Dual energy X-ray absorptiometry

  • Introduced in 1987
  • Most widely available bone density technique
  • Two X-ray beam with differing kVp (30-50 keV and >70 keV) used, this enables soft tissue subtraction
  • BMD measured in given area of bone as g/cm3.
  • Spatial resolution 1 to 0.5 mm
  • Low radiation dose: 1-6 µSv
Anatomical locations
  • L1-L4 spine
  • Proximal femur
  • Distal radius
  • Z-scores: SD compared with an age-matched reference population
  • T-scores: SD compared with a young adult reference population
Definition using scores
  • Postmenopausal women and men > 50 years old
    • Osteoporosis: T-score at or below -2.5
    • Osteopenia: T-score between -1.1 and -2.4
  • Premenopausal women and men < 50 years old, children
    • Z-score less than -2 is defined as “below the expected range for age”
  • 2D image of a 3D bone. Overestimates fracture risk in short individuals with small bones due to lower areal BMD
  • Spine and hip DXA is sensitive to artefacts in degenerative disease, causing falsely increased areal BMD
  • Structures overlying the spine affect calculations – calcification of aorta, fractures (false elevation of BMD) or laminectomy (false reduction)
  • False high BMD – Paget disease, sclerotic metastasis, calcified lymph nodes, navel rings
  • Strontium ranelate use increased BMD due to high-atomic number strontium in the bones