🦴 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.
đź“Š 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)
| Feature | DEXA | Bone Scintigraphy |
|---|---|---|
| Main Purpose | Measures bone mineral density (osteoporosis risk) | Detects areas of abnormal bone turnover (metastases, fractures, infection) |
| Tracer / Radiation | Low-dose X-rays only | Radioactive tracer (99mTc-bisphosphonate) |
| Key Output | T-score / Z-score | “Hot spots” (↑ uptake) or “cold spots” (↓ uptake) |
| Best For | Osteoporosis, fracture risk prediction | Bone metastases, Paget’s disease, stress fractures |
| Exam Pearl | DEXA = Density | Bone scan = Metabolism |