A bone density test measures the density of minerals (such as calcium) in your bones using a special X-ray. 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 . 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 increase bone density and strength by exercising, lifting weights or using weight machines, getting enough calcium and vitamin D, and taking certain medicines.
There are several different ways to measure bone density.
Dual-energy X-ray absorptiometry (DXA). This is the most accurate way to measure bone density. 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. DXA 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 DXA.
Peripheral dual-energy X-ray absorptiometry (P-DXA). P-DXA is a type of DXA test. It measures the density of bones in the arms or legs, such as the wrist. It can't measure the density of the bones most likely to break, such as the hip and spine. P-DXA machines are portable units that can be used in a doctor's office. P-DXA also uses very low doses of radiation, and the results are ready faster than standard DXA measurements. P-DXA is not as useful as DXA for finding out how well medicine used to treat osteoporosis is working.
Dual photon absorptiometry (DPA). This test uses a radioactive substance to measure bone density. It can measure bone density in your hip and spine. DPA also uses very low doses of radiation but has a slower scan time than the other methods.