What is the recommended imaging protocol for assessing lung segments using High-Resolution Computed Tomography (HRCT) scan of the chest?

HRCT Chest Imaging Protocol for Lung Segment Assessment

For assessing lung segments with HRCT, acquire volumetric thin-section images (≤1.5 mm) at full inspiration in the supine position without contrast, using a high spatial frequency reconstruction algorithm, and evaluate five standardized anatomical levels covering upper, middle, and lower lung zones. 1

Technical Acquisition Requirements

Core Protocol Parameters

• No intravenous contrast – lung parenchyma has inherently high contrast, making contrast administration unnecessary 1, 2
• Volumetric scanning at full inspiration to capture the entire chest 1
• Thin-section reconstruction ≤1.5 mm to maximize spatial resolution for detailed parenchymal evaluation 1, 2
• High spatial frequency (bone) algorithm for optimal lung parenchyma visualization 1
• Supine positioning as the standard acquisition position 1

Optimizing Image Quality

• Minimize motion artifacts by using the shortest rotation time (0.5 seconds) and highest pitch 1
• Patient breathing instruction is critical before and during examination, as depth of inspiration significantly affects lung attenuation and interpretation 1
• Shortest acquisition time to reduce movement-related degradation 1

Standardized Anatomical Levels for Lung Segment Evaluation

The recommended approach uses five specific anatomical regions that adequately cover all lung zones: 1

1. Region 1: Level of the aortic arch (upper zone)
2. Region 2: 1 cm below the carina (upper-middle zone)
3. Region 3: Right pulmonary venous confluence (middle zone)
4. Region 4: Midpoint between regions 3 and 5 (middle-lower zone)
5. Region 5: 1 cm above the right hemidiaphragm cupola (lower zone)

These five levels provide systematic coverage of upper, middle, and lower lung zones for comprehensive assessment 1

Additional Imaging Considerations

When to Add Expiratory Imaging

• Expiratory acquisition is recommended if inspiratory images are inconclusive 1
• Primary indication: identifying air trapping in conditions like hypersensitivity pneumonitis or connective tissue disease-associated ILD 1
• Expiratory volumetric HRCT offers powerful adjunct capability for small airway disease detection 3

When to Add Prone Positioning

• Prone images are useful for investigating abnormalities in the dorsal lower lobes seen on supine images 1
• Key purpose: distinguishing position-dependent atelectasis from true interstitial changes or fibrosis 1, 2
• This prevents the common pitfall of misinterpreting dependent atelectasis as pathologic fibrosis 2

Multiplanar Reconstructions

• Evaluate sagittal and coronal reconstructions to increase diagnostic confidence 1
• Modern volumetric acquisition with thin slices enables near-isotropic voxels for high-quality multiplanar reformats 1

Quantification Method for Lung Segment Involvement

Semi-Quantitative Visual Assessment

Divide each lung into five segments, with each segment representing 10% of lung parenchyma: 1

• Half a segment = 5% involvement
• Quarter segment = 2.5% involvement
• Visually estimate percentage involvement at each of the five anatomical levels
• Average the percentages from all five levels to obtain overall extent 1

Features to Assess at Each Level

Evaluate and quantify these specific patterns: 1

• Honeycombing extent
• Traction bronchiectasis/bronchiolectasis (grade 0-3: none, mild, moderate, severe) 1
• Reticulation extent
• Ground-glass opacities when associated with fibrosis/architectural distortion
• Emphysema extent (specifically document if ≥15% or <15% of total lung volume) 1

Critical Comparison Principle

• Compare all features on corresponding anatomical levels between examinations 1
• Side-by-side comparison of serial studies is strongly recommended to reduce variability and increase reproducibility 1

Common Pitfalls to Avoid

• Inadequate inspiration: Variable lung attenuation from inconsistent breath-holding leads to misinterpretation 1
• Skipping prone images: Dependent atelectasis in posterior lung bases can mimic or obscure early fibrosis 1, 2
• Using contrast unnecessarily: Adds no diagnostic value for parenchymal lung assessment and increases cost/risk 1, 2
• Inadequate image quality: If technical requirements aren't met, repeat the examination rather than risk misdiagnosis 1
• Ignoring clinical context: HRCT interpretation requires integration with medical history, clinical data, and prior imaging 1

Follow-Up Imaging Considerations

• Perform serial imaging at the same facility with the same scanner whenever possible to ensure consistency 1
• For progressive disease monitoring, follow-up within 12 months is advisable; anticipate earlier if clinical or functional decline occurs 1
• HRCT has 95.7% sensitivity and 63.8% specificity for detecting significant ILD (≥20% lung involvement) 1