Ch03: Chest Radiology — Radiological Signs of Lung Disease（胸部基本征象）

# Ch03: Chest Radiology — Radiological Signs of Lung Disease（胸部基本征象）

Preparatory Mindset

Chest radiology signs are the "vocabulary" of lung imaging. Master the six categories of radiological signs: air-space opacification, pulmonary collapse (atelectasis), spherical/nodular opacities, cavitation or calcification, linear opacities, and widespread small opacities. Each sign points to a mechanism and a short differential; combine signs to reach a diagnosis.

Core Concepts

1. Air-space opacification

Replacement of air in the alveoli by fluid or other material (pus, blood, etc.).

• Patchy consolidation: one or more patches of ill-defined opacity. Etiology: pneumonia, infarction, contusion, immunological disorders.
• Lobar consolidation: consolidation of a whole lobe or the majority of a lobe; etiology = bacterial pneumonia. Produces an opaque lobe except for air bronchograms; silhouette sign — the boundary between the affected lung and adjacent heart/mediastinum/diaphragm is invisible.
• Air bronchogram: alveoli are filled with fluid, so the air in the bronchi contrasts with the fluid in the adjacent lung (hallmark of air-space disease).
• Cavitation (abscess formation): may occur within consolidated areas in bacterial/fungal infections; communication with the bronchial tree allows the liquid center to be coughed up and replaced by air → air-fluid level.

2. Pulmonary collapse (atelectasis)

Loss of volume of a lobe or lung.

- RUL: opacity at right apex + elevated horizontal fissure + tracheal/mediastinal deviation. - RML: opacity + loss of right heart border. - RLL: opacity + loss of the right diaphragm/heart border (through-heart triangular opacity). - LUL: veil-like opacity of the left hemithorax; loss of upper 2/3 of left mediastinal/heart border. - LLL: triangular opacity seen through the heart, lateral border = displaced oblique fissure.

• Causes: bronchial obstruction (tumor, foreign body, mucus plug), pneumothorax or pleural effusion, linear atelectasis (hypoventilation).
• Imaging features: displacement of mediastinal structures toward the collapsed lobe; opacification of the collapsed lobe; the silhouette sign; fissure displacement; vascular crowding; elevation of the ipsilateral diaphragm.
• The silhouette sign localizes the collapse: collapse of anterior lobes (upper/middle) obliterates mediastinal/heart outlines; lower lobe collapse obscures the diaphragm and descending aorta.
• Lobe-specific appearances:
• Linear (discoid) atelectasis: horizontal band/disc of collapse, commonest from hypoventilation (postoperative/post-traumatic pain).

3. Spherical opacities (lung mass / nodule)

Assess: size, shape, calcification, cavitation, chest-wall involvement (e.g. rib erosion), comparison with previous films.

• Size: a solitary mass >4 cm without calcium is nearly always primary carcinoma, lung abscess, or rarely round pneumonia.
• Shape: primary carcinomas are rounded with lobulated, notched or infiltrating outline — even one irregular segment raises suspicion.
• Growth: failure to grow over ≥18 months is a strong pointer to benign tumor/inactive granuloma; an enlarging mass is likely bronchial carcinoma or metastasis.
• CT roles: demonstrate calcification; solitary vs multiple (metastases); localize for biopsy; stage primary carcinoma; follow-up of incidental benign nodules.
• Fleischner Society Guidelines 2017 for incidental nodule follow-up (size + risk-stratified).

4. Cavitation and calcification

- Benign: diffuse, central, popcorn (hamartoma), laminar. - Malignant: eccentric, ground-glass (rarely visible). - Small calcified nodule → fungal granuloma (histoplasmosis).

• Cavitary lesion DDx: cavitary lung cancer (thick irregular wall, spiculated margins, solid mural component) vs lung abscess (smooth inner wall, air-fluid level) vs reactivation TB (smooth inner wall, tree-in-bud).
• Wall thickness rule: >15 mm max wall thickness → malignant; ≤4 mm max wall thickness → benign.
• Calcification patterns (benign vs malignant):

5. Line or band-like opacities

• Septal lines: interlobular septa (connective tissue planes containing lymph vessels) normally invisible. Thickening → interstitial pulmonary edema (Kerley B lines: horizontal, ≤2 cm, at the lung periphery) or lymphangitis carcinomatosis.
• Pleuropulmonary scars and linear (discoid) atelectasis: usually clinically insignificant; scars from prior infection/infarction reach the pleura with pleural thickening.
• Emphysematous bullae: thin line opacities bounding air-spaces devoid of vessels.

6. Widespread small pulmonary opacities

• Nodular: discrete small round opacities (2–3 mm).
• Reticular: net-like pattern of small lines.
• Reticulonodular: both patterns (e.g. fibrosing alveolitis).
• Honeycomb pattern: small rounded air-spaces with thick walls (end-stage interstitial fibrosis, e.g. UIP/IPF; subpleural distribution).

Summary of radiological signs of lung disease

Air-space opacification (patchy/lobar, air bronchogram, cavitation) → pulmonary collapse (atelectasis with silhouette sign) → spherical/nodular opacities (mass vs nodule, growth, calcification) → cavitation/calcification (wall thickness, calcification pattern) → linear opacities (Kerley B, scars, bullae) → widespread small opacities (nodular/reticular/reticulonodular/honeycomb).

High-Yield Points

• Key Point: Air bronchogram = air-space disease (alveoli filled, bronchi patent).
• Key Point: Silhouette sign = loss of a mediastinal/heart/diaphragm border; localizes the lesion (anterior = heart border, posterior/basal = diaphragm/descending aorta).
• Key Point: Atelectasis pulls structures toward it (mediastinum, fissures); consolidation does not cause volume loss.
• Key Point: Cavitary nodule: wall >15 mm → malignant; ≤4 mm → benign.
• Key Point: Kerley B lines = horizontal lines ≤2 cm at the lung periphery (interstitial edema or lymphangitis carcinomatosa).
• Key Point: Calcification: popcorn = hamartoma (benign); eccentric = suspect malignancy.
• Key Point: Nodule stable ≥18 months → benign; enlarging → carcinoma/metastasis.

Topic Summary

Six categories of lung signs — air-space opacification, collapse, spherical opacities, cavitation/calcification, linear opacities, and widespread small opacities — cover most of chest radiology. Air bronchogram and silhouette sign identify air-space disease and its location; atelectasis is distinguished by volume loss; nodule assessment uses size, shape, calcification and growth; cavitary wall thickness and calcification pattern separate benign from malignant. Kerley B lines and honeycombing characterize interstitial disease.