Bone Densitometry (DXA)
Dual-energy X-ray absorptiometry. Quantifies bone mineral density (BMD) by comparing tissue attenuation at two X-ray energies — bone attenuates differently from soft tissue at each energy, so the differential isolates bone signal. Reports BMD in g/cm² and derived T-score / Z-score.
Physics
- X-ray tube generates two energy spectra (switching or K-edge filtering).
- Fan-beam geometry scans across the anatomy.
- Paired photon-counting detectors register attenuation at both energies.
- Dual-energy math isolates bone attenuation from soft-tissue attenuation.
- Region-of-interest analysis (spine L1-L4, femoral neck, total hip, total body) reports BMD.
Clinical outputs
- BMD (g/cm²) — absolute measurement
- T-score — standard deviations vs young-adult reference population. Osteoporosis = T ≤ −2.5. Osteopenia = −1.0 ≥ T > −2.5.
- Z-score — standard deviations vs age-matched reference. Used in premenopausal women + pediatric + secondary osteoporosis workup.
- Body composition (optional license) — lean mass, fat mass, BMD distribution
- Atypical femur fracture assessment — screening for bisphosphonate-associated fractures
- Vertebral fracture assessment (VFA) — lateral spine imaging for fracture detection
Key specs
- Scan regions — lumbar spine, hip (femoral neck / total), total body
- Dual-energy method — kVp switching or K-edge filtering
- Scan time — 30 sec to 5 min depending on protocol
- Patient dose — very low (~1–10 μSv, orders of magnitude below CT or chest X-ray)
- Body composition — optional licensed application
- Fan-beam — modern DXA; pencil-beam is legacy
Clinical applications
- Postmenopausal osteoporosis screening (primary)
- Osteoporosis monitoring / treatment response
- Secondary osteoporosis workup
- Pediatric BMD (Z-score interpretation)
- Body composition research + clinical
- Sports + nutrition (lean/fat distribution)
Systems
- Hologic Horizon family
- GE Lunar iDXA (competitor)
Manufacturer-specific calibration
BMD absolute values are not interchangeable across Hologic vs GE Lunar — different calibration phantoms + reference populations. Serial monitoring should be on the same vendor and ideally the same physical unit. Cross-calibration studies exist; clinicians use them cautiously.
Regulatory
- State radiation registration (low dose but still X-ray equipment)
- ACR DXA accreditation for CMS reimbursement
- Annual quality control per International Society for Clinical Densitometry (ISCD) guidelines
- Daily scanner QC (phantom scan)
Service reality
- Detector drift — daily phantom QC catches it
- C-arm motor belt wear — scan-line tracking errors
- Software licensing — body composition + atypical femur + VFA are separately licensed
- Patient table cable flex — intermittent communication faults