Two broad, partly-pathologic, partly-radiographic categories:
Airspace (alveolar) disease
Interstitial (infiltrative) disease
Learning the distinction narrows differentials and speeds correct diagnosis.
Box 4.1 canonical list (remember that several entities may show mixed patterns):
Airspace Diseases
Acute: pneumonia, pulmonary alveolar edema, hemorrhage, aspiration, near-drowning
Chronic: adenocarcinoma (former BAC subtype), alveolar proteinosis, sarcoidosis, lymphoma
Interstitial Diseases
Reticular: pulmonary interstitial edema, interstitial pneumonia, scleroderma, sarcoid
Nodular: bronchogenic carcinoma, metastases, silicosis, miliary tuberculosis, sarcoid
Opacity morphology:
Fluffy, cloud-like, hazy
Confluent—merge imperceptibly; edges indistinct
Distribution: diffuse (e.g., pulmonary edema) or focal/lobar/segmental (e.g., pneumococcal pneumonia)
Key signs:
Air-bronchogram (almost pathognomonic): black branching air-filled bronchi become visible because surrounding alveoli are replaced by material > air.
Bronchi normally invisible (thin walls, air-inside, air-outside)
Visualized when surroundings become water/soft-tissue density.
Silhouette sign: loss of normal interface when two like densities abut (e.g., RML pneumonia erasing right heart border).
What can fill alveoli?
Fluid (pulmonary edema)
Blood (hemorrhage)
Gastric acid/juice (aspiration)
Inflammatory exudate (pneumonia)
Water (near-drowning)
Summary box (Box 4.2):
Fluffy, fuzzy, confluent opacities
Indistinct margins
± air-bronchograms
± silhouette sign
Pulmonary alveolar edema
Bilateral, perihilar, bat-wing/angel-wing; heart often enlarged
Usually no air-bronchograms (bronchi also fluid-filled)
<48\,\text{h} clearing once treated
Frequently accompanied by pleural effusions + fissure fluid
Pneumonia
\approx90\% of community-acquired lobar/segmental = Streptococcus pneumoniae
Patchy → lobar consolidation; contains air-bronchograms
Clears in <10\,\text{d} (pneumococcal may clear in \le 48\,\text{h})
Aspiration
Manifests in most dependent lung at time of event
Bedridden: lower lobes or posterior upper lobes
RLL > LLL
Right main bronchus is SHORTER, WIDER, MORE VERTICAL
Radiographic appearance + timeline depend on material & infection:
Neutralized gastric fluid/water: clears in 24\text{–}48\,\text{h}
Infected material: may last weeks
Lung interstitium composed of:
connective tissue, lymphatics, vessels, bronchi—the scaffolding around airspaces.
Radiographic appearance: discrete “particles/packets” separated by aerated lung, usually with sharp margins.
Key features (Box 4.3):
Three basic patterns: reticular (lines), nodular (dots), reticulonodular (lines + dots)
Can be focal or diffuse
Usually no air-bronchograms
Pattern illustrations (Fig 4.8):
Reticular = criss-cross lines
Nodular = myriad small dots
Reticulonodular = combination
Heavy ILD may look confluent → mimics airspace disease on CXR due to summation.
Tip: scrutinize lesion edge/periphery for underlying lines/dots; if uncertain order chest CT (resolves superimposition).
Pulmonary interstitial edema (precursor to alveolar edema)
Etiologies: ↑capillary pressure (CHF), ↑permeability (allergy), ↓lymphatic drainage (lymphangitic spread)
4 Key findings:
fluid in major/minor fissures
peribronchial cuffing
pleural effusions
Kerley B lines
Clinical–radiologic dissociation: few rales despite dramatic film
Rapid therapeutic clearing <48\,\text{h}
Interstitial pneumonias
Usual interstitial pneumonia (UIP)
Older men, smoking & GERD associations
CXR: may be normal early; then fine → coarse reticulation, honeycombing, basal volume loss
CT: subpleural basilar honeycombing, reticulation, traction bronchiectasis
End-stage entity: idiopathic pulmonary fibrosis
Nonspecific interstitial pneumonia (NSIP)
Commonest CT pattern in connective-tissue disease (e.g., scleroderma)
Prognosis > UIP
CT: basilar ground-glass ± subpleural sparing; traction bronchiectasis late
Lymphangitic carcinomatosis (see nodular section – metastasis)
Bronchogenic carcinoma
Four major cells: adenocarcinoma, squamous, small cell, large cell
Adenocarcinoma often a solitary peripheral nodule
CXR: sharp discrete nodule; CT: spiculation/irregular borders
Metastases to the lung
Hematogenous: ≥2 nodules; “cannonballs” when large
Classically from breast, colorectal, renal, bladder, testicular, head/neck, sarcoma, melanoma
Lymphangitic spread
Blood → interstitium → lymphatics OR retrograde from hilar node obstruction
Mimics interstitial edema but segmental/lobar/unilateral
Kerley lines, fissural fluid, effusion; common primaries: breast, lung, stomach, pancreas, occasionally prostate
Direct extension
Least common; subpleural mass with adjacent rib destruction; pleura usually resistant
Sarcoidosis
Classic adenopathy: bilateral hilar + right paratracheal
Pulmonary parenchymal disease in ~50 %; mixture of lines and dots
Staging evolution:
Stage I – adenopathy only
Stage II – adenopathy + ILD
Stage III – ILD persists, nodes regress
Stage IV – pulmonary fibrosis
Majority of parenchymal cases eventually resolve
Global epidemiology: Up to \frac13 of world population infected; \approx10\% progress to active disease.
Often clinically silent; UL > LL involvement
Manifestations:
Lobar pneumonia (esp. with hilar/mediastinal nodes)
Isolated unilateral hilar/mediastinal adenopathy (children)
Large, often asymptomatic pleural effusion (adults)
Cavitation rare.
Predominates in adults; favors apical & posterior UL segments, superior LL segments.
Pathology: caseous necrosis, tubercles (macrophages + Langerhans giant cells + lymphocytes + fibroblasts)
Imaging patterns:
Thin-walled cavities, smooth inner margin, no air-fluid level (bilateral UL common)
Cavitary pneumonia
Transbronchial spread → opposite LL or other lobes
Asymptomatic bronchiectasis (traction or endobronchial)
Late bronchostenosis ⇒ chronic lobar atelectasis (middle-lobe syndrome)
Tuberculoma: solitary nodule ± satellite lesions
Pleural effusion = empyema (worse prognosis than primary effusion)
Hematogenous dissemination; insidious systemic symptoms (fever, chills, night sweats)
Radiology:
Initial \sim1\,\text{mm} nodules → may enlarge 2\text{–}3\,\text{mm} untreated
Clearing rapid with therapy
Rarely calcifies
Differential (Box 4.4): miliary TB, histoplasmosis, sarcoidosis, pneumoconioses (silicosis)
Edge analysis: confluent-looking opacity—examine periphery for hidden ILD pattern.
Air-bronchogram ≠ pathognomonic for pneumonia but strongly indicates airspace process; not expected in pulmonary edema (bronchi fluid-filled).
Bat-wing pattern ≈ pulmonary edema; add cardiomegaly, pleural effusions, fissure fluid to support CHF.
RML silhouette of right heart border localizes airspace disease to right middle lobe on CXR.
Close-up shows fluffy, confluent opacity + air-bronchograms → airspace disease.
With cough/fever history, most likely diagnosis = pneumonia.
Parenchymal lung disease divides into airspace vs interstitial; recognizing pattern streamlines diagnosis.
Airspace: fluffy, confluent, indistinct; may show air-bronchograms or silhouette sign.
Interstitial: sharp packets—reticular, nodular, or reticulonodular; seldom air-bronchograms.
Tuberculosis, sarcoidosis, some metastases can display mixed or evolving patterns—correlate clinically and with CT.