Dual Energy X-ray Absorptiometry (DEXA Scan)

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

Objective

Dual Energy X-ray Absorptiometry (or DEXA, or bone densitometry) is used primarily for osteoporosis tests. 2 narrow x-ray beams are emitted at a 90 degree angles across the patient. The most commonly imaged areas are he hip (head of the femur), lower back (lumbar spine) or heel (calcaneum), 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 the bone mineral density.

Although not common, it can be used to measure total body fat. The test is noninvasive, fast, and more accurate than a regular X-ray. It involves an extremely low level of radiation.

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 DEXA scan 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

Interpretation of scores

Bone density test results are reported using T-scores. T-scores are in reference 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.
• 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.

2 types of DXA scans

1. Central DXA
   • The National Osteoporosis Foundation recommends a central DEXA test of the radius bone in the forearm when testing can’t be done on the hip and spine
   • This scan will assist a diagnosis of osteoporosis which is a risk factor for fractures as well as predicting future breaks in other bones.
   • For this scan you remain fully clothed as long as there are no buttons or zippers in the way of the area to be scanned.
   • The scan takes less than 15 minutes and is painless.
   • For the most accurate comparison between scans, have the test done at the same location with the same equipment if possible.

1. Screening Tests or peripheral tests
   • pDXA (peripheral dual energy x-ray absorptiometry)
   • QUS (quantitative ultrasound)
   • pQCT (peripheral quantitative computed tomography)

These measure bone density in the lower arm, wrist, finger or heel. 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.