Dual Energy X-ray Absorptiometry (DEXA Scan)

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Top Contributors - Carin Hunter, Lucinda hampton, Tarina van der Stockt and Kim Jackson  

Objective

Dual Energy X-ray Absorptiometry (or DEXA, or bone densitometry) is used primarily for osteoporosis tests. It is not projection radiography, as the X-rays are emitted in 2 narrow beams that are scanned across the patient, 90 degrees from each other. Usually the hip (head of the femur), lower back (lumbar spine) or heel (calcaneum) are imaged, and the bone density (amount of calcium) is determined and given a number (a T-score). It is not used for bone imaging, as the image quality is not good enough to make an accurate diagnostic image for fractures, inflammation etc. It can also be used to measure total body fat, though this isn't common. The radiation dose received from DEXA scans is very low, much lower than projection radiography examinations.

Intended Population

The World Health Organization (WHO) established dual x-ray absorptiometry tests scans (DEXA) of the central skeleton is the best test for assessing bone mineral density

National Osteoporosis Foundation recommends that you have a bone density test if:

• you are a woman age 65 or older
• you are a man age 70 or older
• you break a bone after age 50
• you are a woman of menopausal age with risk factors
• you are a postmenopausal woman under age 65 with risk factors
• you are a man age 50-69 with risk factors

A bone density test may also be necessary if you have any of the following:

• an X-ray of your spine showing a break or bone loss in your spine
• back pain with a possible break in your spine
• height loss of ½ inch or more within one year
• total height loss of 1½ inches from your original height

Method of Use

Bone density test results are reported using T-scores. T-scores are relative to how much higher or lower your bone density is compared to that of a healthy 30 year old adult

• A t-score reflects the difference between the measured bone mineral density and the mean value of bone mineral density in young adults.
• It is measured in standard deviations.
• The WHO has defined normal bone mineral density for women as a t-score within one standard deviation of the young adult mean.
• Scores between negative 1 and negative 2.5 reflect a diagnosis of osteopenia.
• Scores below negative 2.5 reflect a diagnosis of osteoporosis.

DEXA stands for dual energy X-ray absorptiometry. This technique was introduced for commercial use in 1987. It sends two X-ray beams at different peak energy frequencies to the target bones.

One peak is absorbed by soft tissue and the other by bone. When the soft tissue absorption amount is subtracted from the total absorption, the remainder is your bone mineral density.

The test is noninvasive, fast, and more accurate than a regular X-ray. It involves an extremely low level of radiation.

2 types of DXA scans

• Central DXA

NOF recommends a bone density test of the hip and spine using a central DXA machine to diagnose osteoporosis. DXA stands for dual energy x-ray absorptiometry. When testing can’t be done on the hip and spine, NOF suggests a central DXA test of the radius bone in the forearm. In some cases, the type of bone density testing equipment used depends on what is available in your community.

Healthcare providers measure bone density in the hip and spine for several reasons. First, people with osteoporosis have a greater chance of fracturing these bones. Second, broken bones in the hip and spine can cause more serious problems, including longer recovery time, greater pain and even disability. Bone density in the hip and spine can also predict the likelihood of future breaks in other bones.

With most types of bone density tests, a person remains fully dressed, but you do need to make sure no buttons or zippers are in the way of the area to be scanned. The test usually takes less than 15 minutes. Bone density tests are non-invasive and painless. This means that no needles or instruments are placed through the skin or body. A central DXA uses very little radiation. You are actually exposed to 10–15 times more radiation when you fly roundtrip between New York and San Francisco.

When repeating a bone density test, it is best to use the same testing equipment and have the test done at the same place each time. This provides a more accurate comparison with your last test result. Although it is not always possible to have your bone density test at the same place, it is still important to compare your current bone density scores to your previous scores.

Standard x-rays cannot be used in place of bone density tests. Unlike bone density tests, X-rays are not able to show osteoporosis until the disease is well advanced. However, X-rays can be used in addition to a DXA to detect broken bones in the spine or elsewhere.

• Screening Tests

Also called peripheral tests, screening tests measure bone density in the lower arm, wrist, finger or heel. The types of peripheral tests are:

• • pDXA (peripheral dual energy x-ray absorptiometry)
  • QUS (quantitative ultrasound)
  • pQCT (peripheral quantitative computed tomography)

Screening tests can help identify people who are most likely to benefit from further bone density testing. They are also useful when a central DXA is not available. These tests are often done at health fairs and in some medical offices. Screening tests cannot accurately diagnose osteoporosis and they should not be used to see how well an osteoporosis medicine is working.

If you have a peripheral bone density test, you should follow up with your healthcare provider. Discuss whether you need additional testing, such as a central DXA test of the hip and/or spine. The results of a peripheral test cannot be compared with the results of a central DXA.

People of Larger Size. Most central DXA machines cannot measure bone density in the hip and spine of patients who weigh more than 300 pounds. Some newer machines can measure bone density in people who weigh up to 400 pounds, but these machines are not widely available. When the hip and spine cannot be measured, some healthcare providers recommend a central DXA test of the radius bone in the forearm and a peripheral bone density test of the heel or another bone. Doing both of these tests might provide more complete information.

Evidence

Reliability and Validity

We examined data for density changes by bone compression after TAA, and found “good” results for internal reliability but only “acceptable” results for external reliability.

Repeatability of total body DEXA measurements was excellent for bone mineral content (r = 0.99), LM (r = 0.99), fat tissue mass (r = 1.00), and bone mineral density (r = 0.98) in supine scanning. Change of position from supine to prone slightly decreased the reproducibility of total body measurements. Reproducibility of regional measurements was inferior to total body results; especially in the upper extremity, the repeated automatic LM measurements in supine-supine positions produced r values as low as 0.74 but increased to 0.93 after manual adjustment of the ROIs. To obtain maximal reliability of the composition measurements, we recommend manual checking of machine-made ROIs and, if needed, manual adjustment to avoid measurement errors.

Conclusions

The results were reliable and reproducible.