A bone density test measures the mineral density (such as calcium) in your bones using a special X-ray, computed tomography (CT) scan, or ultrasound. From this information, an estimate of the strength of your bones can be made.
Calcium is constantly being added to and taken away from bone. When calcium is taken away faster than it is added, the bones become lighter, less dense, and more porous. This makes the bones weaker and increases their risk of fracture.
Loss of bone mass occurs as part of the natural process of aging. 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, 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 older than age 65.
Regular X-rays cannot detect mild bone loss. A bone must lose at least a quarter of its weight before a regular X-ray can detect the problem.