The bone density test or scan, is designed to check for osteoporosis( a disease that occurs when the bones become thin and weak). Osteoporosis happens when the bones lose calcium and other minerals that keep the bones strong. Osteoporosis begins after menopause in many women, and gets worse after age 65, often resulting in serious fractures. A bone density scan measures the strength of your bones and determines the risk of fracture.
Today, most people will get a bone density scan from a machine using a technology called Dual Energy X-ray Absorptiometry or DEXA for short. This machine takes a picture of the bones in the spine, hip, total body and wrist, and calculates their density. If a DEXA machine is not available, bone density scans can also be done with dual photon absorptiometry (measuring the spine, hip and total body) and quantitative computed tomography scans (measuring the spine). Bone density scanners that use DEXA technology to just measure bone density in the wrist (called pDEXA scans) provide scans at some drugstores. Yet these tests are not as accurate as those that measure density in the total body, spine or hip--where most fractures occur
To take a DEXA bone density scan, the patient lies on a bed underneath the scanner, a curving plastic arm that emits x rays. These low-dose x rays form a fan beam that rotates around the patient. During the test, the scanner moves to capture images of the patient's spine, hip or entire body. A computer then compares the patient's bone strength and risk of fracture to that of other people in the United States at the same age and to young people at peak bone density. Bones reach peak density at age 30 and then start to lose mass. The test takes about 20 minutes to do and is painless.
The DEXA bone scan exposes the patient to only a small amount of radiation--about one-fiftieth that of a chest x ray, or about the amount you get from taking a cross-country airplane flight.