Dual-energy X-ray absorptiometry (DXA)
Dual-energy X-ray absorptiometry (DXA) is the main method for diagnosing osteoporosis in adults. It assesses projection bone mineral density in g/cm².
The scan is performed in standard regions:
- Lumbar spine (L1–L4).
- Proximal femur (femoral neck).
- Distal third of the radius.
The diagnosis of osteoporosis is established at a T-score ≤ -2.5. However, this indicator is not always optimal for initiating therapy, as fracture risk strongly depends on age. For a comprehensive assessment, the FRAX tool is used to calculate the 10-year risk of major osteoporotic fractures.
Limitations of DXA: The method generates a 2D image. In short stature individuals, projection BMD will be falsely underestimated. Degenerative spinal changes (occurring in 60% of patients over 70) with sclerosis, calcifications, metal implants, and strontium ranelate intake lead to a false increase in readings. Prior laminectomy, conversely, underestimates BMD.
Quantitative computed tomography (QCT)
Quantitative CT calculates true volumetric density (in mg/cm³) using a bone-equivalent calibration phantom. The method allows separate analysis of the cortical and more metabolically active trabecular tissue.
Advantages of QCT over DXA:
- High sensitivity for monitoring BMD changes (e.g., during glucocorticoid or parathyroid hormone therapy).
- Absence of artifacts in obesity (BMI > 25 kg/m²), where overlying soft tissues falsely increase DXA readings.
- Effectiveness in patients with severe degenerative spinal diseases (e.g., Forestier disease).
Important: The WHO T-score is not applicable to QCT as it causes overdiagnosis. Assessment is based on absolute values: normal ends at 120 mg/cm³, and osteoporosis is diagnosed at a density of less than 80 mg/cm³ (according to ACR).
Vertebral morphometry
Standardized assessment of vertebral height and shape is necessary to detect compression fractures (from T4 to L4). Classic radiography requires strict positioning: improper centration (normal is at T7 for the thoracic and L3 for the lumbar spine) causes false biconcavity of the endplates ("soda can" effect).
A modern alternative is vertebral fracture assessment on a densitometer (VFA, Vertebral Fracture Assessment). Compared to X-ray, this method delivers 100 times less radiation, visualizes the entire spine in a single image without projection distortions, and simultaneously measures BMD.
Alternative diagnostic methods
- Quantitative ultrasound (QUS): Usually performed on the calcaneus. Sound velocity (decreases in osteoporosis) and attenuation (increases) are evaluated. The method is inexpensive, but temperature-dependent and unsuitable for WHO criteria. It remains primarily research-oriented.
- Radiogrammetry: Assessment of cortical thickness on radiographs (usually of the 2nd metacarpal bone). The introduction of digital technologies (DXR) has reduced method error to less than 1%.
- HR-pQCT: High-resolution peripheral CT (<80 µm). Evaluates trabecular microarchitecture of the extremities with minimal radiation exposure (<3 µSv), but the scan takes a long time and does not cover the central skeleton.
Briefly: answers to topical questions
Which serum bone turnover markers are used for early evaluation of osteoporosis treatment efficacy?
Specific biochemical bone remodeling markers in the blood are used for early evaluation of osteoporosis treatment efficacy.
- C-terminal telopeptide (β-CTx / CTX) is a bone resorption marker evaluated when prescribing antiresorptive therapy.
- N-terminal propeptide of type 1 procollagen (P1NP) is a bone formation marker evaluated when prescribing anabolic therapy.
These parameters are considered the most stable, exhibit good reproducibility, and are recommended for most patients. Laboratory monitoring is performed initially and 3 months after initiating therapy to assess treatment response and patient adherence.
Which clinical risk factors are included in the FRAX algorithm to calculate the 10-year fracture risk?
The FRAX algorithm uses clinical risk factors combined with or without femoral neck bone mineral density data. Sources do not provide a complete list of included factors, but the following parameters are mentioned:
- Glucocorticoid use — the fact of intake is considered, but the algorithm does not evaluate the specific drug dose.
- Rheumatoid arthritis — the presence of the disease is a risk factor and is also used as a surrogate marker in type 2 diabetes mellitus.
The algorithm does not account for the number of cigarettes smoked, alcohol consumption volume, and the impact of type 2 diabetes mellitus.
Which specific bones and skeletal regions are evaluated during high-resolution peripheral quantitative computed tomography (HR-pQCT)?
Extremity bones are evaluated during high-resolution peripheral quantitative computed tomography.
- Distal radius — scanned to assess cortical porosity and trabecular microarchitecture.
- Distal tibia — imaged to model biomechanical strength.
The method is limited to examining the peripheral skeleton and does not directly assess bone quality in typical osteoporotic fracture sites, such as the lumbar spine and proximal femur.
The two-vertebra rule: reliable interpretation of lumbar spine DXA requires at least two intact (unaffected) vertebrae. Vertebrae with fractures, hemangiomas, or pronounced sclerosis are excluded from analysis.
Class and exam questions
Can the WHO T-score be applied when evaluating QCT results?
No, using a T-score ≤ -2.5 for quantitative CT is unacceptable as it leads to the overdiagnosis of osteoporosis. Absolute threshold values are used (less than 80 mg/cm³).
How quickly can the efficacy of osteoporosis treatment be assessed?
Using DXA, treatment response is monitored every 1–2 years. A faster response (in 3–4 months) can be confirmed by measuring serum bone turnover markers.
Why is QCT preferred over DXA in obesity?
In high BMI (> 25 kg/m²), excess soft tissue attenuates the DXA X-ray beam, leading to a false overestimation of bone density. QCT lacks this flaw and measures true volumetric density.
What else to study on this topic
- Mechanisms of artifact formation in DXA (impact of strontium, calcifications, laminectomy)
- Principles of the FRAX algorithm and inclusion of clinical risk factors
- Peripheral DXA and cortical bone assessment in hyperparathyroidism
- Vertebral fracture classification methods (6-point method and semiquantitative scale)
- Biomechanical bone parameters in pQCT studies
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