Quantitative Computed Tomography (qCT) Explained

What is Quantitative Computed Tomography?

Quantitative Computed Tomography (qCT) is a specialized imaging technique based on conventional computed tomography (CT). Unlike standard CT scans, qCT is used to generate precise quantitative measurements – meaning concrete numerical values – such as bone mineral density (BMD), body composition, or tissue density. It is considered one of the most accurate methods for assessing bone structure and is primarily used in the diagnosis and monitoring of osteoporosis.

How It Works

qCT uses X-rays that are directed through the body from multiple angles. A computer program then reconstructs three-dimensional cross-sectional images. By comparing the scan data with a simultaneously scanned calibration phantom – a reference object with known density – the measured absorption values can be converted into absolute density values (mg/cm³).

There are two main variants:

• Peripheral qCT (pqCT): Examines smaller body regions such as the forearm or lower leg. Radiation exposure is low.
• Axial qCT: Examines the spine or hip and provides particularly clinically relevant measurements for osteoporosis diagnosis.

Clinical Applications

qCT is used across several medical specialties:

• Osteoporosis diagnosis: Measurement of volumetric bone mineral density (vBMD) in trabecular and cortical bone separately – making it more precise than DXA (Dual-energy X-ray Absorptiometry).
• Fracture risk assessment: Calculation of bone load capacity using finite element analysis.
• Body composition analysis: Measurement of fat, muscle, and bone tissue proportions.
• Oncology: Assessment of bone metastases and their response to treatment.
• Endocrinology: Monitoring of conditions that affect bone metabolism, such as hyperparathyroidism.

Advantages Over Other Methods

Compared to the widely used DXA scan, qCT offers several advantages:

• Three-dimensional (volumetric) measurement of bone density in mg/cm³
• Separate assessment of trabecular (spongy) and cortical (hard) bone
• Not influenced by calcifications or degenerative changes in the spine
• High sensitivity for early bone changes

Disadvantages and Limitations

qCT also has some limitations that should be considered in clinical decision-making:

• Higher radiation exposure compared to DXA
• Higher costs and limited availability
• Longer examination time
• Requires specialized calibration equipment

How the Examination Is Performed

The examination is typically performed on an outpatient basis in a radiology practice or clinic. The patient lies on an examination table that passes through the circular CT gantry. A calibration phantom is scanned simultaneously alongside the patient. The examination usually takes only a few minutes. No special preparation is required apart from removing metal objects.

Interpreting the Results

Measurements are reported in mg of hydroxyapatite per cm³ (mg/cm³). For the lumbar spine, the following reference values apply:

• Normal: above 120 mg/cm³
• Osteopenia: 80–120 mg/cm³
• Osteoporosis: below 80 mg/cm³

These values serve as a guide and are always interpreted within the overall clinical context.

References

1. Engelke K. et al. - Clinical Use of Quantitative Computed Tomography-Based Advanced Techniques in the Management of Osteoporosis in Adults: the 2015 ISCD Official Positions. Journal of Clinical Densitometry, 2015.
2. World Health Organization (WHO) - Assessment of Fracture Risk and Its Application to Screening for Postmenopausal Osteoporosis. WHO Technical Report Series, 1994.
3. Adams J.E. - Quantitative computed tomography. European Journal of Radiology, 2009; 71(3):415-424.