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A bone mineral density (BMD) test measures the density of minerals (such as calcium) in your bones using a special X-ray or computed tomography (CT) scan. This information is used to estimate the strength of your bones.

We all lose some bone mass as we age. Bones naturally become thinner (called osteopenia) as you grow older because existing bone is broken down faster than new bone is made. As this occurs, our bones lose calcium and other minerals and become lighter, less dense, and more porous. This makes the bones weaker and increases the chance that they might break (fracture).

With further bone loss, osteopenia leads to osteoporosis camera. So the thicker your bones are, the longer it takes to get osteoporosis. Although osteoporosis can occur in men, it is most common in women older than age 65.

If your bone density is lower than normal, you can take steps to increase your bone strength and reduce your chances of having a fracture. Some ways to increase bone density and strength include combining calcium and vitamin D supplements with weight-bearing exercise (such as walking), weight training (such as lifting weights or using weight machines), and using medicines such as calcitonin (Miacalcin), alendronate (Fosamax), risedronate (Actonel), or ibandronate (Boniva).

There are several different ways to measure BMD.

    Dual-energy X-ray absorptiometry (DEXA). This is the most accurate way to measure BMD. It uses two different X-ray beams to estimate bone density in your spine and hip. Strong, dense bones allow less of the X-ray beam to pass through them. The amounts of each X-ray beam that are blocked by bone and soft tissue are compared to each other. DEXA can measure as little as 2% of bone loss per year. It is fast and uses very low doses of radiation. Single-energy X-ray absorptiometry (SXA) may be used to measure heel and forearm bone density, but SXA is not used as often as DEXA. See a picture of a DEXA X-ray of the hips camera or a DEXA X-ray of the spine camera.