Dual-energy x-ray absorptiometry

Last revised by Charisma DeSai on 11 Sep 2023

Dual-energy x-ray absorptiometry (DEXA or DXA) is a technique used to aid in the diagnosis of osteopenia and osteoporosis

Radiographic features

Values are calculated for the lumbar vertebrae and femur preferentially, and if one of those sites is not suitable (e.g. artifact, patient mobility), or if there is a history of hyperparathyroidism, the forearm can be used 1. Inclusion of forearm measurements is also recommended for patients with high BMI.5 The preferred sites for children are the lumbar spine and total body less head (TBLH)6.

Bone mineral density (BMD) is calculated in g/cm2, and then compared against two reference population giving two scores 1-3:

  • T-score: comparison by standard deviation (SD) to a young adult population, matched for sex and ethnicity (used for postmenopausal women and men >50 years) and classified by WHO criteria 4

    • ≥-1.0: normal

    • <-1.0 to >-2.5: osteopenia

    • ≤-2.5: osteoporosis

    • ≤-2.5 plus fragility fracture: severe osteoporosis

  • Z-score: compared by SD to an age, sex, and ethnicity population (used for premenopausal women, men <50 years, and children instead of T-score WHO criteria 4, 6)

    • ≤-2.0: below expected range/low bone density for age5, and a cause should be sought

Practical points

  • if automatic segmentation is performed, ensure that it has been performed correctly (i.e. not including ilium on lumbar spine or ischium on hip calculations)

  • similarly, ensure the bone edges have been accurately detected (this is a particular problem in individuals with very low bone mineral density, where software can not detect a clear density difference between soft tissue and bone)

  • valid comparison to reference data requires adequate positioning - deviations from optimum positioning may falsely decrease (e.g. spinal rotation) or increase (e.g. insufficient internal rotation of the femur) measured bone mineral density

  • scores in the lumbar spine can be increased in the setting of degenerative sclerotic change, ankylosing spondylitis, osteoporotic spinal compression fracture, etc. (where there is a focal abnormality, e.g. a vertebral fracture at L2, the affected level should be excluded from the analysis)

  • for diagnosis, the region with the lowest T-score from the lumbar spine (L1-L4 or L2-L4), hip (lowest value at the femoral neck or total hip) or radius (mid-third/33% radius) is used to classify bone mineral density per WHO criteria 4

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