Bone density scanning, also called dual-energy x-ray absorptiometry (DEXA) or bone densitometry, is an enhanced form of x-ray technology that is used to measure bone loss.
DEXA 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.
DEXA has also been called dual energy X-ray absorptiometry, or DXA. DEXA bone density scan provides an estimate of the strength of your bones. DEXA can measure as little as 2 percent of bone loss per year. It is fast and uses very low doses of radiation.
When is a Bone Density Scan Needed?
DEXA 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.
DEXA is also effective in tracking the effects of treatment for osteoporosis and other conditions that cause bone loss.
The DEXA 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.
Before Your Bone Density Scan
On the day of the exam you may eat normally. You should not take calcium supplements for at least 24 hours before your exam. You should wear loose, comfortable clothing, avoiding garments that have zippers, belts or buttons made of metal. Objects such as keys or wallets, jewelry, removable dental appliances, eye glasses and any metal objects that would be in the area being scanned should be removed.
Inform your physician if you recently had a barium examination or have been injected with a contrast material for a computed tomography (CT) scan or radioisotope scan. You may have to wait 10 to 14 days before undergoing a DEXA test.
Women should always inform their physician and x-ray technologist if there is any possibility that they are pregnant. Many imaging tests are not performed during pregnancy so as not to expose the fetus to radiation. If an x-ray is necessary, precautions will be taken to minimize radiation exposure to the baby.
During Your Bone Density Scan
DEXA is most often performed on the lower spine and hips. In children and some adults, the whole body is sometimes scanned.
You will lie on your back on a padded table. An x-ray generator is located below the patient and an imaging device, or detector, is positioned above. You can usually remain fully clothed. You may lie with your legs straight or with your lower legs resting against a platform built into the table. The machine will scan your bones and measure the amount of radiation they absorb. Testing at least two different bones each time – preferably the hip and spine – is the most reliable way of measuring BMD. It is best to test the same bones and to use the same measurement technique and BMD equipment each time.
Usually there is no discomfort during a bone mineral density test. Bone density tests are a quick and painless procedure
The DEXA technique, which scans the hip and lower spine, takes only about 20 minutes to perform.