What is involved in a bone density scan using dual-energy X-ray absorptiometry (DXA)?

Bone Density Scan (DXA): Procedure and Purpose

A bone density scan, also known as dual-energy X-ray absorptiometry (DXA), is a non-invasive imaging test that uses two X-ray beams at different energy levels to measure bone mineral density by subtracting the patient's soft-tissue absorption. 1

How DXA Works

• DXA uses two X-ray beams at different energy levels to measure bone mineral density (BMD) at specific sites in the body, primarily the lumbar spine, hip, and distal forearm 1
• The technology works by subtracting the patient's soft-tissue absorption to provide a precise measurement of skeletal BMD 1
• The effective radiation dose is very low, approximately equivalent to a chest X-ray 1
• DXA is considered the gold standard for bone density measurement due to its high precision, accuracy, and reproducibility 1

What Happens During a DXA Scan

• Before the scan, your weight and height will be measured using a stadiometer 1
• You will be positioned on a padded table while a scanner arm passes over your body 1
• You must remain still during the scanning process to ensure accurate results 1
• The scan is painless and typically takes 10-20 minutes to complete 1
• Proper positioning is critical for accurate results, so the technologist will carefully position your body for each measurement site 1

Body Sites Measured

• The primary measurement sites include:
  • Lumbar spine (lower back) 1
  • Proximal femur (hip) 1
  • Forearm (in certain cases) 1
• Some DXA machines can also perform Vertebral Fracture Assessment (VFA), which is a lateral spine image to evaluate for the presence of vertebral fractures 1, 2

Results and Interpretation

• Results are reported as T-scores and Z-scores 1
• T-scores compare your BMD to that of a young adult of the same gender at peak bone mass 1
• Z-scores compare your BMD to people of the same age and gender 1
• According to WHO criteria:
  • Normal: T-score greater than -1.0 1
  • Osteopenia (low bone mass): T-score between -1.0 and -2.4 1
  • Osteoporosis: T-score equal to or less than -2.5 1

Quality Control and Accuracy

• DXA facilities perform regular quality control procedures to ensure accuracy 1
• A spine phantom is typically scanned daily or at least three times weekly to verify machine calibration 1
• Each facility should determine its precision error and calculate the least significant change (LSC) for accurate monitoring of changes in bone density over time 1

Advantages Over Other Bone Density Techniques

• DXA has several advantages over alternative bone densitometry techniques:
  • Established WHO T-score definition for interpreting results 1
  • Proven ability to predict fracture risk 1
  • Effectiveness in targeting anti-fracture therapies 1
  • Ability to monitor response to treatment 1
  • Low radiation exposure compared to other methods 1

Common Pitfalls and Limitations

• BMD measurements can be affected by:
  • Degenerative changes in the spine that may falsely elevate readings 1
  • Scoliosis which can affect accuracy 1
  • Previous fractures or surgical hardware in the measurement area 1
• Weight limitations exist (typically between 120-150 kg depending on scanner type) 1
• Changes in weight or fat mass between scans may affect precision and accuracy 1