When is Spiral (Computed Tomography) CT used as a diagnostic tool?

Spiral CT: Diagnostic Applications and Limitations

Spiral CT is primarily used as a diagnostic tool for pulmonary embolism (PE), particularly for detecting central or segmental thrombi, and has largely replaced ventilation-perfusion (V/Q) scanning in many clinical settings due to its speed, accuracy, and ability to visualize the pulmonary vasculature directly. 1

Primary Applications of Spiral CT

Pulmonary Embolism Detection

• Central and Lobar Emboli: Excellent sensitivity (95% or higher) for detecting emboli in main, lobar, and segmental pulmonary arteries 1
• Alternative to V/Q Scanning: Level B recommendation as an alternative to V/Q scanning, particularly with thin collimation (1-2mm image reconstruction) 1
• Preferred in Specific Populations: Especially valuable in patients with conditions that result in nondiagnostic V/Q scans:
  • Patients with significant cardiopulmonary disease
  • Patients with chronic obstructive pulmonary disease
  • Patients with infiltrates on chest radiography 1

Technical Advantages

• Continuous Data Acquisition: Records data with continuously moving table and rotating radiation source 1
• Single Breath Hold: Most or all of the thorax can be scanned during a single breath hold (typically 20-60 seconds) 2, 3
• Volume Data Set: Provides complete anatomical volume that can be reconstructed in multiple planes 4, 2

Clinical Protocol for PE Diagnosis

1. Contrast Administration: 140-180 ml of contrast medium with 270-300 mg iodine/ml administered at 4-5 ml/s 1
2. Image Interpretation:
   • Use both soft tissue (mediastinum) and pulmonary parenchymal windows
   • Side-by-side analysis of images with different window settings
   • Cine-mode viewing for dynamic impression of pulmonary arteries 1
3. Diagnostic Findings:
   • Low attenuation filling defects within vessels
   • Complete filling defects leaving distal vessel unopacified 1

Limitations and Caveats

Diagnostic Accuracy

• Subsegmental Emboli: Limited sensitivity (approximately 75%) for subsegmental PE 1
• Variable Sensitivity: Overall sensitivity ranges from 53-89%, specificity from 78-100% 1
• Isolated Subsegmental PE: A normal spiral CT does not rule out isolated subsegmental PE, which occurs in 6-17% of cases 1

Interpretation Challenges

• Breathing Artifacts: Can create pseudo-hypoattenuating areas mimicking clots 1
• Perivascular Tissue: May be confused with intravascular thromboembolic material 1
• Technical Factors: Image quality depends on scanner technology, operator experience, and patient factors 1

Advanced Applications and Developments

CT Venography

• Level C recommendation: Spiral CT with delayed CT venography may increase detection of significant thromboembolic disease 1

Multidetector CT (MDCT)

• Newer generation scanners with thin collimation (1-2mm) provide improved visualization of subsegmental vessels 1
• Allows for better understanding of the diagnostic utility of CT pulmonary angiogram 1

3D Imaging Applications

• Improved lesion detection through retrospective centering of slices 5
• Enhanced spatial resolution when images are reconstructed at increments less than half the distance traveled during one 360° tube rotation 5

Practical Considerations

• Patient Positioning: Requires patient to lie supine and hold breath for a few seconds 1
• Contrast Requirements: Approximately 100 mL of contrast material administered intravenously 1
• Cost-Effectiveness: Spiral CT is considered cost-effective, with strategies including spiral CT angiography showing the lowest cost per life saved 1

Spiral CT has revolutionized the diagnosis of pulmonary embolism and continues to evolve with technological advancements, offering improved resolution and diagnostic accuracy while maintaining efficiency and patient comfort.