Bone densitometry scan - 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.

Your results

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.