What is a Bone Density Scan (DXA)?
Bone density scanning, also called dual-energy x-ray absorptiometry (DXA) or bone densitometry, is an enhanced form of x-ray technology that is used to measure bone loss. DXA is today’s established standard for measuring bone mineral density (BMD).
An x-ray (radiograph) is a noninvasive medical test that helps physicians diagnose and treat medical conditions. Imaging with x-rays involves exposing a part of the body to a small dose of ionizing radiation to produce pictures of the inside of the body. X-rays are the oldest and most frequently used form of medical imaging.
DXA is most often performed on the lower spine and hips. In children and some adults, the whole body is sometimes scanned. Peripheral devices that use x-ray or ultrasound are sometimes used to screen for low bone mass. In some communities, a CT scan with special software can also be used to diagnose or monitor low bone mass (QCT). This is accurate but less commonly used than DXA scanning.
When bone density testing is performed
DXA is most often used to diagnose osteoporosis, a condition that often affects women after menopause but may also be found in men and rarely in children. Osteoporosis involves a gradual loss of calcium, as well as structural changes, causing the bones to become thinner, more fragile and more likely to break.
DXA is also effective in tracking the effects of treatment for osteoporosis and other conditions that cause bone loss.
The DXA test can also assess an individual’s risk for developing fractures. The risk of fracture is affected by age, body weight, history of prior fracture, family history of osteoporotic fractures and life style issues such as cigarette smoking and excessive alcohol consumption. These factors are taken into consideration when deciding if a patient needs therapy.
Bone density testing is strongly recommended if you:
- are a post-menopausal woman and not taking estrogen.
- have a personal or maternal history of hip fracture or smoking.
- are a post-menopausal woman who is tall (over 5 feet 7 inches) or thin (less than 125 pounds).
- are a man with clinical conditions associated with bone loss.
- use medications that are known to cause bone loss, including corticosteroids such as Prednisone, various anti-seizure medications such as Dilantin and certain barbiturates, or high-dose thyroid replacement drugs.
- have type 1 (formerly called juvenile or insulin-dependent) diabetes, liver disease, kidney disease or a family history of osteoporosis.
- have high bone turnover, which shows up in the form of excessive collagen in urine samples.
- have a thyroid condition, such as hyperthyroidism.
- have a parathyroid condition, such as hyperparathyroidism.
- have experienced a fracture after only mild trauma.
- have had x-ray evidence of vertebral fracture or other signs of osteoporosis.
The Lateral Vertebral Assessment (LVA), a low-dose x-ray examination of the spine to screen for vertebral fractures that is performed on the DXA machine, may be recommended for older patients, especially if:
- they have lost more than an inch of height.
- have unexplained back pain.
- if a DXA scan gives borderline readings.
In most cases, you will be asked to undress and put on a hospital gown. All jewellery has to be removed prior to the test. There are several procedures which can be used to measure bone density, including:
- Dual energy x-ray absorptiometry (DEXA) – x-ray beams of differing energy are used to detect bone and soft tissue density separately. This technique can be used to measure bone density in the spine, hip, forearm and the total body. It is one of the most common methods to determine bone density as it is fast and highly accurate.
- Single energy x-ray absorptiometry – a single x-ray beam is used to measure bone density at peripheral sites like the forearm and heel. In this technique, the area to be tested is wrapped in a tissue-like substance or immersed in water to improve the quality of the results.
- Ultrasound – measurements taken during an ultrasound may provide data on the structural integrity of bone. New ultrasound devices such as quantitative ultrasound (QUS) can estimate bone density of the heel within minutes, providing an automatic print-out of results.
Immediately after the procedure
Most bone density scans take around half an hour or so and you can go home straight away. You will have to make a follow-up appointment with your doctor to discuss your results.
What the measurements mean
Bone density measurements are reported as a value in g/cm2 and as a T-score or Z-score, which describes your bone density measurement in relation to other people in a similar group – known as the ‘reference population’.
The reference population for the T-score is young adults of the same sex as the patient, while the reference population for the Z-score is a group of the same age and sex as the person being tested. Osteoporosis is diagnosed if your T-score is -2.5 or less.
Possible complications for bone density testing
Bone density testing is a safe procedure and the dose of radiation is roughly the same as you would receive from the general environment in about one day.
Taking care of yourself at home
Bone density testing is a safe and painless procedure with no after-effects. You can resume your normal activities as soon as the test is finished.
If you have osteoporosis, for example, you may need to have repeat bone density tests to make sure your treatment is helping to preserve the integrity of your bones. Try to have all future tests at the same testing facility using the same machine. This will rule out the possibility of any measurement differences between machines or facilities.
Other types of tests
Standard x-rays can only detect osteoporosis when around one-third of the bone mass has already gone. This means that x-rays cannot detect osteoporosis in its earlier stages. However, your doctor may request regular x-rays to make sure your symptoms aren’t caused by some other kind of condition or to check for bone fractures.