Bone density scanning, also called dual-energy x-ray absorptiometry (DXA) or bone densitometry, is an enhanced form of x-ray technology that is used to measure bone loss. DXA 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.
DXA is most often performed on the lower spine and hips. In children and some adults, the whole body is sometimes scanned. Peripheral devices that use x-ray or ultrasound are sometimes used to screen for low bone mass. In some communities, a CT scan with special software can also be used to diagnose or monitor low bone mass (QCT). This is accurate but less commonly used than DXA scanning.
DXA is also effective in tracking the effects of treatment for osteoporosis and other conditions that cause bone loss.
The DXA 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:
The Lateral Vertebral Assessment (LVA), a low-dose x-ray examination of the spine to screen for vertebral fractures that is performed on the DXA machine, may be recommended for older patients, especially if:
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, dentures, 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 DXA 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.
Central DXA devices measure bone density in the hip and spine and are usually located in hospitals and medical offices. Central devices have a large, flat table and an “arm” suspended overhead.
Peripheral devices measure bone density in the wrist, heel or finger and are often available in drugstores and on mobile health vans in the community. The pDXA device is much smaller than the Central DXA device, weighing only about 60 pounds. It is a portable box-like structure with a space for the foot or forearm to be placed for imaging. Other portable technologies such as specially designed ultrasound machines, are also sometimes used for screening.
The DXA 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.
DXA machines feature special software that compute and display the bone density measurements on a computer monitor.
In the Central DXA 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 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 technologist will walk behind a wall or into the next room to activate the x-ray machine.
The peripheral tests are simpler. The finger, hand, forearm or foot is placed in a small device that obtains a bone density reading within a few minutes.
An additional procedure called Lateral Vertebral Assessment (LVA) is now being done at many centers. LVA is a low-dose x-ray examination of the spine to screen for vertebral fractures that is performed on the DXA machine.
The LVA test adds only a few minutes to the DXA procedure.
The DXA bone density test is usually completed within 10 to 30 minutes, depending on the equipment used and the parts of the body being examined.
You will probably 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 DXA 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.
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.