A dual energy X-ray (DEXA, usually referred to as DXA) scan is a type of X-ray that measures the amount of calcium in bones. This measurement is often referred to as bone mineral density (BMD). DXA scans are most commonly used for diagnosing osteoporosis (weakened bones that may fracture easily) and assessing the risk of osteoporosis developing. They can also be used to detect other bone disorders and conditions, such as osteopenia, and to measure the relative amounts of body fat and muscle.
A DXA scan is a quick and painless way of measuring BMD. It is more sensitive than a normal X-ray when identifying low bone density. It uses a much lower level of radiation, equivalent to less than one day's exposure to natural background radiation.
You may be offered a DXA scan if you are considered at high risk of having or developing osteoporosis. The scans are also used to monitor the effectiveness of treatment for conditions such as osteoporosis.
DXA scans are sometimes called bone densitometry scans, QDR scans or BMD measurement.
How bone density is measured
During a DXA scan, X-rays are passed through the body. Some radiation will be absorbed in bone and soft tissue, and some will travel through the body. Special detectors in the DXA scanner measure how much radiation passes through the bones. This information is sent to a computer.
Measurements are then compared to the normal range of bone mineral density in healthy people of the same gender and ethnic background.
Types of scan
There are two different types of DXA scan:
- Axial or central DXA scan: a large scanning arm that passes over the body to measure bone density in the centre of your skeleton, such as your hip and lower spine.
- Peripheral DXA scan (pDXA): a large scanning arm or a smaller portable device used to measure bone mineral density in peripheral parts of your skeleton, such as your wrist, heel or hand.
Central DXA devices are more commonly used to diagnose osteoporosis. However, peripheral devices have the advantage of being small and portable.
- The spine supports the skeleton and surrounds and protects the delicate spinal cord and nerves. It is made up of 33 bones called the vertebrae.
- An X-ray is a painless way of producing pictures of inside the body using radiation.