dual x-ray absorptiometry
Meaning of dual x-ray absorptiometry: The standard bone density scan, and one that reports mineral spread across an area rather than through a true volume.
Definition of dual x-ray absorptiometry
Dual x-ray absorptiometry is a scan that measures how much mineral a bone contains by passing two x-ray beams of different energies through it. Because bone and soft tissue absorb the two beams in different proportions, the soft tissue can be calculated and subtracted, leaving a figure for bone alone. It is the standard way of assessing thinning of the skeleton.
Why two beams instead of one?
A single beam cannot tell dense bone apart from a thick layer of muscle and fat sitting over it, since both weaken the beam. Using two energies gives two separate readings of the same path, and that is enough to work out how much of the absorption belongs to soft tissue and remove it. The remainder is bone density. Scanning takes minutes, involves lying still on a table, and delivers a radiation dose far below that of an ordinary chest x-ray. The lower spine and the hip are the usual sites, the hip being the better predictor of a future break.
What do the results mean?
Two comparisons are reported rather than a raw number. One compares the measurement against the average peak bone mass of a healthy young adult of the same sex, and the recognized definition of osteoporosis rests on that comparison falling below an agreed point. The other compares it against people of the same age and sex, and a low value there suggests some cause beyond ordinary aging. Neither figure measures the internal architecture of bone, and a large share of fractures occur in people whose reading sits above the threshold, which is why the result is weighed alongside age, previous fractures and other risks.
How is it different from a bone scan?
| Test | What is measured | What it shows |
|---|---|---|
| Dual x-ray absorptiometry | Mineral per unit of projected area | How thin the skeleton has become |
| Bone scan | Uptake of an injected tracer | Places where bone is actively remodeling |
| Measurement from a CT scan | Mineral per unit of true volume | Density independent of bone size |
The names are close enough to be confused, yet the two answer unrelated questions. The middle row involves a radioactive tracer and highlights injury, infection or spread of cancer without saying anything about strength. The last row exposes the main weakness of the first: an area measurement makes a physically larger bone read as denser, so results run high in big frames and low in small ones.
Also known as
Used in a sentence
Dual x-ray absorptiometry of the hip and lumbar spine was arranged.
Good to know
The result expresses mineral spread over a projected area rather than through a true volume, so a larger bone reads as denser, and spinal arthritis, healed fractures or calcium in the aorta can raise a spine reading without adding any strength.
Word origin
Where “dual x-ray absorptiometry” comes from
Absorptiometry joins the Latin absorbere, to swallow up, with the Greek metron, a measure; dual comes from the Latin duo, two.