What is a Bone Density Scan (DEXA)?
Bone density scanning, also called dual-energy x-ray absorptiometry (DXA or 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).
What are some common uses of the procedure?
DEXA bone densitometry is most often used to diagnose osteoporosis, a condition that often affects women after menopause but may also be found in men. Osteoporosis involves a gradual loss of calcium, 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.
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 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.
How should I prepare for the procedure?
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 may be asked to wear a gown during the exam. You may also be asked to remove jewelry, eye glasses and any metal objects or clothing that might interfere with the x-ray images.
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 any possibility that they are pregnant. Many imaging tests are not performed during pregnancy because radiation can be harmful to the fetus. If an x-ray is necessary, precautions will be taken to minimize radiation exposure to the baby.
How does the procedure work?
The DEXA machine sends a thin, invisible beam of low-dose x-rays with two distinct energy peaks through the bones being examined. One peak is absorbed mainly by soft tissue and the other by bone. The soft tissue amount can be subtracted from the total and what remains is a patient's bone mineral density.
How is the procedure performed?
This examination is usually done on an outpatient basis.
The patient lies on a padded table.
To assess the spine, the patient's legs are supported on a padded box to flatten the pelvis and lower (lumbar) spine. To assess the hip, the patient's foot is placed 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.
The patient must hold very still and may be asked to keep from breathing for a few seconds while the x-ray picture is taken to reduce the possibility of a blurred image.
The DEXA bone density test is usually completed within 10 to 30 minutes, depending on the equipment used and the parts of the body being examined.
Who interprets the results and how do I get them?
A radiologist, or other physician specifically trained to supervise and interpret radiology examinations, will analyze the images and send a signed report to your primary care or referring physician, who will share the results with you.
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. The T score 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 this score is unusually high or low, it may indicate a need for further medical tests.
What are the benefits vs. risks?
- DEXA bone densitometry is a simple, quick and non-invasive procedure.
- The amount of radiation used is extremely small—less than one-tenth the dose of a standard chest x-ray.
- DEXA bone density testing is the most accurate method available for the diagnosis of osteoporosis and is also considered an accurate estimator of fracture risk.
- DEXA equipment is widely available, making DEXA bone densitometry testing convenient for patients and physicians alike.
- X-rays usually have no side effects.
- There is always a slight chance of cancer from radiation. However, the benefit of an accurate diagnosis far outweighs the risk.
- The effective radiation dose from this procedure is about 0.01 mSv, which is about the same as the average person receives from background radiation in one day. Thus, if there is an increased risk of cancer, the risk of might be equivalent to what is incurred by living one extra day.
- Women should always inform their physician or x-ray technologist if there is any possibility that they are pregnant.
- No complications are expected with the DEXA procedure.
What are the limitations of a DEXA Scan?
- A DEXA test cannot predict who will experience a fracture but can provide indications of relative risk (i.e, "What are the odds?").
- A test done on a peripheral location, such as the heel or wrist, may help predict the risk of fracture in the spine or hip. But because bone mass tends to vary from one location to the other, measuring the heel is not as accurate as measuring the spine or hip.