a DEXA scan is appropriate, and also uses what is known as the DEXA femoral neck bone mineral density T score to calculate the risk of fracture.
Osteoporosis can affect people of both sexes and all ages, although older, post-menopausal women are particularly at risk. This is because after the menopause the level of oestrogen declines, resulting in a decrease in bone density.
The denser your bones, the stronger and less likely they are to fracture (break). Osteoporosis doesn’t cause any symptoms until a bone is broken. It used to be difficult to measure bone density and identify those at risk of developing osteoporosis until a fracture occurred.
However, by using bone densitometry techniques such as DEXA scans, it’s now possible to measure bone density before someone gets a fracture.
Read more about when DEXA scans are used.
Measuring bone density
During a DEXA scan, X-rays will be passed through your body. Some radiation will be absorbed by the bone and soft tissue, and some will travel through your body.
Special detectors in the DEXA scanner 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.
Read more about how DEXA scans are carried out.
DEXA scans use a much lower level of radiation than standard X-ray examinations, 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 DEXA scan will vary depending on the area of the body being examined, but is very low and less than two days’ exposure to natural background radiation (NBR).
By comparison, a chest X-ray uses the equivalent of about five days’ exposure to NBR, and a flight to North America is equivalent to about a week’s exposure to NBR.
Despite being very safe procedures, DEXA scans and X-rays aren’t recommended for pregnant women, as X-rays can damage an unborn child.
Read more about your health during pregnancy.
When a DEXA (DXA) scan is used
DEXA (DXA) scans are used to measure bone density and assess the risk of bone fractures.
They’re often used to help diagnose bone-related conditions such as osteoporosis (or the risk of developing it).
Total body DEXA scans can also be used to measure body composition (the amount of bone, fat and muscle in the body). This type of scan is routinely used in children, but is still a research application in adults.
Identifying bone conditions
Unlike ordinary X-ray images, DEXA scans can measure very small reductions in bone mineral density. This makes it possible to diagnose osteoporosis in its early stages before fractures occur.
Doctors use the results of DEXA scans to help them decide whether treatment for low bone mineral 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 DEXA scan is recommended
A DEXA scan may be recommended if you have an increased risk of developing a bone condition such as osteoporosis. Your risk is increased if you:
- have had a fracture after a minor fall or injury
- are a woman who has had an early menopause, or you’ve had your ovaries removed at a young age (before 45) and haven’t had hormone replacement therapy (HRT)
- are a post-menopausal woman and you smoke or drink heavily, have a family history of hip fractures or you have a body mass index (BMI) of less than 21
- are a man or a woman with a condition that leads to low bone density, such as rheumatoid arthritis
- are a woman who has large gaps between periods (more than a year)
- are a man or a woman taking oral glucocorticoids for three months or more (glucocorticoids are used to help treat inflammation, but can also cause weakened bones)
A DEXA scan isn’t the only way of measuring bone strength or fragility. Other risk factors need to be taken into consideration, such as family history and medication use, to determine who’s at risk of developing bone fractures.
All of the risk factors will need to be considered before a DEXA scan is recommended and before any treatment is started.
Some people will need to have a DEXA scan to confirm that their risk of developing bone fractures is high enough to need treatment. For others, particularly the elderly, the risk of fracture may be so high that there’s no need for them to have a DEXA scan before treatment is prescribed.
Interpreting the results of a DEXA 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. Sometimes, spinal abnormalities or a previous spinal fracture can give a false result.
A DEXA scan won’t show whether low bone mineral density is due to osteoporosis (too little bone) or osteomalacia (too little calcium in bone, usually due to a lack of vitamin D).
How a DEXA scan is performed
A DEXA (DXA) scan is a quick and painless procedure that involves lying on your back on an X-ray table so that an area of your body can be scanned.
No special preparations are needed before having a DEXA scan.
You may be able to remain fully clothed, depending on the area of your body being scanned. You will, however, 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.
The DEXA scan
Unlike a magnetic resonance imaging (MRI) scan or a computerised tomography (CT) scan, a DEXA scan doesn’t involve being enclosed inside a tunnel or a ring, so you won’t feel claustrophobic. Instead, you lie on your back on a flat and open X-ray table. You’ll need to keep very still during the scan so that the images produced aren’t 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 – this is done to check for osteoporosis (weak and brittle bones). However, as bone density varies in different parts of the skeleton, more than one part of your body may be scanned. For some conditions, the forearm is scanned.
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 will measure the amount of X-rays that have passed through your body. This information will be used to produce an image of the scanned area.
A DEXA scan usually takes around five minutes, although it depends what part of the body is being scanned. You’ll be able to go home after you’ve had it done.
A DEXA 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 then calculated as a standard deviation (SD) score. This measures the difference between your bone density and the expected value in terms of the natural spread of values in the healthy population.
The difference between your measurement and that of a young healthy adult is known as a T score, and 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 decreased bone mineral density (BMD) compared to peak bone mass (PBM)
- 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 DEXA scan won’t necessarily predict whether you will 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 – will also determine if you are likely to sustain a fracture.
Your doctor will consider all of your individual risk factors before deciding if treatment is necessary.