Equipment and Instrumentation
Principle of DXA
DXA measures areal bone mineral density (BMD, g/cm²) by passing two photon spectra through tissue and solving the attenuation equations for soft tissue and bone. Systems either switch kVp or filter the beam with a K-edge filter (cerium or samarium) to create low- and high-energy components; the low-energy beam is preferentially absorbed by calcium hydroxyapatite. Detector data yield bone mineral content (BMC, g) and projected area (cm²), and BMD = BMC ÷ area. Because area is a two-dimensional projection, BMD is areal, not volumetric - large bones read denser and small bones read less dense, a classic exam distractor.
Hardware
Pencil-beam units sweep a narrow collimated beam with a single detector. Fan-beam units use a wide fan and a multi-element array for faster scans, with slight magnification that software corrects. Peripheral devices (heel, forearm) are portable but cannot image hip or spine and are unsuitable for monitoring.
Calibration
Daily phantom scanning verifies calibration stability, and results are tracked on control charts. Reference databases (for example NHANES III for hip) convert BMD into T-scores.
Worked example: L1–L4 BMD is 0.750 g/cm²; the young-adult mean is 1.000 g/cm² with an SD of 0.120. T = (0.750 − 1.000) ÷ 0.120 = −2.08, which falls in the osteopenia range (−1.0 to −2.5), not osteoporosis.