Which CT‑derived trabecular bone density (CTXA) measurement should I select for FRAX risk assessment, and can I use the CTXA T‑score for this purpose?

CTXA Selection and T-Score Use for FRAX Assessment

You can use the femoral neck T-score derived from CTXA directly in the FRAX calculator, as CTXA produces T-scores that are equivalent to DXA for this purpose 1, 2.

Which CTXA Measurement to Select

When performing CTXA for FRAX risk assessment, measure the femoral neck region of the proximal femur 1, 2. This is the standard site that FRAX requires for hip BMD input.

Key Technical Points:

• Femoral neck CTXA BMD measurement is now officially included in the WHO FRAX tool 2
• CTXA T-scores show good correlation with DXA (r² = 0.824), with only a small bias of +0.18 T-score units (DXA slightly higher) 2
• The standard deviation between methods is approximately 0.45 T-score units 2
• CTXA demonstrates 94.9% sensitivity for discriminating normal from low bone mineral density 2

Can You Use the CTXA T-Score?

Yes, you can directly use the CTXA-derived T-score in FRAX, but with important caveats:

Critical Distinction:

The ACR guidelines explicitly state that T-scores from volumetric QCT (quantitative CT with calibration phantom) do NOT apply to WHO definitions of osteoporosis 1. However, CTXA is different from QCT - CTXA is a projectional technique that mimics DXA methodology and produces T-scores that ARE compatible with WHO criteria and FRAX 1, 2.

What CTXA Actually Measures:

• CTXA uses routine contrast-enhanced CT scans to measure areal BMD at the proximal femur
• It can be performed opportunistically during routine CT examinations without additional radiation exposure 2
• The measurement correlates well with DXA femoral neck values 2

Common Pitfalls to Avoid

1. Don't confuse CTXA with QCT: QCT measures volumetric BMD (mg/mL) and uses different thresholds (<80 mg/mL for osteoporosis per ACR) 1. QCT T-scores cannot be used in FRAX.

2. Don't use Hounsfield Unit (HU) measurements directly in FRAX: While HU measurements from opportunistic CT show promise for osteoporosis screening (thresholds around 96-110 HU for proximal femur) 3, 4, these are not validated for direct FRAX input. FRAX requires either a proper T-score or can be calculated without BMD.

3. Timing matters: Ensure CTXA and clinical assessment occur within a reasonable timeframe - the validation study used scans within 100 days of DXA 2.

4. BMI considerations: BMI independently affects both CT attenuation values and bone density parameters, so consider this when interpreting borderline results 4.

Practical Algorithm

For FRAX calculation using CTXA:

1. Obtain CTXA measurement of the femoral neck from routine contrast-enhanced CT
2. Use the reported femoral neck T-score directly in the FRAX calculator
3. Input all other FRAX variables (age, sex, weight, height, clinical risk factors)
4. Calculate 10-year fracture probability
5. Apply NOF treatment thresholds: treat if hip fracture risk ≥3% or major osteoporotic fracture risk ≥20% 1

If only HU values are available (not formal CTXA):

• Use FRAX without BMD input, relying solely on clinical risk factors
• Consider obtaining formal DXA or CTXA if the clinical decision is uncertain
• HU values <96 in proximal femur suggest high osteoporosis likelihood 3