WHAT IS BONE DENSITOMETRY?
A DEXA scan is a painless test that detects the density of bones. DEXA stands for dual-energy X-ray absorptiometry.
It is a fast and accurate test, and is preferred over normal x-rays for detecting bone density, because it is more sensitive. For example, normal x-rays can only detect osteoporosis (weakened bones) when around one-third of the bone mass has already gone. DEXA scans measure the calcium content in the bones – this cannot be evaluated in an ordinary x-ray.
These scans are sometimes called bone densitometry scans, QDR scans, or BMD (bone mineral density) measurement. They are usually used to scan the lumbar region (the lower back) and the hips.
DEXA scans help find out whether you have osteoporosis or are at risk of developing it. They can also be used to detect other bone disorders and conditions, and to monitor the relative amounts of body fat and muscle in the body.
There are two different types of DEXA scanning devices, central and peripheral. Central DEXA devices are more sensitive than peripheral ones. Bone mass often varies between parts of the body, so it is more accurate to measure the spine or/and hips htan a heel or wrist. The peripheral devices may help predict the risk of fracture in your spine or hip but cannot accurately follow changes in your bones during certain types of treatment (e.g. chemotherapy).
WHEN SHOULD IT BE DONE?
A DEXA scan involves a low dose of x-rays (less than a normal x-ray) being passed across the body. The x-rays are separated into beams of differing intensity. This enables the scan to detect the density of bone and soft tissue separately.
The scan usually takes between 10 and 15 minutes. In order to check for osteoporosis, the doctor will scan the lower back and hips.
No preparation for the test is required, and depending on which part of the body is being scanned, the patient may be able to remain fully dressed, in clothes without metal fastenings. It is a painless procedure and the patient can go straight home afterwards.
WHY IS IT NECESSARY?
By assessing the bone mineral density of the patient, a DEXA scan can work out whether the readings are significantly lower than normal for their age and sex, and if they are therefore more likely to break a bone. This technique is used to detect osteoporosis in its early stages, as it is very sensitive to slight changes in density.
Conditions other than osteoporosis can also cause low bone density levels and may also cause fractures. These include osteopenis (a milder form of bone loss), brittle bone disease, secondary prostate cancer, Paget’s disease and osteomalacia (vitamin D deficiency).
All patients on coricosteroids should have their bone strength measured with a DEXA scan because these medicines can weaken the bones.
People with a spinal deformity or who have had previous spinal surgery may gain little from a DEXA scan. The presence of vertebral compression fractures or osteoarthritis may interfere with the accurancy of the results.
WHO CAN USE IT?
This scan is a safe procedure but because DEXA scanners are relatively costly, they are not freely available throughout the UK, so high-risk groups are prioritised for scans, anyone can make an appointment with us.
The groups most likely to require a DEXA scan include:
- Women of menopausal age, who smoke or drink heavily, have a history in their family of hip fractures, or a body mass index (BMI) of less than 21
- Those who take oral steroids or corticosteroids for a long period (these medicines can contribute to weakening bones)
- Older women who have had bad fractures
- Women who have early menopause or ovaries removed, under the age of 45
- Men with very high consumption of alcohol
Pregnant women are advised against bone density measuring. The scan may damage the unborn child, in the same way to ordinary x-rays.