What is a Bone Density Scan (DEXA)?
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. X-rays are the oldest and most frequently used form of medical imaging.
DEXA is most often performed on the lower spine and hips. If surgery has been performed on the spine or hips, or if compression fractures make the bone density inaccurate the forearm is also scanned. In children and some adults, the whole body is sometimes scanned.
What are some common uses of the procedure?
DEXA 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, 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.
How should I prepare?
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 that would be in the area being scanned should be removed.
You may be asked to remove some or all of your clothes and to wear a gown during the exam. You may also be asked to remove jewelry 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 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.
What does the equipment look like?
Central DEXA devices, like the machine at our facility measure bone density in the hip and spine. It has a large, flat table and an “arm” suspended overhead. The arm will move over you during the exam, in the area of interest. The technologist will be nearby, at the computer where your images will appear.
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.
DEXA machines feature special software that compute and display the bone density measurements on a computer monitor.
How is the procedure performed?
This examination is usually done on an outpatient basis.
In the Central DEXA examination, which measures bone density in the hip and spine, the patient lies on a padded table. An x-ray generator is located below the patient and an imaging device, or detector, is positioned above.
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.
You must hold very still while the x-ray picture is taken to reduce the possibility of a blurred image. Each scan takes about 3 to 5 minutes to complete. The total time of your exam will be approximately 10 to 15 minutes.
You will be asked to fill out a questionnaire that will help the doctor determine if you have medical conditions or take certain medications that either increase or decrease your risk of a fracture. The World Health Organization has recently released an online survey that combines the DEXA results and a few basic questions and can be used to predict 10-year hip fracture risk for post-menopausal women. This will be coming into more use in the next few years.
What will I experience during and after the procedure?
Bone density tests are a quick and painless procedure.
Routine evaluations every two years may be needed to see a significant change in bone mineral density, decrease or increase. Few patients, such as patients on high dose steroid medication, may need follow-up at six months.
Who interprets the results and how will I get them?
A radiologist, a 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 discuss 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 (low bone mass). 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.
Small changes may normally be observed between scans due to differences in positioning and usually are not significant.
What are the benefits vs. risks?
- DEXA bone densitometry is a simple, quick and noninvasive procedure.
- No anesthesia is required.
- The amount of radiation used is extremely small—less than one-tenth the dose of a standard chest x-ray, and less than a day’s exposure to natural radiation.
- 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.
- No radiation remains in a patient’s body after an x-ray examination.
- X-rays usually have no side effects in the diagnostic range.
- There is always a slight chance of cancer from excessive exposure to radiation. However, the benefit of an accurate diagnosis far outweighs the risk.
- 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.
A Word About Minimizing Radiation Exposure
Special care is taken during x-ray examinations to use the lowest radiation dose possible while producing the best images for evaluation. National and international radiology protection councils continually review and update the technique standards used by radiology professionals.
State-of-the-art x-ray systems have tightly controlled x-ray beams with significant filtration and dose control methods to minimize stray or scatter radiation. This ensures that those parts of a patient’s body not being imaged receive minimal radiation exposure.
What are the limitations of DXA Bone Densitometry?
- A DEXA test cannot predict who will experience a fracture but can provide indications of relative risk.
- Despite its effectiveness as a method of measuring bone density, DEXA is of limited use in people with a spinal deformity or those who have had previous spinal surgery. The presence of vertebral compression fractures or osteoarthritis may interfere with the accuracy of the test; in such instances, CT scans may be more useful.
- 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. These tests are not helpful in following response to treatment, however, and if they indicate that drug therapy is needed, a baseline central DEXA scan should be obtained.