DEXA Scan
Dual-energy X-ray absorptiometry — the gold standard imaging technique for measuring bone mineral density. DEXA scans use two X-ray beams at different energies to measure bone and soft tissue composition. They are used to diagnose osteoporosis, monitor treatment response, and assess body composition.
Technical Context
DEXA technology: two X-ray beams at different energy levels (typically 40 keV and 70-100 keV) pass through the body; differential attenuation by bone and soft tissue allows calculation of bone mineral content (BMC, in grams) and bone area (in cm²), yielding areal BMD (BMC/area, in g/cm²). DEXA also measures body composition: fat mass, lean mass, and bone mineral content for whole body and regional (android, gynoid, arms, legs, trunk) analysis. Radiation dose: very low (~1-10 μSv per scan, comparable to 1-2 days of background radiation). Precision: coefficient of variation approximately 1-1.5% at spine, 1.5-2.5% at hip. Sources of error: degenerative changes (osteophytes artificially elevate spine BMD), aortic calcification, fracture compression, obesity (affecting beam hardening). For clinical trials of bone-active peptide drugs, DEXA is the standard endpoint for demonstrating BMD changes, with fracture incidence as the definitive clinical endpoint.