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Chest X-ray: Alveolar vs Interstitial Disease

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Interstitium is the scaffolding that supports the alveolar walls and surrounds both the alveoli and the terminal bronchioles. Neither alveoli nor interstitium is visible on a chest X-ray when normal. It is necessary to analyze whether the pattern of diffuse opacification in the lung field is alveolar or interstitial.

Terms:
  1. Fine or small nodules: tiny opacities
  2. Reticular: mesh or basket-like – fine or coarse lines.
  3. Reticulo-nodular: a combination of both reticular and nodular pattern
  4. Septal lines: fine thread-like lines produced by fluid or thickening of the septa between the lobules of the lung. Kerley B lines are one of the commonest septal lines mentioned around in the rounds and textbooks.
  5. Kerley B lines: fine horizontal lines approximately 1 cm long, situated perpendicular to the lateral pleural surface – commonly seen just above the costophrenic angles on a frontal CXR
  6. Air bronchogram: air-filled bronchi (dark) being made visible by the opacification of surrounding alveoli (grey/white)

Difference between alveolar vs interstitial shadow:

Alveolar pattern Interstitial pattern
Usual shadows Fluffy or blobby Small nodules
Ill-defined margins Linear/reticular
Coalescing/merging Linear/reticular with septal lines
Segmental/lobar Reticulo-nodular
Additional features Air bronchogram Reduced lung volume (extensive disease)
Honey-comb pattern (end-stage disease)

Differential diagnosis:

These two entities may be present simultaneously but generally, one of them is present dominantly.

Dominant alveolar pattern

1. Adults:

Air bronchogram seen in pneumonia (alveolar disease)

2. Infants:

Dominant Interstitial pattern:
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