What is a DEXA scan?
A dual energy X-ray absorptiometry (DEXA) scan, (or bone density scan) is a technique used to measure bone density. The scan is quick and painless and involves the patient lying on their back on an X-ray table. There is no tunnel to pass through as with an MRI or CT scan, so there is no feeling of claustrophobia.
What is the procedure for a DEXA scan?
The scan takes around 15 minutes. Whilst lying down on an X-ray table, a scan will be carried out by a radiographer. A large scanning arm passes over the body to measure bone density in the centre of the skeleton. A low-dose beam of X-rays passes through the part of the body being examined, which is usually the lumbar spine and both hips. Bone density varies throughout different parts of the skeleton and more than one of part of the body may be scanned.
Some of the X-rays that pass through the body are absorbed by the bone and soft tissue. An X-ray detector inside the scanning arm is able to measure the amount of X-rays that have passed through the body, which then produces an image of the scanned part of the body that is sent to a computer.
What do my DEXA scan results show?
The bone density measurements will be compared with the bone density of a young healthy adult (known as a T score) or with an adult of your age, gender and ethnicity (known as a Z score). There are special detectors in the in the scanner that measures the amount of radiation that has passed through the bones.
The comparison of bone density with a healthy person’s is calculated as a standard deviation (SD) score. It measures the difference between the patients current bone density and the expected value in terms of the natural spread of values in the healthy population.
The T score shows that:
- Above -1 SD is normal
- Between -1 and -2.5 SD is mildly reduced bone mineral density compared to peak bone mass
- At or below -2.5 SD is defined as osteoporosis
If the Z score is below -2 then the bone density is lower than it should be. The bone mass density shows a good indication of bone strength but it does not mean it can predict whether a fracture will occur.
Who should have a DEXA scan?
A DEXA scan is advised for those who have a bone fracture following a minor injury. It is also advised if a patient is considered at increased risk of osteoporosis. The thinning of the bones means that the patient is more likely to have a fracture. If the doctor thinks a patient is at risk of developing osteoporosis they may use the Fracture Risk Assessment Tool (FRAX) risk calculator. This calculates the likeliness of bone fracture following a minor knock. If the risk level is medium then a DEXA scan is necessary. The scan can also monitor if a current treatment plan for osteoporosis is working.