A bone-density scan answers a narrower question than a regular X-ray: how much mineral is packed into a measured area of bone? The usual method is dual-energy X-ray absorptiometry, or DXA. It is most often used at the hip and lower spine.
Why two X-ray energies?
A DXA scanner sends a very low-dose beam with two energy peaks through the body. Bone and soft tissue absorb those energies differently. Software estimates the soft-tissue contribution and subtracts it, leaving a measurement of bone mineral density. The result is a number, not a photograph of every microscopic detail of the bone.
What the result can and cannot tell you
Clinicians use the measurement to help diagnose osteoporosis, estimate fracture risk, and monitor treatment. Age, previous fractures, and other health factors still matter: DXA alone cannot predict exactly who will break a bone. Arthritis, vertebral fractures, or earlier spinal surgery can also complicate interpretation at the spine.
DXA uses ionizing radiation, but the dose is small. A healthcare professional weighs its use in context, including pregnancy. For a broader look at the imaging principle, see how a digital X-ray detector forms an image.