Bone Mineral Density
A bone mineral density (BMD) test measures the
density of minerals (such as
calcium) in your bones using a special
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
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 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
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 or a
DEXA X-ray of the spine .
Peripheral dual-energy X-ray absorptiometry (P-DEXA). P-DEXA is a type of DEXA test. It measures the
density of bones in the arms or legs, such as the wrist-it cannot measure the
density of the bones most likely to break, such as the hip and spine. P-DEXA
machines are portable units that can be used in a doctor's office. P-DEXA also
uses very low doses of radiation, and the results are ready faster than
standard DEXA measurements. P-DEXA is not as useful as DEXA 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 BMD in your hip and spine.
DPA also uses very low doses of radiation but has a slower scan time than the
Quantitative computed tomography (QCT). This is a type of CT scan that measures the density of
a bone in the spine (vertebra). A form of QCT called peripheral QCT (pQCT)
measures the density of bones in your arms or legs, usually your wrist. QCT is
not usually used because it is expensive, uses higher radiation doses, and is
less accurate than DEXA, P-DEXA, or DPA.