A DEXA (dual-energy X-ray absorptiometry) scan is a type of imaging test that measures bone mineral density (BMD). It uses a very small dose of ionizing radiation to produce pictures of the inside of the body, usually the lower (or lumbar) spine and hips, to measure bone loss. Bone density scans are also called dual X-ray absorptiometry (DEXA or DXA) scans. The test is simple, quick, and noninvasive.
What is a DEXA Scan Used For?
DEXA scans are primarily used to:
- Diagnose osteoporosis, a condition characterized by decreased bone density and increased risk of fractures.
- Assess an individual's risk for developing osteoporotic fractures.
- Monitor the effectiveness of osteoporosis treatment.
- Measure regional and whole-body fat and lean mass in patients with conditions like malabsorption, cancer, or eating disorders.
Who Should Get a DEXA Scan?
Screening for osteoporosis is recommended for specific patient populations, including:
- All women age 65 or older and men age 70 or older should be screened for asymptomatic osteoporosis.
- Women younger than 65 at risk for osteoporosis include the following groups:
- Those with estrogen deficiency
- Those with a history of maternal hip fracture before 50
- Those with low body mass (<127 pounds or body mass index (BMI) <18.5 kg/m²)
- Those with a history of amenorrhea for 1 year or more before 42
- Women younger than 65 or men younger than 70 with the following risk factors should also be screened.
- Current cigarette smoker
- Loss of height
- Thoracic kyphosis
- Individuals at any age with bone mass osteopenia or fragility fractures on imaging studies.
- Individuals 50 or older who develop wrist, hip, spine, or proximal humerus fractures with minimal or no trauma.
- Individuals of any age who develop 1 or more insufficiency fractures.
- Individuals receiving (or expected to receive) glucocorticoid therapy equivalent to 5 mg or more of prednisone daily for 3 or more months.
- Individuals considering pharmacological therapy for osteoporosis.
- Hypogonadal men 18 or older and men with surgically or chemotherapeutically induced castration.
- Children or adolescents with medical conditions associated with abnormal BMD.
- Individuals receiving (or expected to receive) glucocorticoid therapy for more than 3 months
- Individuals receiving radiation or chemotherapy for malignancies
- Individuals with endocrine disorders known to adversely affect BMD, such as hyperparathyroidism, hyperthyroidism, growth hormone deficiency, or Cushing syndrome
- Individuals with bone dysplasias known to have excessive fracture risk, such as osteogenesis imperfecta and osteopetrosis, or high bone density
- Individuals with medical conditions that could decrease BMD include the following conditions:
- Chronic renal failure
- Rheumatoid arthritis and other inflammatory arthritides
- Eating disorders, including anorexia nervosa and bulimia
- Organ transplantation
- Prolonged immobilization
- History of a gastric bypass for obesity
Your healthcare provider may recommend a DEXA scan if you have any of these other risk factors. DEXA scans should be used when the health benefits outweigh the risks.
Types of DEXA Scans
There are two main types of DEXA scans:
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- Central DEXA: This is the most common and accurate method. It measures bone density in the hip and spine using a central device located in hospitals and medical offices.
- Peripheral DEXA (p-DEXA): This scan measures bone density in the wrist, hand, or foot using a portable p-DEXA device. These devices are often available in drugstores and on mobile health vans.
Preparing for a DEXA Scan
Generally, little to no special preparation is required for a DEXA scan. However, you should:
- 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.
- Tell your doctor and the technologist if there is a possibility you are pregnant. Doctors will not perform many tests during pregnancy to avoid exposing the fetus to radiation. If an x-ray is necessary, the doctor will take precautions to minimize radiation exposure to the baby.
- You may be told to stop taking calcium supplements 24 to 48 hours before your test.
- Wear loose, comfortable clothing, avoiding garments that have zippers, belts, or buttons made of metal. You may need to remove some clothing and/or change into a gown for the exam. Leave jewelry at home.
- On the day of the exam you may eat normally.
What to Expect During a DEXA Scan
During a central DEXA scan:
- You will lie on your back on a padded table.
- For spine assessment, your legs will be supported on a padded box to flatten the pelvis and lower (lumbar) spine.
- For hip assessment, your foot will be placed in a brace that rotates the hip inward.
- The arm of the scanning machine will pass over your hips and spine. At the same time, another scanning machine will pass under you. The images from the two machines will be combined and sent to a computer.
- You must hold very still and may need to hold your breath for a few seconds while the technologist takes the x-ray. This helps reduce the possibility of a blurred image.
- The test usually takes 10 to 30 minutes.
Peripheral DEXA scans are simpler and may involve placing your wrist, hand, or foot in a portable box-like device for imaging.
Understanding DEXA Scan Results
Bone density results are typically given as T-scores and Z-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. It is primarily used for postmenopausal women and men age 50 or older.
- A T-score of -1.0 or higher is considered normal.
- A T-score between -1.1 and -2.4 is classified as osteopenia (low bone mass).
- A T-score of -2.5 or less is defined as osteoporosis.
- Less than or equal to −2.5 plus fragility fracture: severe osteoporosis
- 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. It is primarily used for premenopausal women, men under age 50, and children.
If your results show you have low bone density, your provider will recommend steps to prevent further bone loss. Your provider may recommend how often you need to have repeat scans, based on your risk of bone fractures.
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Benefits and Risks of DEXA Scans
Benefits
- Accurate and reliable measurement of bone density.
- Early detection of osteoporosis and fracture risk.
- Monitoring the effectiveness of osteoporosis treatment.
- Low radiation exposure compared to other imaging techniques.
- Simple, quick, and noninvasive procedure.
Risks
- Radiation Exposure: DEXA scans use a very small dose of ionizing radiation. The amount of radiation used in DEXA scans is very low and similar to the amount of radiation used in x-rays. We are all exposed to ionizing radiation every day from the natural environment. 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. No radiation stays in your body after an x-ray exam. There is always a slight chance of cancer from excessive exposure to radiation.
- Pregnancy: DEXA scans are not recommended during pregnancy due to the potential risk of radiation exposure to the developing baby. Even low doses of radiation could harm a developing baby. Women should always tell their doctor and x-ray technologist if they are pregnant.
- Limited Use in Certain Conditions: DEXA may be of limited use in people with a spinal deformity or those who have had previous spinal surgery.
- Variability Between Machines: Results and readings from DXA scans can vary between manufacturers of the scanning machines.
Factors Affecting Accuracy
Several factors can affect the accuracy of DEXA scan results, including:
- Patient Positioning: Incorrect positioning can lead to inaccurate measurements.
- Artifacts: Metal objects, such as jewelry, surgical clips, and vascular prostheses, can interfere with the scan.
- Demographics: Errors in assigning demographics can affect the T-score, as it is correlated to reference populations in gender and race.
- Structural Changes: Structural changes, such as osteophytes, calcifications, or fractures, are more common in the lumbar spine compared to in the proximal femur and potentially cause an artefactual component of BMD.
- Extremes of BMI: Extremes of high or low BMI may adversely affect the ability to obtain accurate and precise measurements.
Alternatives to DEXA Scans
While DEXA is the gold standard for measuring bone density, alternative methods exist:
- Quantitative Computed Tomography (QCT): QCT uses CT scans with special software to diagnose or monitor low bone mass.
- Quantitative Ultrasound (QUS): QUS is a non-ionizing method for determining BMD, particularly useful in children.
Cost and Insurance Coverage
The cost of a DEXA scan varies depending on your insurance coverage. Most insurance plans, including Medicare, cover DEXA scans with low or no out-of-pocket costs.
The Role of Exercise and Physical Therapy
Targeted exercises and physical therapy have proven to be especially effective in slowing and reversing osteoporosis. Several types of exercise are recommended for patients with osteoporosis to counter bone mass reduction. Combining multiple types of exercise can significantly affect BMD at three sites-the femoral neck, greater trochanter, and especially the spine. A meta-analysis of postmenopausal women found that resistance training increased lumbar BMD, and adding low-moderate impact exercises such as jogging, walking, and stair climbing was even more effective in preserving BMD at both lumbar and femoral levels.
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