DEXA Bone Densitometry
The Gold Standard for Bone Density Measurement
WHO Classification of Bone Density
Normal: T-score at or above -1.0 SD
Osteopaenia (low bone mass): T-score between -1.0 and -2.5 SD
Osteoporosis: T-score at or below -2.5 SD
Severe (established) Osteoporosis: T-score at or below -2.5 SD WITH one or more fragility fractures
Key: The T-score threshold of -2.5 was derived from population data showing fracture risk increases exponentially below this level
Critical Must-Knows
- DXA measures areal bone mineral density (aBMD) in g/cm² using two X-ray beams of different energies to separate bone from soft tissue.
- T-score compares the patient's BMD to the mean of a young adult reference population. Each 1 SD decrease doubles fracture risk.
- WHO classification: Normal = T-score at or above -1.0, Osteopaenia = T-score between -1.0 and -2.5, Osteoporosis = T-score at or below -2.5.
- FRAX integrates BMD with clinical risk factors to estimate 10-year probability of major osteoporotic and hip fractures.
- Z-score (not T-score) is used in premenopausal women, men under 50, and children — compares to age-matched controls.
Examiner's Pearls
- "DXA measures AREAL density (g/cm²), not TRUE volumetric density (g/cm³) — this means it overestimates BMD in large bones and underestimates in small bones.
- "Spine DXA is falsely elevated by: degenerative osteophytes, compression fractures, aortic calcification, scoliosis — always check the image.
- "For monitoring treatment response, the Least Significant Change (LSC) must be exceeded — typically 3-5% for spine, 4-6% for hip.
- "The hip (femoral neck specifically) is the best predictor of hip fracture — it should always be included in DXA scanning.
- "Vertebral fracture assessment (VFA) can be performed simultaneously with DXA and identifies prevalent fractures that change management.
Exam Warning
DXA and osteoporosis management are commonly examined in both clinical and viva settings. You must be able to: explain the physics of DXA, interpret T-scores and Z-scores, discuss sources of error that falsely elevate or lower BMD, explain the FRAX algorithm, and outline treatment thresholds in Australian practice. A common viva trap is using T-scores in premenopausal women or men under 50 (where Z-scores should be used).
NO-OSWHO Classification
Memory Hook:NO-OS: Normal (above -1), Osteopaenia (-1 to -2.5), Osteoporosis (below -2.5), Severe (below -2.5 + fracture).
DOCSSources of Falsely Elevated DXA
Memory Hook:DOCS: when spine DXA looks surprisingly good, check for these four common causes of false elevation.
FRAX-10FRAX Clinical Risk Factors
Memory Hook:FRAX: Fracture history, RA/risks, Alcohol/smoking, eXcessive steroids — the key clinical inputs to the 10-year fracture risk calculator.
Overview
Dual-energy X-ray Absorptiometry (DXA or DEXA) is the gold standard for measuring bone mineral density (BMD) and diagnosing osteoporosis. It is the most widely validated, precise, and clinically applicable bone density measurement technique, forming the basis of the WHO diagnostic classification and treatment guidelines worldwide.
DXA uses two X-ray beams of different energies (typically 40 and 70 keV) to separately measure the attenuation by bone mineral and soft tissue. By mathematically combining the two measurements, the contribution of soft tissue is eliminated, leaving a measurement of bone mineral content that is divided by the projected bone area to give areal bone mineral density (aBMD) in g/cm².
Why BMD Matters
BMD is the single strongest predictor of fracture risk that can be measured clinically. For each 1 standard deviation decrease in BMD, fracture risk approximately doubles. However, BMD alone does not capture all fracture risk — clinical risk factors (age, prior fracture, glucocorticoids, family history) modify fracture probability independently of BMD. This is why FRAX was developed: to integrate BMD with clinical risk factors into a single 10-year fracture probability estimate.
Areal vs Volumetric Density
DXA measures areal BMD (g/cm²) — bone mineral content divided by the projected two-dimensional area. This means DXA inherently measures a combination of bone density and bone size. Large bones will have higher areal BMD than small bones even if their true volumetric density (g/cm³) is identical. This is clinically relevant: DXA may overestimate BMD in tall individuals with large vertebrae and underestimate it in short individuals with small vertebrae. Quantitative CT (QCT) can measure true volumetric BMD but is not used for routine clinical diagnosis.
Clinical Imaging
Imaging Gallery
Systematic Approach
Systematic DXA Interpretation
Systematic DXA Interpretation Framework
| Step | Assessment | Key Considerations |
|---|---|---|
| 1. Review the image | Check the DXA scan image for artefacts and positioning errors | Exclude vertebrae with compression fractures, osteophytes, or overlying calcification from the analysis |
| 2. Identify the correct score | T-score for postmenopausal women and men over 50. Z-score for premenopausal women, men under 50, and children | Using the wrong score is a common error — T-scores are meaningless in young patients |
| 3. Apply WHO classification | Normal (above -1.0), Osteopaenia (-1.0 to -2.5), Osteoporosis (at or below -2.5) | The lowest T-score at any measured site determines the overall diagnosis |
| 4. Calculate FRAX if indicated | Enter BMD and clinical risk factors into the FRAX calculator | FRAX gives 10-year probability of major osteoporotic fracture and hip fracture separately |
| 5. Apply treatment thresholds | Australian guidelines: treat if FRAX 10-year hip fracture risk above 3% or major fracture risk above 20% | Also treat if T-score at or below -2.5, or if fragility fracture present regardless of T-score |
| 6. Plan monitoring | Repeat DXA at 1-2 years to assess treatment response or disease progression | Change must exceed the Least Significant Change (LSC) to be clinically meaningful — typically 3-5% change |
DXA Physics and Technique
Dual-Energy X-ray Absorptiometry Physics
DXA exploits the principle that bone mineral and soft tissue have different X-ray absorption characteristics at different energies. By measuring the attenuation of two X-ray beams of different energies (typically approximately 40 keV and approximately 70 keV), the system mathematically separates the contribution of bone from soft tissue.
How it works:
- An X-ray tube generates a broadband X-ray beam
- The beam is filtered to produce two distinct energy peaks (K-edge filtration with cerium or samarium filters, or rapid kV switching)
- Each energy beam is attenuated differently by bone mineral (calcium hydroxyapatite) and soft tissue
- Detectors measure the transmitted intensity at both energies
- Mathematical algorithms separate bone mineral content (BMC in grams) from soft tissue
- BMC is divided by the projected bone area to give areal BMD (g/cm²)
Measurement sites:
- Lumbar spine (L1-L4): Anteroposterior projection. Most responsive to treatment changes but susceptible to artefact from degenerative disease.
- Proximal femur: Total hip and femoral neck are measured. Femoral neck is the most important predictor of hip fracture. Less susceptible to degenerative artefact than the spine.
- Distal radius (1/3 radius): Used when the spine and hip cannot be measured (e.g., bilateral hip replacements, severe spinal degeneration).
Radiation dose: DXA produces an extremely low radiation dose — approximately 0.001 mSv per scan (less than one day of background radiation). This makes it safe for serial monitoring.
FRAX and Clinical Decision-Making
FRAX Fracture Risk Assessment Tool
FRAX is a computer-based algorithm developed by the WHO Collaborating Centre that calculates the 10-year probability of hip fracture and major osteoporotic fracture (hip, spine, forearm, or proximal humerus) based on individual patient risk factors, with or without BMD.
FRAX Input Variables
| Variable | Description | Impact on Risk |
|---|---|---|
| Age | Patient age in years (valid for ages 40-90) | Fracture risk increases exponentially with age independent of BMD |
| Sex | Male or female | Women have higher absolute fracture risk at any given T-score |
| Body mass index | Weight in kg / height in m² | Both very low BMI (less than 20) and very high BMI modify risk |
| Prior fragility fracture | Any previous osteoporotic fracture | One of the strongest risk factors — approximately doubles 10-year risk |
| Parental hip fracture | Mother or father had a hip fracture | Strong genetic risk factor independent of BMD |
| Current smoking | Active smoker at the time of assessment | Independent risk factor for fracture (also reduces BMD) |
| Glucocorticoids | Current or recent use (prednisolone 5mg or more daily for 3+ months) | Major secondary osteoporosis risk factor |
| Rheumatoid arthritis | Confirmed diagnosis of RA | Independent risk factor beyond its effect on BMD |
| Secondary osteoporosis | Type 1 diabetes, osteogenesis imperfecta, untreated hyperthyroidism, hypogonadism | Increases risk independent of BMD |
| Alcohol (3+ units/day) | Excessive alcohol intake | Dose-dependent increase in fracture risk |
| Femoral neck BMD (optional) | Can calculate with or without BMD | Adding BMD significantly improves prediction accuracy |
Australian Treatment Thresholds
Australian guidelines recommend pharmacological treatment for osteoporosis when: (1) T-score at or below -2.5 at the spine, total hip, or femoral neck. (2) A minimal trauma (fragility) fracture has occurred, regardless of T-score. (3) FRAX 10-year probability of hip fracture exceeds 3%, or major osteoporotic fracture exceeds 20% (when FRAX is available for the Australian population). (4) Glucocorticoid-induced osteoporosis: treatment recommended when prednisone 7.5mg or more daily for 3+ months if T-score is below -1.5.
Evidence Base
BMD and Fracture Risk Prediction
- Each 1 SD decrease in BMD approximately doubles the relative risk of fracture (RR 1.5-2.6 depending on site).
- Hip BMD was the best predictor of hip fracture (RR 2.6 per SD decrease).
- Spine BMD was the best predictor of vertebral fracture.
FRAX Development and Validation
- FRAX was validated across 12 prospective cohorts comprising over 250,000 person-years of follow-up.
- 10-year fracture probability was well-calibrated against observed fracture rates in validation cohorts.
- FRAX with BMD performed significantly better than BMD or clinical risk factors alone.
DXA and FRAX are well-validated for fracture risk prediction and treatment guidance.
Australian Context
In Australia, DXA scanning is widely available through private radiology practices and public hospitals. Medicare funds DXA under specific clinical indications, including: age 70 years or over, established osteoporosis with fracture, long-term glucocorticoid use, primary hyperparathyroidism, conditions associated with bone loss (e.g., hypogonadism, coeliac disease), and monitoring patients on osteoporosis treatment.
The Australian and New Zealand Bone and Mineral Society (ANZBMS) provides clinical practice guidelines for osteoporosis management that align with FRAX-based treatment thresholds adapted for the Australian population. PBS-funded osteoporosis treatments (bisphosphonates, denosumab, teriparatide, romosozumab) require documented DXA evidence of osteoporosis or the presence of a fragility fracture.
The Australian FRAX model is calibrated to Australian fracture incidence data, providing population-specific risk estimates. Treatment thresholds used in Australian practice include a 10-year hip fracture probability of 3% or more or a 10-year major osteoporotic fracture probability of 20% or more.
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
"You receive a DXA report for a 68-year-old postmenopausal woman showing a lumbar spine T-score of -1.8 and a femoral neck T-score of -2.7."
"A 35-year-old premenopausal woman on long-term oral prednisolone for rheumatoid arthritis has a DXA showing a lumbar spine T-score of -2.8."
"An examiner asks you to explain why the DXA-measured T-score at the lumbar spine may be falsely normal or elevated in an elderly patient with multiple clinical risk factors for osteoporosis."
DEXA Bone Densitometry — Exam Day Reference
High-Yield Exam Summary
WHO Classification (T-scores)
- •Normal: T-score at or above -1.0
- •Osteopaenia: T-score -1.0 to -2.5
- •Osteoporosis: T-score at or below -2.5
- •Severe: T-score at or below -2.5 PLUS fragility fracture
When to Use Z-score
- •Premenopausal women, men under 50, and children
- •Z-score at or below -2.0 = 'below expected range for age'
- •Z-score above -2.0 = 'within expected range'
- •T-scores are NOT VALID in these populations
Sources of False Elevation (DOCS)
- •Degenerative osteophytes (10-15% overestimation)
- •Overlying calcification (aortic calcification)
- •Compression fractures (same mineral in smaller area)
- •Scoliosis and surgical hardware
FRAX
- •10-year probability of major osteoporotic fracture and hip fracture
- •Inputs: age, sex, BMI, prior fracture, parental hip fracture, smoking, steroids, RA, alcohol
- •Treatment threshold: hip fracture risk above 3% or major fracture above 20%
- •Use country-specific (Australian) FRAX model
Monitoring
- •Repeat DXA every 1-2 years on the SAME machine
- •Change must exceed LSC (typically 3-5% spine, 4-6% hip) to be meaningful
- •Spine is most responsive to treatment changes
- •Use hip as primary site in elderly patients with degenerative spine