What is Radiography-Based (X-ray) Bone Densitometry?
Also called bone mineral density testing, the procedure measures how much bone material there is per square centimeter in your bones. Every day physicians use X-rays to view and evaluate bone fractures and other injuries of the musculoskeletal system. However, a plain X-ray test is not the best way to assess the amount of calcium and bone minerals packed into a small segment of your bone. To detect osteoporosis accurately, doctors use an enhanced form of X-ray technology called dual-energy X-ray absorptiometry (DXA or DEXA). DEXA is a quick, painless procedure. Measurement of the lower spine and hips are most often done. 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 experienced a fracture after only mild trauma.
- Have had x-ray evidence of vertebral fracture or other signs of osteoporosis.
How should I prepare for the procedure?
On the day of the exam eat normally, but don’t take calcium supplements for at least 24 hours beforehand. Wear loose, comfortable clothing, avoiding garments that have zippers, belts or buttons made of metal.
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 or x-ray technologist if there is a possibility they are pregnant.
What will I experience during the X-ray procedure?
The (DEXA) bone density test takes 10 to 30 minutes, depending on the equipment used and the parts of the body being examined. You may be asked to undress and put on a hospital gown. Then you’ll lie on a padded table with an x-ray generator below and a detector (an imaging device) above.
Most often, doctors focus on bone loss in the spine and hip where most osteoporosis-related fractures happen. During an examination of the spine, your legs will be supported on a padded box to flatten your pelvis and lower (lumbar) spine. To assess your hip, the technologist will place your foot in a brace that rotates the hip inward. In both cases the detector is slowly passed over the area, generating images on a computer monitor. It is important that you stay as still as possible during the procedure to ensure a clear, useful image. No anesthesia is required. The procedure is painless and radiation exposure is minimal.
Who interprets the results and how do I get them?
The results of a DEXA bone density exam are interpreted by a radiologist, who is a physician specially trained to diagnose conditions and diseases by obtaining and interpreting medical images. The radiologist will send an interpretation of your results and a signed report to your primary care physician who will work with you to develop a treatment plan. Usually available within a few days, your test results will be in the form of two scores:
T score — This number shows the amount of bone you have compared with a young adult of the same gender with peak bone mass. A score above -1 is considered normal. A score between -1 and -2.5 is classified as osteopenia, the first stage of bone loss. A score below -2.5 is defined as osteoporosis. It is used to estimate your risk of developing a fracture.
Z score — This number reflects the amount of bone you have compared with other people in your age group and of the same size and gender. If it is unusually high or low, it may indicate a need for further medical tests.
* Many DEXA scans are interpreted by other physicians such as rheumatologists and endocrinologists.