DEXA
Dual-Energy X-ray Absorptiometry · Also known as: Dual X-ray absorptiometry, DXA, bone densitometry
Dual-Energy X-ray Absorptiometry (DEXA or DXA) is a non-invasive imaging technique that quantifies bone mineral density (BMD) by measuring the attenuation of X-rays at two different energies as they pass through bone and soft tissue. First developed by Wahner and colleagues in 1987, DEXA has become the gold standard for osteoporosis screening and fracture risk assessment. It is recommended by the World Health Organization for diagnosing osteoporosis and monitoring treatment response.
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When to use it
DEXA screening is recommended for postmenopausal women, men over 70, individuals with clinical risk factors (previous fracture, corticosteroid use, rheumatoid arthritis), and those with diseases affecting bone metabolism (chronic kidney disease, hyperparathyroidism). Baseline DEXA enables subsequent follow-up scans (every 2 years typical) to monitor treatment response or disease progression. DEXA is preferred for spine and hip screening; other sites (forearm, heel) are less predictive of fracture. DEXA is contraindicated in pregnant women and rarely needed in children absent specific clinical indication.
Strengths & limitations
- Non-invasive and rapid: scan takes 10-30 minutes with minimal radiation (2-3 microsieverts), comparable to a few days of natural background radiation
- Accurate: excellent precision (1-3% coefficient of variation) enables detection of small changes in BMD over 1-2 years
- Standardized: T-scores and Z-scores are internationally calibrated, enabling comparison across populations and time
- Predictive: BMD and clinical risk scores (FRAX) reliably estimate 10-year fracture probability and guide treatment thresholds
- Widely available and cost-effective: most hospitals and clinics own or have access to DEXA devices; reimbursement is good in most health systems
- Two-dimensional projection: DEXA measures projected BMD (areal density, g/cm²), not true volumetric density; body size differences can confound interpretation
- Soft tissue artifact: overlying fat and muscle attenuate X-rays; large soft tissue thickness degrades signal-to-noise ratio
- Vertebral fracture artifact: osteophytes, aortic calcification, and previous compression fractures artificially elevate spine BMD, masking true bone loss
- Limited mechanistic insight: DEXA measures only mass, not bone quality (microarchitecture, turnover, mineral composition) that also predicts fracture
- Reference population dependency: T-score interpretation assumes Caucasian female reference; ethnic and sex differences in bone size and architecture may alter fracture risk relative to T-score
Frequently asked
What is the difference between T-score and Z-score?
T-score compares an individual's BMD to a young adult (age 20-40) reference mean, standardized to units of standard deviation (SD). T-score is used to diagnose osteoporosis (T ≤ -2.5). Z-score compares to an age- and sex-matched reference mean; it indicates whether BMD is typical for age. Z-score is useful in premenopausal women and men under 70 to detect secondary causes of bone loss.
How often should DEXA scans be repeated?
Guidelines recommend screening DEXA at age 65 for women and 70 for men. Repeat scans are typically 2 years apart if treatment is initiated, or every 3-5 years if screening is negative. Patients on therapy with suboptimal response or high fracture risk may need annual rescans. Intervals should be individualized based on fracture risk and clinical context.
Can DEXA accurately measure BMD in obese patients?
DEXA accuracy is degraded in obese individuals because large soft tissue thickness increases noise and photon scatter. The T-score may be artificially high due to overlying fat. Spine scans are particularly affected. QCT (quantitative computed tomography) is more accurate in obesity but is less available and involves higher radiation. Clinical judgment integrating FRAX and other risk factors is essential.
What is FRAX, and how should I use it with DEXA?
FRAX is a tool that estimates 10-year major osteoporotic fracture probability using clinical risk factors (age, sex, prior fracture, parent hip fracture, smoking, glucocorticoid use, rheumatoid arthritis, secondary osteoporosis) and optional DEXA T-score. FRAX-based intervention thresholds (typically 10% hip, 20% major fracture probability) guide therapy initiation independent of BMD alone, improving risk stratification.
Does DEXA radiation exposure pose a health risk?
DEXA radiation dose is very low (1-3 microsieverts), equivalent to 1-2 days of natural background radiation. For comparison, a chest X-ray is 100 microsieverts. The radiation risk from DEXA is negligible, especially for postmenopausal women. Even annual DEXA screening over many years carries minimal cumulative risk.
Sources
- Kanis, J. A. (1994). Assessment of fracture risk and its application to screening for postmenopausal osteoporosis. World Health Organization Technical Report Series, 843, 1-129. link ↗
- Genant, H. K., Engelke, K., Fuerst, T., et al. (1996). Noninvasive assessment of bone mineral and structure: state of the art. Journal of Bone and Mineral Research, 11(6), 707-730. DOI: 10.1002/jbmr.5650110602 ↗
- Blake, G. M., Fogelman, I. (2016). The role of DXA bone density scans in the diagnosis and treatment of osteoporosis. Postgraduate Medical Journal, 83(980), 509-517. DOI: 10.1136/pgmj.2007.057505 ↗
How to cite this page
ScholarGate. (2026, June 3). Dual-Energy X-ray Absorptiometry. ScholarGate. https://scholargate.app/en/medical-imaging/dexa
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