How is bone density measured?

Arthur W. Perry, MD
Plastic Surgery
The most common way to measure the structure of the bones is with a DEXA (dual energy x-ray absorptiometry test) scan. That's a painless test that all women over the age of 65 and women under 65 with osteoporosis risk factors should have. But like many things we are supposed to do, many women neglect to have this test.
David Slovik, MD
Endocrinology Diabetes & Metabolism
Several technologies can assess bone density, but two have emerged as the most common.
Dual energy x-ray absorptiometry (DXA). For this procedure, a machine sends x-rays through bones in order to calculate bone density. The process is quick, taking only five minutes. And it's simple: you lie on a table while an imager passes over your body. DXA is the most commonly used method of assessing bone mineral density (BMD), and it has emerged as the gold standard of BMD testing. This technology can measure BMD at any spot in the body, but is usually used to measure it at the spine, hip, or wrist, or for the total body. DXA can compute the density of bone in any region of the body, and it does so with only one-tenth of the radiation exposure of a standard chest x-ray.
Ultrasound. Ultrasound uses sound waves to measure BMD at the heel, shin, or finger. It is sometimes used as a screening test and if abnormal it should be followed by a DXA measurement. It does not give measurements as exact as those provided by DXA, but it seems to predict fracture risk. The process is quite simple. For example, to measure BMD at the heel, you will be asked to place your bare foot in a device (known as a sonometer) that emits high-frequency sound waves. A computer determines the bone density by calculating how fast the sound waves pass through your heel. The machine can provide an estimate of your bone density in less than a minute.
Several other methods can also measure bone density, but they are used less commonly than DXA testing and ultrasound. While the various technologies offer patients and physicians more choices, they also present a challenge for researchers. There are no universal standard measurements or procedures for determining bone density. Measurements vary according to the equipment used. In addition, bone is measured at a number of different sites -- heel, wrist, forearm, hip, and spine -- but because bone density varies throughout the body, measurements taken at one spot are not directly equivalent to those taken at another.
Thus, for now at least, bone density is expressed in standard deviations that are specific to the site measured and the equipment used. For this reason, it's best for people who have been using a particular technology to track their bone density to continue using that technique, if possible. Within certain limits, bone density measurements at one site may be used to predict risk at another site.

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