(Dual energy x-ray absorptiometry [DEXA, P-DEXA], bone densitometry, dual photon absorptiometry)
Bone mineral density (BMD) is a test that measures the amount of calcium in a specific region of the bones. From this information, an estimate of the strength of your bones can be made.
Minerals (such as calcium) are constantly being added to and taken away from bone. When minerals are taken away faster than they are added, the bones become lighter, less dense, and more porous. This makes the bones weaker and increases their risk of fracture.
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, the bones lose minerals (such as calcium), heaviness (mass), and structure, making them weaker. With further bone loss, osteopenia develops into osteoporosis. The thicker your bones are, the longer it takes to develop osteoporosis . Although osteoporosis can occur in men, it is most common in women who have gone through menopause.
Ordinary X-rays cannot detect mild bone loss. A bone must lose at least a quarter of its weight before a standard X-ray can detect the problem.
Bone density measurements can be done on several bones in the body. Controversy exists over which bones are best to use for BMD measurements. The bones most commonly used are those in the lower spine and hip. These bones generally have the greatest amount of bone loss and are at the highest risk of fracture. In special cases, bones in the wrist may be used for BMD measurements.
Several different techniques can be used to measure BMD.
Our bone mineral densitometry services include:
- Dual energy X-ray absorptiometry (DEXA). DEXA uses two different X-ray beams. The amounts of each X-ray beam that are blocked by bone and soft tissue are compared to estimate the bone density. DEXA is the most accurate method for measuring BMD. It is fast and uses very low doses of radiation. DEXA measures BMD on bones of the spine and hip. Under good conditions, DEXA can measure as little as 2% of bone loss per year.
- Peripheral dual energy X-ray absorptiometry (P-DEXA). P-DEXA is a modification of the DEXA technique. It measures bone density in outlying (peripheral) areas of the body, such as the wrist. P-DEXA machines are portable units that can be used in a doctor's office. P-DEXA also uses very low doses of radiation, but the results are usually obtained faster than conventional DEXA measurements. The disadvantages of P-DEXA include an inability to measure density of the bones most likely to fracture (the hip and spine) because of osteoporosis and its limited usefulness (compared to DEXA) for monitoring the effect of medication used to treat osteoporosis.
If bone density is lower than normal, action can be taken to increase bone strength and reduce the risk of fracture. Some ways to increase bone density and strength include taking calcium and vitamin D supplements, doing weight-bearing exercise (such as walking), weight training (such as lifting weights or using weight machines), hormone replacement therapy (usually for women past menopause), and using medications such as calcitonin (Miacalcin), alendronate (Fosamax), risedronate (Actonel), or raloxifene (Evista).
Before being screened for osteoporosis, you may want to give some thought to what you will do if the results of bone mineral testing indicate you are at risk for developing osteoporosis.
Radiation dose from a bone mineral density X-ray
To learn about the radiiation exposure from a bone mineral density scan, view the Bone Mineral Density Patient X-ray Radiation Dose Brochure
For more information on radiation safety, visit our Radiation Safety webpage.