What is a DEXA (Dual-Energy X-ray Absorptiometry) scan?

DEXA Scan: A Comprehensive Overview

A DEXA (Dual-Energy X-ray Absorptiometry) scan is the gold standard for measuring bone mineral density, body composition, and diagnosing osteoporosis, using very low radiation doses to differentiate between bone, fat, and lean tissue. 1

What is a DEXA Scan?

DEXA is an advanced imaging technology that:

• Uses two different energy X-ray beams to measure tissue density
• Provides precise measurements of bone mineral density (BMD), lean tissue mass, and fat mass
• Requires minimal radiation exposure (approximately 1μSv, which is very low) 1
• Takes only minutes to complete and is non-invasive
• Is considered one of the "gold standards" for body composition assessment 1

Clinical Applications

Bone Health Assessment

• Diagnoses osteoporosis according to WHO classification:

  T-score	Diagnosis
  > -1.0	Normal
  -1.0 to -2.4	Osteopenia
  ≤ -2.5	Osteoporosis

• Monitors changes in bone density over time to assess treatment efficacy

• Evaluates fracture risk, especially in postmenopausal women

Body Composition Analysis

• Measures total body fat percentage and distribution
• Quantifies lean muscle mass
• Assesses regional fat distribution (though cannot differentiate between subcutaneous and visceral fat) 1

Special Populations

• Recommended for women taking aromatase inhibitors or experiencing chemotherapy-induced premature menopause (every 2 years) 1, 2
• Used for screening in high-risk populations (those on long-term corticosteroids, with low body weight, or physical inactivity) 2

Advantages of DEXA

• High precision and accuracy for bone mineral measurements
• Quick scan time (typically 10-20 minutes)
• Non-invasive procedure
• Low radiation exposure compared to other imaging techniques
• Ability to measure both total body and regional composition 1
• Easily applied for both healthy individuals and patients 1

Limitations

• Cannot differentiate between subcutaneous and visceral fat 1
• May underestimate body fat at low body fat percentages and overestimate at higher body fat percentages 1
• BMD measurements can be falsely elevated due to degenerative changes like osteophytes 1
• Less accurate in patients with scoliosis 1
• Requires specialized equipment and trained technicians 1

Technical Considerations

• Follow-up DEXA scans should be performed on the same machine as baseline scans 2
• Comparison between scans should use absolute BMD values (g/cm²), not T-scores 2
• Common errors include not accounting for artifacts or degenerative changes 2
• Different manufacturers' machines may give slightly different results 1

Alternative Methods

While DEXA is considered the gold standard for BMD measurement, alternative methods include:

• Quantitative CT (qCT) and Opportunistic CT (oCT): Can provide bone density information with Hounsfield unit values 1
• MRI: Emerging as a potential alternative for assessing bone quality 1
• Bioelectrical impedance: Less accurate but more accessible for body composition 1

Clinical Recommendation

For most accurate assessment of bone health and body composition, DEXA remains the preferred method due to its precision, low radiation exposure, and established clinical protocols. Follow-up scans should typically be performed every 2 years, or annually for high-risk patients such as those on glucocorticoid therapy 2.