Dual X-ray absorptiometry (DEXA)

🦴 DEXA (Dual-Energy X-ray Absorptiometry) is the gold standard for measuring Bone Mineral Density (BMD). ✅ It is used to diagnose osteoporosis, assess fracture risk, and monitor response to treatment. ⚡ Unlike a bone scan (scintigraphy), DEXA measures bone density, not metabolic activity.

🔎 Introduction

• DEXA uses two low-dose X-ray beams at different energies to estimate bone density.
• It is fast, safe, and involves minimal radiation exposure (similar to a day’s background radiation).
• Most useful for elderly patients, postmenopausal women, and those at risk of fragility fractures.

🧾 Preparation

• Avoid calcium supplements for 24 hours before scan to reduce interference.
• Clothing: Wear loose, metal-free clothing (no belts, zips, or buttons).
• Precautions: Not performed in pregnancy due to X-ray exposure.

⚙️ Procedure

• The patient lies flat on a table while a scanner arm passes over target bones.
• Dual-energy beams: One beam is absorbed by soft tissue, the other penetrates bone → software subtracts soft tissue signal → calculates BMD.
• Key measurement sites:
  • Lumbar Spine: Best for trabecular bone, sensitive to early bone loss.
  • Hip: Strong predictor of hip fracture risk in elderly patients.
  • Forearm: Used when hip/spine cannot be measured (e.g., hip replacement, severe arthritis).
• Duration: ~10–20 minutes, painless, no contrast required.

📊 Interpretation of Results

• T-score: Compares patient’s BMD to a healthy 30-year-old of same sex.
  • Normal: > -1.0
  • Osteopenia: -1.0 to -2.5
  • Osteoporosis: ≤ -2.5
• Z-score: Compares to peers of same age/sex (used in children, men <50, premenopausal women).

🔬 Post-Procedure

• No recovery needed – patients resume normal activity immediately.
• Follow-up: Repeat scans usually every 1–2 years depending on risk and therapy.
• Used to monitor bisphosphonate, denosumab, or hormone therapy response.

🆚 DEXA vs Bone Scan (Key Differences)

📚 References

• NHS: DEXA Scan
• Radiopaedia: DEXA