What is a bone mineral density scan?
A dual-energy X-ray absorptiometry scan (DXA), or bone density scan, is a special type of X-ray that measures bone mineral density (BMD). It provides information about bone strength or fragility and the risk of fractures or broken bones. The higher the density, generally, the lower the risk of fracture.
The spine and one or both hips are routinely scanned. The forearm might also be scanned if either the hip or spine is unavailable (usually due to surgery). As any condition affecting bone density tends to affect the whole skeleton, a snapshot of a few sites is sufficient to establish the overall bone density. The BMD at the hip and spine has been shown to be the best way of predicting the risk of fracture.
There are no after effects of a BMD Scan.
A BMD scan varies between individual scanning machines, and can take from 10 minutes to 30 minutes.
Generally, the risks of a BMD scan by DXA are very small. The radiation doses used are extremely small and significantly less than those used in normal X-ray images, allowing the radiographer who carries out the scan to remain in the room, without the need for lead protection.
A DXA scan is currently the best test for measuring the amount of bone (density) in the spine, hip or wrist. The result is a comparison of your bone density with both the young normal population (of the same sex), and to an age- and sex-matched population. The BMD provides information about fracture risk and bone loss. It can be used to monitor response to treatment, with the usual frequency of scanning being 2 years.
More accurate assessment of BMD is obtained when the measurements are repeated at the same location and on the same machine.
A radiographer skilled in DXA scans will carry out the scan, and ensure you are positioned correctly and you are comfortable. In most instances the technologist will remain in the room with you for the duration of the scan.
The images taken by the radiographer will be reviewed by a radiologist (specialist doctor), or another medical specialist trained in assessing the results of BMD, and a computerised assessment report will be provided to your referring doctor immediately.
Why would my doctor refer me to have this procedure?
You might be referred for this test if you have:
Osteoporosis is a condition that increases with age. It is a major cause of weak bones, causing fractures resulting from minor injury, which could be preventable with treatment. Osteoporosis, in the absence of fracture, has no symptoms. A number of medical conditions and medications can increase bone loss, making it important to diagnose osteoporosis early, to prevent fractures from occurring. The DXA scan measures the bone mineral content and provides information to your doctor as to whether you have lost a small amount of bone (osteopenia) or a more significant amount (osteoporosis), compared with a young normal population, and people of the same age and sex as you. This informs your doctor about your risk of having a fracture, and assists in monitoring bone loss and in planning any preventative therapy or medical treatment.
No preparation is required for this procedure. You do not need to fast, and you can take all your medications as usual. There are no tunnels or confined spaces, no injections and the procedure is not painful. It is helpful, but not essential, to wear loose fitting, comfortable clothing without metal buttons, buckles, fasteners or zippers, as metal objects interfere with the scan. A gown is provided if clothing needs to be removed. A DXA scan does involve a very small dose of radiation, which makes this test unsuitable for women who are, or might be, pregnant.
If you have had spinal surgery, particularly with metallic implants, or hip surgery (hip replacements, screws or pins) you will need to inform the radiographer carrying out the scan who might decide to avoid that area. Any radiological investigation using contrast media (Barium enemas, IVP’s and CT scans) or nuclear medicine test might interfere with the accuracy of the DXA scan if carried out within the last week. This can usually be discussed at the time you book your DXA scan appointment.
On arrival for your DXA scan you will be asked to fill in a questionnaire to provide the latest software with your pertinent medical information. Your height and weight will be measured. This allows the computer to generate information about your bone density.
The most important aspect of a DXA scan is to position the hips and spine in the same way each time you are scanned, so that results are accurate and comparable at each visit. To achieve this, when scanning the spine, a cushioned box will be placed under your knees. The cushioned box allows the small of your back or lower spine to lie flat on the table. To scan the hip, this box is removed and a frame made up of a flat sheet of Perspex with a triangle at one end will be placed between your feet. The frame allows the leg being scanned to be positioned accurately. The foot is strapped to the triangle by Velcro, and the knee can also be held in place by a Velcro strap to keep the leg still. Generally, neither of these positioning manoeuvres are uncomfortable or painful.
A number of factors can affect bone health. For example:
It is important that you discuss the results with the doctor who referred you, either in person or on the telephone, so that they can explain what the results mean for you.
Further information about a BMD scan:
Although osteoporosis is the most common cause for weak bones, there are other less common conditions that have the same effect. If your doctor is concerned about this, you might be referred for more specialised images of the bones (e.g. CT scans or X-rays), blood tests or referred to a BMD medical specialist for further advice.
Your bones are continuously changing — new bone is made and old bone is broken down. When you're young, your body makes new bone faster than it breaks down old bone, and your bone mass increases.
Most people reach their peak bone mass around age 30. After that, bone remodeling continues, but you lose slightly more bone mass than you gain.
How likely you are to develop osteoporosis — a condition that causes bones to become weak and brittle — depends on how much bone mass you attain by the time you reach age 30 and how rapidly you lose it after that. The higher your peak bone mass, the more bone you have "in the bank" and the less likely you are to develop osteoporosis as you age.