Bone densitometry is used to assess bone mass by measuring bone mineral density (which is the amount of bone per unit of skeletal area). It is the most sensitive screening tool to detect osteoporosisa disorder characterized by fragile, weak bones due to a drop in bone mass and increased risk of fracture. The following techniques are used to measure bone mineral density.
Dual energy x-ray absorptiometry (DEXA) is considered the gold standard for quantifying bone mass in the spine and hip bone. In this technique, an x-ray tube emits 2 x-ray beams, which pass through the bone and are picked up by a detector. A computer is used to analyze the resulting images and calculate bone density based on the amount of radiation absorbed by the bone—denser bones absorb more radiation.
Peripheral DEXA (pDEXA) uses a small, portable device that works similar to DEXA but is used to examine bone density in peripheral bones, such as the wrist.
Single x-ray absorptiometry (SXA) also uses a small, portable device that works similar to DEXA, but emits a single x-ray beam. It is used to examine the heel bone or forearm.
Quantitative ultrasound (QUS), the only technique for measuring bone density that does not use x-rays, works by transmitting high-frequency sound waves through the heel bone to a signal receiver.
Quantitative computed tomography (QCT) uses a standard CT scannerfitted with special equipment and software to measure bone mass—to deliver x-rays to the spine at many different angles.
Purpose of Bone Densitometry
- To screen people at high risk for osteoporosis, such as postmenopausal women or individuals on long-term therapy with corticosteroid drugs
- To detect suspected osteoporosis in people who have experienced recent unexplained bone fractures
- DEXA is the only method that may be used to make a definitive diagnosis of osteoporosis and to monitor the response to treatment
- To detect and evaluate other disorders affecting bone mass, such as the inherited disorder osteogenesis imperfecta
Who Performs Bone Densitometry
- A physician or a technician
Special Concerns about Bone Densitometry
- DEXA is the most accurate method for measuring bone density in the spine and hip bone, the most common sites of fracture. It is of limited use, however, in people with a spinal deformity or those who have had previous spinal surgery. In addition, the presence of vertebral compression fractures or degenerative disease such as osteoarthritis may interfere with the accuracy of the test.
- QCT is only used in selected individuals due to its limited availability, relatively high cost, and greater radiation exposure. It is the only method that can evaluate spongy bone tissue in the spine, which may be useful when assessing the response to therapy for osteoporosis.
- Because pDEXA, SXA, and QUS are performed by small, portable instruments and are less expensive than DEXA or QCT, many physicians now use these methods to screen patients in their offices.
- Pregnant women should not undergo x-ray tests because exposure to ionizing radiation may harm the fetus.
- Because residual barium in the gastrointestinal tract can interfere with test results, DEXA should not be performed until about 10 days after an imaging study using this contrast agent.
- Calcium in blood vessels overlying the area being scannedsometimes found in atherosclerotic plaquesmay falsely increase bone density.
- Movement during the scan may alter the test results.
Before Bone Densitometry
- Do not take calcium supplements for 24 hours before the test.
- Remove all metallic objects (such as belt buckles, zippers, jewelry, coins, and keys) that might obstruct the scanning path.
- You may be able to wear your street clothes, but for DEXA or QCT you may be asked to disrobe and put on a hospital gown.
- Tell your doctor if you recently had a test that involves contrast material, such as a barium swallow, barium enema, computed tomography (CT) scan or radioisotope scan.
- Tell your doctor if you are pregnant or may be pregnant.
What You Experience during Bone Densitometry
- You will lie on a padded table with an x-ray generator below and a detector camera above.
- To examine your spine, your legs will be supported on a padded box to flatten your pelvis and lower (lumbar) spine. The detector device is slowly passed over the lumbar area, producing images projected onto a computer monitor.
- To assess the hip, your foot is placed in a brace that rotates the hip inward, and the scanning procedure is repeated.
- The scan may take up to 30 minutes.
- You place your finger or hand in a small scanning device and a bone density reading is obtained within a few minutes.
- You immerse your heel or forearm in a tub of water. A scanning device obtains a bone density reading through the water within a few minutes.
- An ultrasound device held next to your heel obtains a reading in about 10 seconds.
- You will lie on your back on a narrow table that is then advanced into the CT scanner. The scanner, which encircles you, rotates around you taking pictures at different intervals and from various angles. You will feel the table move during the test.
- You must remain as still as possible because any movement can distort the scan.
- The test typically takes 15 to 30 minutes.
>Risks and Complications of Bone Densitometry
- Ultrasound imaging does not expose you to any radiation.
- DEXA, pDEXA, and SXA involve only minimal exposure to radiation. Although radiation exposure is still minimal with QCT, you will receive a higher dose (50 to 100 times greater than DEXA).
After Bone Densitometry
- You may resume your normal activities immediately after the test.
Bone Densitometry Results
- A physician reviews the test data to determine your bone mineral status. In some cases, a follow-up test is performed about 2 years after the first scan to evaluate bone loss rate.
- If the findings indicate bone loss due to osteoporosis, your doctor will prescribe a treatment program—which typically includes exercise, calcium and vitamin D supplements, and sometimes hormone replacement or other medications—to help forestall further bone loss and fractures.
The Johns Hopkins Consumer Guide to Medical Tests
Simeon Margolis, M.D., Ph.D., Medical Editor
Updated by Remedy Health Media