Bone Densitometer (DXA)

Overview

A bone densitometer measures how much mineral is packed into bone, the number used to diagnose osteoporosis and estimate fracture risk. It does this by shining a very low-dose X-ray beam at two different energies through the skeleton and comparing how much each is absorbed, which separates the contribution of bone from the surrounding soft tissue. The patient lies still on a table while a scanning arm rasters the beam across the region of interest.

The patient rests on the Scan Table, a radiolucent carbon-fiber surface that the imaging hardware travels beneath and above. The Scanning Arm carries the Dual-Energy X-Ray Source under the table and the Dual-Energy Detector over it, kept in alignment as motorized drives sweep them across the body. The Collimator shapes the beam into a pencil or fan, and the Acquisition Computer turns the two-energy transmission data into a density map and the clinical T- and Z-scores shown on the Operator Display.

How it works

The core idea is two-material decomposition. Bone and soft tissue attenuate X-rays differently, and that difference changes with photon energy, so measuring transmission at two energies gives two equations that solve for bone mineral content independent of the overlying fat and muscle. The Dual-Energy X-Ray Source produces those two bands either by rapidly switching the tube voltage or by passing the beam through the K-Edge Filter Wheel whose rare-earth K-edge splits the spectrum, and the Dual-Energy Detector sorts the arriving photons into low- and high-energy bins.

Precision matters more than raw resolution here, because the clinical value is tracking small changes in density over years. The Scanning Arm rasters the beam line by line under the Position Encoder so every measurement is mapped to an exact location, and the Scan Control Board synchronizes the source pulses with detector sampling so the two energies register to the same pixel. Dose is kept to a few microsieverts by running the tube at very low current, the long scan compensates with integration time rather than intensity. The Acquisition Computer then segments the bone regions, computes areal density, and compares it against young-adult and age-matched reference populations to report the T- and Z-scores clinicians act on.