🩻 Chest Imaging Interpretation A Primer for Medical Students · CXR & HRCT Patterns

1. Systematic Approach to Chest X‑Ray (CXR)

ABCDE Method A systematic review prevents missed findings.

Letter	Structure	What to Assess
A	Airway	Trachea midline? Deviation suggests tension pneumothorax, mass, or atelectasis.
B	Bones	Ribs, clavicles, spine, shoulders. Fractures, lytic lesions, rib notching (coarctation).
C	Cardiac silhouette & Mediastinum	Cardiothoracic ratio <50%. Aortic knob, AP window, hilar size and density.
D	Diaphragm	Right higher than left by ~1.5 cm. Flattening = hyperinflation. Free air under diaphragm?
E	Everything Else (Lung fields)	Compare apices to bases, side‑to‑side. Look for opacity, lucency, nodules, lines.
F	Foreign bodies / Tubes / Lines	ETT depth (3‑5 cm above carina), CVC tip (SVC/RA junction), chest tube position.

2. Common CXR Patterns & Their Meaning

Increased Opacity (White)

Pattern	Description	Common Causes	Key Features
Consolidation	Alveoli filled with fluid/pus/cells	Pneumonia, pulmonary edema, hemorrhage	Air bronchograms (patent airways surrounded by opaque alveoli), silhouette sign
Atelectasis	Collapsed lung	Mucus plug, pleural effusion, tumor obstruction	Volume loss: fissure displacement, diaphragmatic elevation, mediastinal shift toward opacity
Pleural Effusion	Fluid in pleural space	CHF, pneumonia, malignancy, PE, cirrhosis	Blunted costophrenic angle, meniscus sign. Supine CXR: hazy hemithorax.
Interstitial Opacities	Thickening of interstitium	Pulmonary edema (Kerley B lines), ILD, viral pneumonia	Reticular (lines), nodular, or reticulonodular patterns
Mass / Nodule	Discrete opacity <3 cm (nodule) or >3 cm (mass)	Lung cancer, granuloma, metastasis, hamartoma	Assess margins (spiculated = malignant), calcification (benign), growth over time

Increased Lucency (Black)

Pattern	Description	Common Causes
Pneumothorax	Air in pleural space	Spontaneous (tall thin male, bleb rupture), traumatic, iatrogenic
Tension Pneumothorax	One‑way valve → progressive air trapping	Tracheal deviation away from lucent side, depressed hemidiaphragm, hypotension
Hyperinflation	Increased lung volumes bilaterally	COPD (flattened diaphragms, ↑ retrosternal airspace, bullae)
Pneumomediastinum	Air tracking along mediastinal structures	Esophageal rupture (Boerhaave), bronchial tear, subcutaneous emphysema

Silhouette Sign

• Loss of normal border between two structures of similar density.
• Right heart border obliterated: Right middle lobe consolidation.
• Left heart border obliterated: Lingular consolidation.
• Diaphragm obliterated: Lower lobe consolidation or effusion.
• Aortic knob obliterated: Left upper lobe / anterior mediastinal mass.

3. High‑Resolution CT (HRCT): Key Patterns in ILD

Pattern	Appearance	Differential Diagnosis
Ground‑Glass Opacity (GGO)	Hazy increased attenuation with visible bronchovascular markings	PJP, early IPF, hypersensitivity pneumonitis, pulmonary edema, viral pneumonia (COVID‑19)
Consolidation	Dense opacity obscuring vessels and airways	Pneumonia, COP, adenocarcinoma (mucinous), lymphoma
Reticulation	Fine linear or curvilinear opacities (net‑like)	IPF (UIP), NSIP, connective tissue disease‑ILD
Honeycombing	Clustered cystic airspaces with thick walls (end‑stage fibrosis)	IPF (UIP pattern), chronic hypersensitivity pneumonitis, sarcoidosis (stage IV)
Traction Bronchiectasis	Dilated airways due to surrounding fibrosis pulling them open	IPF, fibrotic NSIP, chronic HP
Crazy Paving	GGO with superimposed interlobular septal thickening	PJP, pulmonary alveolar proteinosis, lipoid pneumonia, ARDS
Tree‑in‑Bud	Centrilobular nodules with branching linear opacities	Endobronchial infection: TB, MAC, aspiration, cystic fibrosis
Mosaic Attenuation	Patchwork of different lung densities; due to air trapping or vascular disease	Hypersensitivity pneumonitis, chronic PE, constrictive bronchiolitis

4. Disease‑Specific Imaging Findings

Pneumonia

• Lobar pneumonia (S. pneumoniae): Dense consolidation with air bronchograms, usually one lobe.
• Bronchopneumonia (S. aureus, H. flu): Patchy, multifocal consolidation, often lower lobes.
• Atypical pneumonia (Mycoplasma, Chlamydia): Interstitial pattern, reticulonodular opacities, worse than physical exam.
• Aspiration pneumonia: Dependent segments: RLL superior segment or posterior upper lobes (supine patient).
• Round pneumonia: Spherical consolidation in children (S. pneumoniae).

Tuberculosis (TB)

• Primary TB: Ghon focus (peripheral nodule) + ipsilateral hilar adenopathy = Ghon complex. Lower lobe predominance.
• Reactivation (Post‑primary) TB: Upper lobe cavitary lesion (apical/posterior segments), often with satellite nodules and "tree‑in‑bud."
• Miliary TB: Innumerable tiny nodules (1‑3 mm) throughout both lungs.
• Rasmussen aneurysm: Pseudoaneurysm of pulmonary artery within a tuberculous cavity → massive hemoptysis.

COPD & Emphysema

• CXR: Hyperinflation (flattened diaphragms, increased retrosternal airspace >2.5 cm), bullae, saber‑sheath trachea.
• CT: Centrilobular emphysema (upper lobe, smoking), Panacinar emphysema (lower lobe, α₁‑AT deficiency), Paraseptal (subpleural, risk of pneumothorax).

Pulmonary Edema

• Cardiogenic (CHF): Cardiomegaly, vascular congestion (cephalization), Kerley B lines (interlobular septal thickening), peribronchial cuffing, pleural effusions (bilateral > right > left).
• Non‑cardiogenic (ARDS): Normal heart size, diffuse bilateral alveolar opacities, air bronchograms, rapid progression.

Pulmonary Embolism (PE)

• CXR: Often normal or nonspecific (atelectasis, small effusion). Westermark sign (oligemia distal to clot), Hampton hump (wedge‑shaped opacity, infarction).
• CT Pulmonary Angiography (CTPA): Filling defect in pulmonary artery (acute PE may show "polo mint" sign).

Sarcoidosis

• CXR Staging (Scadding):
  • Stage I: Bilateral hilar lymphadenopathy (BHL) only.
  • Stage II: BHL + parenchymal infiltrates.
  • Stage III: Parenchymal infiltrates only.
  • Stage IV: Pulmonary fibrosis (upper lobe predominance).
• CT: Perilymphatic micronodules (along fissures, bronchovascular bundles), "galaxy sign."

Pneumothorax Recognition

• Visible visceral pleural line with absent lung markings beyond it.
• Deep sulcus sign (supine CXR: costophrenic angle abnormally deep and lucent).
• Ultrasound: Absence of lung sliding, "barcode sign" on M‑mode (stratosphere sign).

Pleural Effusion

• Upright CXR: blunting of costophrenic angle (requires ~200‑300 mL to see on PA view).
• Lateral decubitus: most sensitive for small effusion (layering fluid).
• Loculated effusion: D‑shaped opacity, does not layer freely.
• Ultrasound: Anechoic or complex fluid; guides thoracentesis.

Lung Cancer

• Adenocarcinoma: Peripheral nodule/mass, may have GGO component (lepidic growth).
• Squamous cell: Central mass, may cavitate.
• Small cell: Central bulky mass with extensive mediastinal adenopathy.
• Pancoast tumor (superior sulcus): Apical opacity with rib destruction, may cause Horner syndrome.
• Solitary Pulmonary Nodule: Assess size, margins (spiculated = malignant), calcification pattern (popcorn = hamartoma; central/diffuse/laminated = benign granuloma), growth on prior imaging.

5. Hilar & Mediastinal Abnormalities

Finding	Possible Causes
Unilateral hilar enlargement	Lung cancer, TB, lymphoma (less common), primary pulmonary artery sarcoma
Bilateral hilar lymphadenopathy (BHL)	Sarcoidosis, TB, lymphoma, silicosis, fungal infection (histoplasmosis)
Eggshell calcification of hilar nodes	Silicosis, coal worker's pneumoconiosis, sarcoidosis (rare)
Anterior mediastinal mass	4 T's: Thymoma, Teratoma (germ cell), Thyroid (substernal goiter), Terrible lymphoma
Middle mediastinal mass	Lymphadenopathy, bronchogenic cyst, aortic aneurysm
Posterior mediastinal mass	Neurogenic tumor (schwannoma, neurofibroma), esophageal duplication cyst
Widened mediastinum (>8 cm on supine CXR)	Aortic dissection/aneurysm, traumatic aortic injury, mediastinal hematoma, lymphoma

6. Pediatric Chest Imaging Pearls

• Thymus: Prominent anterior mediastinal soft tissue in infants; "sail sign" on CXR. Normal, involutes with age.
• Foreign body aspiration: Unilateral hyperlucent lung (air trapping) on expiratory film or decubitus view. Peanut most common.
• Croup (laryngotracheobronchitis): Steeple sign (subglottic narrowing) on AP neck radiograph.
• Epiglottitis: Thumbprint sign (enlarged epiglottis). Do not examine or image supine; secure airway first.
• Bronchiolitis (RSV): Hyperinflation, peribronchial thickening, patchy atelectasis.
• Round pneumonia: Common in children <8 years (due to poor collateral ventilation).

7. Classic Radiologic Signs & Eponyms

Sign	Description	Associated Condition
Air Bronchogram	Air‑filled bronchi visible against opaque lung	Consolidation (pneumonia, edema)
Silhouette Sign	Loss of normal border between two structures	Localizes lobar pathology (see above)
Deep Sulcus Sign	Abnormally deep, lucent costophrenic angle	Pneumothorax (supine patient)
Golden S Sign	Reverse‑S shape of minor fissure	Right upper lobe collapse due to central mass (e.g., lung cancer)
Luftsichel Sign	Crescent of hyperlucent lung between aortic arch and collapsed left upper lobe	Left upper lobe collapse
Continuous Diaphragm Sign	Air outlines entire diaphragm (seen with pneumomediastinum)	Pneumomediastinum
Hampton Hump	Wedge‑shaped peripheral opacity	Pulmonary infarction (PE)
Westermark Sign	Focal oligemia (hyperlucency) distal to PE	Acute PE
Kerley B Lines	Short horizontal lines at lung periphery (interlobular septal thickening)	Pulmonary edema, lymphangitic carcinomatosis

8. Choosing the Right Imaging Study

Clinical Scenario	Initial Imaging	Next Step / Advanced
Suspected pneumonia	CXR (PA & lateral)	CT if complicated (abscess, empyema) or non‑resolving
Suspected PE	CXR (to rule out other causes)	CT Pulmonary Angiography (CTPA) or V/Q scan
Suspected ILD	CXR (often normal or nonspecific)	HRCT (inspiratory + expiratory)
Lung cancer screening	Low‑dose CT (annual, 50‑80 yo, ≥20 pack‑years)	PET‑CT for staging, biopsy
Suspected pneumothorax	CXR (upright PA)	CT if equivocal; Ultrasound in trauma (eFAST)
Pleural effusion	CXR, lateral decubitus	Ultrasound for thoracentesis guidance
Hemoptysis	CXR	CT angiography (bronchial artery mapping) ± bronchoscopy

9. Quick Reference: Pattern Recognition

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