A bone density test or scan measures the strength of an individual's bones and determines the risk of fracture. It 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 them strong. Osteoporosis begins after menopause in many women, and worsens after age 65 in both men and women. It often results in serious fractures. These fractures not only bring disability, but may affect longevity. As many as one-fourth of women who fracture their hip after age 50 die within one year.

Most people today 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...