How do I identify pulmonary nodules on a high‑resolution CT (HRCT) of the thorax?

Identifying Pulmonary Nodules on HRCT Thorax

To identify pulmonary nodules on HRCT, you must first ensure proper imaging technique with thin-section CT (≤1.5 mm, typically 1.0 mm slices) with multiplanar reconstructions, then systematically search for focal non-linear opacities on lung windows, characterizing each nodule by size, attenuation (solid vs. subsolid), calcification pattern, margins, and location. 1

Essential Technical Requirements

Optimal imaging technique is critical for accurate nodule detection:

• Acquire contiguous thin sections ≤1.5 mm (typically 1.0 mm) to enable accurate characterization of small nodules and prevent volume averaging that obscures part-solid morphology and calcification patterns 1
• Reconstruct and archive coronal and sagittal series to facilitate distinction between true nodules and scars, and to improve detection of juxtapleural nodules 1
• Use high-resolution algorithm reconstruction with targeted field of view (9.6-10 cm) for maximum spatial resolution 1
• View images on both lung windows and mediastinal (soft-tissue) windows to assess nodule attenuation and internal characteristics 1
• Apply sharp (edge-enhancing) filters when evaluating lung windows to better assess solid components in subsolid nodules 1

Common pitfall: Thick sections (≥5 mm) effectively preclude accurate characterization of small nodules and may cause you to miss part-solid components or calcification patterns 1

Systematic Nodule Detection Approach

Define what constitutes a nodule:

• A nodule appears as a focal non-linear opacity that can be solid or subsolid (ground-glass opacity) 1
• Measure nodule diameter as the average of length and width, rounded to the nearest millimeter 1
• Distinguish nodules from blood vessels by following structures through multiple slices—vessels branch and connect, while nodules remain discrete 1

Nodule Characterization by Attenuation

Categorize each nodule by attenuation pattern:

Solid Nodules

• Completely obscure the underlying lung parenchyma on thin-section images 1
• Remain visible on mediastinal window settings 1

Ground-Glass Nodules

• Appear as hazy increased opacity that does NOT completely obscure lung parenchyma 1
• Vessels and bronchi remain visible through the nodule 1
• Become partially or completely invisible on mediastinal window settings 1

Part-Solid Nodules

• Contain both solid and ground-glass components 1
• Have patches that completely obscure lung parenchyma (solid) mixed with areas where parenchyma remains visible (ground-glass) 1
• Blood vessels within the nodule appear as solid components but should not be counted as such when distinguishing part-solid from pure ground-glass 1

Critical caveat: Subjective classification of nodules as solid versus subsolid shows high inter- and intraobserver variability—correct classification by all radiologists occurs in only 58% of cases, so use mediastinal windows with sharp filters to objectively assess solid components 1

Calcification Pattern Assessment

Identify benign calcification patterns that require no follow-up:

• Diffuse (complete) calcification: uniform distribution throughout the entire nodule 1, 2
• Central calcification: dense calcification in the center, typical of healed granulomas 1, 2
• Laminated (concentric) calcification: appearing in layers like tree rings, characteristic of granulomas 1, 2
• "Popcorn" calcification: irregular chunky pattern typical of hamartomas 1, 2
• Macroscopic fat: indicates benign hamartoma 1, 2

Critical warning: Eccentric, stippled, or amorphous calcification does NOT indicate benignity and can occur in carcinomas, osteosarcomas, chondrosarcomas, and metastases 2

Technical requirement: Assess calcification on images reconstructed without edge-enhancing filters, and view on both bone and lung windows for nodules <5 mm 1, 2

Nodule Margin and Morphology Assessment

Evaluate margin characteristics that affect malignancy risk:

• Spiculated (irregular, radiating) margins increase malignancy probability with odds ratio 2.2-2.5 1, 3
• Lobulated margins are suspicious, particularly in part-solid nodules 1
• Smooth margins are more common in benign lesions but do not exclude malignancy 3

Assess special morphologic features:

• Cystic components within part-solid nodules are highly suspicious for adenocarcinoma 1
• Perifissural location with triangular/oval shape suggests benign intrapulmonary lymph nodes 2, 3

Location Documentation

Document nodule location systematically:

• Upper lobe location is a risk factor for malignancy that may warrant closer surveillance 1, 2
• Perilymphatic distribution (peribronchovascular, septal, subpleural) suggests sarcoidosis, lymphangitic carcinomatosis, or pneumoconiosis 4
• Centrilobular distribution suggests infectious/inflammatory etiology or pneumoconiosis 4

Size Measurement Protocol

Measure nodules accurately for management decisions:

• Use electronic calipers to measure average of long and short axes 1
• Round measurements to nearest millimeter 1
• Consider volumetric measurement (volume threshold 100-250 mm³) for more reproducible serial assessment, though this is software-dependent 5
• Ensure entire nodule is captured by obtaining images well above and below the nodule when performing targeted HRCT 1

Multiple Nodule Assessment

When multiple nodules are present:

• Document the six largest nodules if more than six noncalcified nodules are present 1
• Use the most suspicious nodule to guide management when multiple nodules of varying sizes exist 1
• Consider multifocal adenocarcinoma when multiple subsolid nodules are present, particularly if at least one is part-solid with suspicious features 1

What NOT to Do

• Do not rely on chest radiography for nodule detection or follow-up, as sensitivity is poor and most nodules <1 cm are not visible 2, 5
• Do not use intravenous contrast for initial nodule identification or characterization, as it adds no value and unnecessary risk 1, 2, 5
• Do not assume any calcification indicates benignity—only the four classic benign patterns (diffuse, central, laminated, popcorn) are definitively benign 2
• Do not evaluate nodules on thick-section CT (≥5 mm)—this precludes accurate characterization and may miss small nodules entirely 1