Dual Energy X-ray Absorptiometry (DEXA Scan)

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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 DEXA scans

• Central DEXA
  • 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.

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

The screening tests measure bone density in the lower arm, wrist, finger or heel. A screening test cannot diagnose osteoporosis but the can identify an individual who would benefit from additional investigations.

Reliability and Validity

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 Results were also "good" for internal reliability but only “acceptable” for external reliability.

Change of position from supine to prone slightly decreased the reproducibility of total body measurements.

Limitations

Most Central DEXA machines cannot measure the hip and spine of a patient that weighs more than 300 pounds or 136kg. In this case, a central DEXA of the radius and the calcaneous can be carries out.