Also called: Bone Densitometry, Radiographic Absorptiometry, DEXA Scan, Dual Energy X Ray Absorptiometry Scan, Absorptiometry, Dual Photon Densitometry
Dual-energy x-ray absorptiometry is a painless, noninvasive imaging test that measures a person’s bone mineral density. More commonly known as a DEXA scan, a bone density test or bone densitometry, it is considered the most accurate tool available to diagnose and monitor osteoporosis.
Most bone densitometry tests send an x-ray beam through the bones. The thin beam contains much less radiation than standard x-rays. The beam has two energy peaks. Soft tissue absorbs one of these peaks, and bone absorbs the other. The amount of radiation absorbed in the tissue is subtracted from the total absorbed, and the remaining amount indicates the patient’s bone density. Some bone density tests involve ultrasound instead of x-rays.
The two major forms of bone densitometry are distinguished by the type of equipment used and the area of the body that is imaged:
- DEXA (or DXA) bone densitometry. This procedure is performed using large equipment known as a central device. It is the more commonly performed type of densitometry and focuses on the lower spine and hips, where fractures associated with osteoporosis most often occur.
- pDEXA bone densitometry. This procedure involves a portable machine known as a peripheral device. It measures bone loss in the fingers, heels or wrists. Because of the portability, it is getting more popular.
Patients are urged to closely follow all preparatory recommendations made by their physician. In most cases, such preparation is minimal. The test itself will unfold differently depending on which procedure take place. When a central device is used, the patient lies down on a padded table for a few minutes while a mechanical arm-like device with an imager passes over the body. When a peripheral device is used, the patient inserts an arm or foot into the device while an image is captured.
After testing is complete, the information obtained during the test is displayed on a computer monitor. A physician known as a radiologist examines the image and makes a diagnosis that is expressed in the form of two scores:
- T score. A number that indicates the density of bone the patient has compared to someone of the same sex who has peak bone mass (usually a healthy 30-year-old adult).
- Z score. A number that indicates the density of bone the patient has compared to someone in the same age group, body size and sex.
After a diagnosis has been suggested, the information is forwarded to the patient’s primary care physician. This physician will then discuss the results with the patient and develop a treatment plan. The test involves only a minimal exposure to radiation and has no significant risks.
About bone densitometry
Dual-energy x-ray absorptiometry is an imaging test that measures a person’s bone mineral density. More commonly known as a DEXA scan, a bone density test, a bone density scan or bone densitometry, it is a painless, noninvasive test that helps measure how many grams of calcium and other minerals are present in a segment of bone.
Bone densitometry is considered to be the most accurate tool available to diagnose and monitor osteoporosis. This disease most often occurs in women after menopause but is also common in elderly men and in certain other populations, such as people who have used corticosteroids or abused alcohol for a long time. Osteoporosis involves a progressive loss of calcium that thins the bones and leaves them more susceptible to fractures. The lower a person’s bone mineral levels, the greater the risk of fractures. Bone densitometry tests also are used to monitor the effectiveness of treatment after osteoporosis has been diagnosed.
During a typical bone densitometry test, an invisible x-ray beam is sent through the bones. The beam is thin and contains a dose of radiation much lower than that used for standard x-rays. This beam has two energy peaks. Soft tissue absorbs one of these peaks, and bone absorbs the other peak. The amount of radiation absorbed by the soft tissue is subtracted from the total absorbed, and the remaining amount represents the measure of a patient’s bone density.
Though most bone densitometry devices use x-rays, others use ultrasound, an imaging technique that uses sound waves. Bone densitometry is not the same as a bone scan, a nuclear medicine test that involves an injection of radioactive dye. Bone scans can be used to detect bone disorders such as fractures, cancer or infections.
Bone densitometry is often used as a preventive measure that allows a physician to assess the health of a person’s bones, especially in people who are at high risk for osteoporosis. Some experts urge all women age 65 or older (or age 60 or older with increased risk factors) to undergo bone densitometry testing.
Other experts recommend that people with certain risk factors undergo this testing. This includes postmenopausal women who are not taking estrogen hormone replacement, who have a personal or maternal history of hip fracture or smoking, or who are tall (over 5 feet 7 inches, or 170 centimeters) or thin (less than 125 pounds, or 56.7). People from a Caucasian or Southeast Asian ancestry also have increased risk of osteoporosis. Men with medical conditions associated with bone loss, such as decreased testosterone (male hypogonadism) due to aging or some treatments for prostate cancer, are also advised to receive bone density testing. Some physicians now recommend routine bone density tests for men starting at age 70 or 80, or beginning at a younger age if there are additional risk factors.
In some cases, bone densitometry may be performed to determine the risk of fractures in people who have other conditions associated with bone loss, such as anorexia, asthma, arthritis, chronic obstructive pulmonary disorder (COPD), diabetes, kidney disease, liver disease, parathyroid conditions or thyroid disorders. People who use certain medications that may cause bone loss (e.g., corticosteroids, certain cancer treatments, antacids that contain aluminum, antiseizure drugs, thyroid replacement drugs, possibly the antidepressants known as selective serotonin reuptake inhibitors) also may be good candidates for bone densitometry. Even some young people, such as teen athletes who undereat and overtrain, can develop osteoporosis. Others who may be considered for this testing include people who:
- Have high bone turnover (revealed through excessive collagen in urine)
- Experience a fracture after a mild injury
- Show evidence of vertebral fracture or other signs of osteoporosis, such as a loss of height or a stooped posture
Bone densitometry assists physicians by determining how much thinning has already taken place, after which the physician can evaluate the patient’s vulnerability to bone fractures. After the health of the bones is known, the physician and patient can work together on a treatment plan to strengthen the bones and to help prevent further bone loss.
In addition, bone densitometry can be used to monitor a patient’s bone density and to track the effectiveness of treatments.
Bone densitometry may not be effective for all patients. The accuracy of testing may be questionable in people who have spinal deformities, have had spinal surgery, or who have vertebral compression fractures or osteoarthritis. In such circumstances, a CAT scan (computed axial tomography) may be preferred instead of bone densitometry.
Types and differences of bone densitometry
The two major forms of bone densitometry are distinguished by the type of equipment used and the area of the body that is imaged:
- DEXA (or DXA) bone densitometry. This procedure is performed using equipment known as a central device. It is the more frequently performed type of densitometry and focuses on the lower spine and hips, where fractures associated with osteoporosis most often occur. Central devices are composed of a table (on which the patient lies down) with an arm-like device suspended overhead. They most often are found in hospitals and physicians’ offices. In addition, a lateral vertebral assessment procedure may be performed using the DXA equipment to screen for vertebral fractures. These fractures are common in older people.
- pDEXA bone densitometry. This procedure involves a portable machine known as a peripheral device. It is used to measure bone loss in the fingers, heels or wrists. Peripheral devices weigh about 60 pounds (27.2 kilograms) and include a space where a patient’s foot or arm is inserted. These devices most often found in locations such as pharmacies or mobile health vehicles. Peripheral devices are not as sensitive as central devices and are not used to monitor a patient’s progress during treatment because they do not detect changes in bone during therapy. The central devices are more sensitive in measuring bone density than peripheral devices.
Less commonly, bone density is assessed with:
- Ultrasound. Imaging with sound waves does not use radiation and may be used to screen small areas such as the heel, but it is less accurate and less comprehensive than a DEXA scan.
- CAT scan (computed axial tomography). A variation known as quantitative computed tomography (QCT) involves more radiation and higher cost than DEXA scans but may be recommended in some cases, such as if a patient has a spinal deformity.
- Dual photon absorptiometry (DPA). This radionuclide (nuclear medicine) test can predict risk of fractures but has disadvantages including limited ability to monitor changes in bone density over time.
Recent research suggests that dental x-rays might help reveal osteoporosis. MRI (magnetic resonance imaging) is also being investigated as a diagnostic tool for the disorder.
Some researchers have proposed the use of DEXA testing for estimating body composition. However, studies indicate that DEXA procedures are not reliable methods of measuring body fat.
Before the bone densitometry test
Patients are urged to closely follow all preparatory recommendations made by their physician. In most cases, such preparation is minimal. For example, patients usually are asked not to take calcium for at least 24 hours prior to the test. Patients also are advised to wear loose clothing without metal belts, buttons or zippers.
Patients are urged to tell their physician if they have recently had a barium examination or if they have been injected with a contrast material during a CAT scan (computed axial tomography) or radionuclide imaging scan. The residue of such injections could interfere with the accuracy of the bone densitometry test. In such situations, patients may be asked to wait several weeks before undergoing bone densitometry.
Women who are pregnant or think they may be pregnant are usually urged not to undergo bone densitometry.
During the bone densitometry test
The test itself will unfold differently depending on which procedure take place. When a central device is used, the patient lies down on a padded table for a few minutes while a mechanical arm-like device containing an imager passes over the body. The body does not come into contact with the device.
When a peripheral device is used, the patient inserts an arm or foot into the device for a few minutes while an image is captured.
In some cases, patients may also undergo a lateral vertebral assessment procedure. This is an additional low-dose x-ray that is used to locate fractures in a patient’s spine. Such fractures often occur in older people, placing them at greater risk for future fractures. Physicians often recommend this extra x-ray for people who have lost more than 1 inch (2.5 centimeters) of height or who have unexplained back pain.
The entire test takes between 10 and 30 minutes. Patients may be asked to hold the body in positions that feel a bit awkward and uncomfortable. It is important that they remain as still as possible during these brief periods. Discomfort associated with bone densitometry is minor, and the test presents few risks to the patient .
After the bone densitometry test
After testing is complete, the information obtained during the test is displayed on a computer monitor. A physician known as a radiologist examines the information. Radiologists are experts in interpreting medical images. The radiologist makes a diagnosis that is expressed in the form of two scores:
- T score. A number that indicates the level of bone density the patient has compared to someone of the same sex who has peak bone mass (usually a healthy 30-year-old adult). Most commonly the femurs (thigh bones) and spine are used to assess density. Differences between an individual’s score and an ideal score are measured in units called standard deviations.
A T score of 0 indicates that the patient’s bone density equals that of a healthy young adult. Higher scores indicate increased bone density, and lower scores suggest lower bone density. Scores above -1 are considered normal, and scores between -1 and -2.5 indicate lower-than-normal bone density (osteopenia), usually a first stage of bone loss. Osteopenia can lead to osteoporosis if left untreated. A score below -2.5 indicates osteoporosis. The lower the score, the more severe the osteoporosis.
- Z score. A number that indicates the level of bone density the patient has compared to someone in the same age group, body size and sex. Whereas T scores are used to diagnose osteoporosis, Z scores can help determine whether factors other than aging – such as an underlying disease or condition – may be responsible for bone loss. Z scores that are significantly higher or lower than the average may indicate the need to undergo further testing.
A patient’s T score and Z score can be used to help determine whether a person has normal bone density or osteoporosis. If bone loss is not severe enough to qualify as osteoporosis, osteopenia may be diagnosed. This low bone mass can result from several factors, including heredity, patient history of less-than-ideal bone mass during youth, presence of various medical conditions or the use of certain types of medications.
Physicians who are specialists in other fields (e.g., rheumatologists, endocrinologists) also may review the images from the test. After all the procedural information has been compiled and diagnosis has been suggested, the information will be forwarded to the patient’s primary care physician. This physician will then discuss the results with the patient and craft an appropriate treatment plan.
Treatment for low bone mass includes changes in diet that emphasize foods rich in calcium or vitamin D. Undertaking weight-bearing exercise (e.g., walking, jogging, dancing) also helps build bone mass, even for people in the senior years. In some cases, a physician may also prescribe medications that help prevent or even reverse osteopenia or osteoporosis.
Questions for your doctor about bone densitometry
Preparing questions in advance can help patients have more meaningful discussions regarding their healthcare. Patients may wish to ask their doctor the following questions about bone densitometry:
- Why do you suggest I schedule a bone density test?
- Which type of bone densitometry do you recommend for me?
- Will my test use x-rays, ultrasound or some other imaging method?
- How long will my test take?
- I recently had a test involving a contrast medium. How long will I have to wait to have my bone densitometry?
- Do I need to restrict my intake of calcium before the test or do anything else to prepare?
- I think I’m pregnant. Should I wait to have bone densitometry?
- How much radiation will be used during my test?
- Will I receive other tests in addition to bone densitometry?
- How long will I have to wait for test results, and who will explain them to me?
- Does my test show that I have osteoporosis, osteopenia or normal bone density? What treatment options, if any, do you recommend for me?
- How often should I be screened or monitored with a bone density test?