Bone Density (DEXA) Scan
What Is A DEXA Scan?
DEXA stands for dual energy X-ray absorptiometry.
A DEXA 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 preparations are needed before you have a DEXA scan. Depending on the part of your body being scanned, you may be able to remain fully clothed. However, you will need to remove any clothes that have metal fastenings. In some cases, it may be necessary for you to wear a gown.
Why Do I Need A DEXA Scan?
A DEXA Scan (also known as DXA scan) is a special type of x-ray which looks to see if there is any evidence of osteoporosis (where the bones are thinning, become weak and fragile and more likely to break). This thinning can sometimes be caused by accumulation of mast cells.
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 the hormone oestrogen falls, resulting in a decrease in bone density.
The denser your bones, the stronger and less likely they are to fracture (break). Osteoporosis does not cause any symptoms until a bone is broken.
In the past it was 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 bone density scans, it is now possible to measure bone density before fractures occur.
The scan is used as a baseline and, if there is evidence of osteoporosis, medication can be started to try and slow the speed at which it develops or reverse it.
A DEXA bone scan can also help detect other bone-related conditions, such as osteopenia (very low bone mineral density) and osteomalacia (softening of the bones caused by a vitamin D deficiency).
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. The measurements will be compared to the normal range for bone density in a healthy adult and someone of the same gender and ethnicity.
Unlike a magnetic resonance imaging (MRI) scan, a DEXA scan does not involve being enclosed inside a tunnel, so you will not feel claustrophobic.
Instead, you lie on your back on a flat X-ray table. You will need to keep very still during the scan so that the images produced are not blurred.
The scan will usually be carried out by a radiographer (a specialist in taking X-ray images). As the scanning arm is slowly moved 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 osteoporosis. However, as bone density varies in different parts of the skeleton, more than one part of your body may be 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 contained in the scanning arm will measure the amount of X-rays that have passed through your body. This information will then be used to produce an image of the scanned area.
A DEXA scan usually takes five minutes or less, depending on the area of your body being examined and whether you are having a central or peripheral scan. During a peripheral scan, a small, portable device will be used to scan an area of your body such as your forearm, heel or hand.
You will be able to go home immediately after having a scan.
A DEXA scan compares your bone density with the bone density of a young healthy adult or an adult of your own age, gender and ethnicity. The difference is then calculated as a standard deviation (SD), which is a measure of variability based on an average or expected value.
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 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 your age.
Although bone mineral density (BMD) readings provide a good indication of bone strength, the results of a DEXA scan will not necessarily predict whether a fracture is likely to occur.
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 previously having had a fall, will also determine your susceptibility to experiencing a fracture. Therefore, your doctor will consider all of your individual risk factors before deciding whether treatment is necessary.
A DEXA scan is a quick and painless bone scan and is more effective than a normal X-ray in identifying low bone density. It also uses a much lower level of radiation than a standard X-ray.
A DEXA scan uses the equivalent of less than 10% of one day’s exposure to natural background radiation. By comparison, a chest X-ray uses the equivalent of about five days' exposure to natural background radiation.
Despite being very safe procedures, DEXA scans and X-rays are not recommended for women who are pregnant. X-rays are not considered to be safe to use during pregnancy because they can cause damage to an unborn child.
Here's a short video (4:15) of a DEXA scan procedure, with further clarifications.