Dual energy X-ray absorptiometry or DEXA is the most common and precise method to measure bone mineral density (BMD), which is the amount of calcium and minerals present in a part of bone. The DEXA scan is a quick and painless way of measuring BMD. It uses a very low dose of X-ray which constitutes about 1/10 of the radiation received from a standard chest X-ray.
Types
There are two different types of DEXA scanning devices:
- Central DEXA: The central DEXA scanner is passed over lower part of your spine and hip when you are lying on a table. It is the most reliable test to predict risk of fractures
- Peripheral DEXA (p-DEXA): The peripheral DEXA device is a smaller machine that is used to measure bone density in the periphery of your skeleton, such as your wrist, heel or finger
Indications
A DEXA scan is used to diagnose osteoporosis, a disease in which bones become fragile and more susceptible to fracture. It is highly recommended in people with increased risk of osteoporosis such as
- Women aged above 65 and men aged above 70
- With broken bone following a minor fall or injury
- Having disorders such as rheumatoid arthritis, chronic kidney disease, and eating disorders
- An early menopause
- Use of corticosteroid therapy for more than three months
- Cigarette smoking
- Previous use of hormonal preparations for prostate cancer or breast cancer
- Loss of height due to vertebral compression fracture
- Family history of osteoporosis
- Frequent alcohol consumption
The DEXA machine sends two X-ray beams with two distinct energy levels through the bone being tested. The amount of X-rays that comes through the bone from each of the two X-ray beams is measured. The bone density is measured based on the difference between the two beams.
Procedure
Normally, there are no restrictions on eating prior to the test. Calcium supplements should be discontinued 24hours before the test. Women who are pregnant should always inform their physician or X-ray technologist.
In the central DEXA examination, you will be asked to lie down still on your back on an X-ray table. The scan usually takes between 10 to 20 minutes. The scanning arm of the device will pass over the region of your body being measured. The bones commonly scanned are the hip and lower spine.
Test results will be in the form of two scores:
- T score: It is the comparison between your bone density with that of a young healthy woman. A T score of -1.0 or above is considered normal. A score between -1 and -2.5 indicates osteopenia (low bone mass). A score below -2.5 indicates osteoporosis
- Z score: It is the comparison between your bone density with that of other people of same age, sex, and race
Certain medical conditions such as lumbar spinal deformity (scoliosis), atherosclerosis (accumulation of calcium in the blood vessels), or multiple fractures can change the results of your DEXA scan.