Approach to HRCT interpretation

Approach to HRCT interpretation:

  1. Dominant pattern?
    – Reticular
    – Nodular
    – High attenuation eg ground glass opacity, consolidation
    – Low attenuation eg cystic, emphysema
  2. Nodules in secondary lobules
    – Centrilobular
    – Perilymphatic
    – Random
  3. Distribution in lung

Reticular pattern (septal thickening)

  • Regular: interstitial pul.edema, lymphangitic carcinomatosis
    Regular septal thickening in interstitial pul.edema
  • Irregular: lymphangitic carcinomatosis, sarcoidosis, silicosis
    Focal thickening in lymphangitic carcinomatosis

Nodular pattern


  • Centrilobular nodules: hypersens.pneumonitis, infectious bronchiolitis (eg RSV), adenocarcinoma in situ
    Centrilobular nodules in hypersens.pneumonitis
    – Tree in bud: nodular branching structure; represents dilated and impacted (mucus or pus-filled) centrilobular bronchioles. Narrows differential to infectious eg TB, ABPA and CF.
    Tree in bud pattern in TB
  • Perilymphatic nodules (located in subpleural or peribronchovascular locations): lymphangitic carcinomatosis, sarcoidosis, silicosis
    Example of sarcoidosis
  • Random: miliary TB, metastasis, fungal infection

High-attenuation pattern

  • Hazy increase in lung opacity without obscuration of underlying vessels (consolidation = obscuration of underlying vessels).Capture
  • Zone predominance can help lower the differential diagnosis:
    – Upper zone: Respiratory bronchiolitis, PCP
    – Lower zone: most of the interstitial lung diseases, drug related pneumonitis etc
  • Extensive GGO is known as crazy paving pattern.

GGO in adenocarcinoma in situ.


  • basically when your alveoli are filled with pus, cells or fluid.
    Eg pneumonia, neoplasm, alveolar edema, hemorrhage

Mosaic attenuation pattern

  • Small airway obstruction: bronchiolitis, CF
    – Mosaic attenuation pattern is seen when expiratory CT is done in cases of small airway obstruction. it represents air trapping.
    – Abnormal areas are the low attenuation areas
  • Vascular occlusion: chronic PE
    – Also known as mosaic perfusion pattern
    – Abnormal areas are the low attenuation areas (areas of oligemia)
  • Parenchymal changes – basically represent GGO

Mosaic attenuation pattern in CF

Low attenuation pattern


  • Cystic, cavitation
  • Honeycombing
  • Emphysema
  • Traction bronchiectasis
  • Bronchiectasis (signet ring sign)

Cystic lung diseases – cysts are radiolucent areas with a wall thickness of less than 4mm (if >4mm are called cavities)

  • LIP
  • Lymphangiomyomatosis
  • Langerhans cell histiocytosis
  • PJP

Cysts in Langerhans cell histiocytosis


CAVITY mnemonic

  • Cancer
  • Autoimmune eg WG, RA
  • Vascular eg septic neoplasm
  • Infectious
  • Trauma eg pneumotocele
  • Youth eg bronchogenic cyst, pulmonary sequestration


  • presence of small cystic spaces lined by bronchiolar epithelium with thickened walls composed of dense fibrous tissue
  • typical feature of UIP



  • areas of low attenuation WITHOUT visible walls, sparing peripheral lungs
  • centrilobular: smoking, panlobular: alpha-1 antitrypsin (especially genotype SZ and ZZ), paraseptal

Centrilobular emphysema sparing the periphery lung regions

Traction bronchiectasis

  • Do you see those thick black lines inside the yellow circle)?
  • Those are airways being tugged open. The lung surrounding those airways retracts as it scars, and as it retracts, the walls of those airways are pulled toward the scar…this is happening on all sides of the airway, and thus, their diameters increase.
  • A lot can cause traction bronchiectasis

NSIP blog pic 14


  • Differential diagnosis: CHADO
    CF, hypogammaglobulinemia, alpha 1 antitrypsin, after infection, ciliary dyskinesia, obstruction
  • Represents localized bronchial dilatation with bronchial wall thickening.
  • Signet rign sign: an axial cut of a dilated bronchus (ring) with its accompanying small artery (signet)

  • 4 main types

    Cystic and cylindrical bronchiectasis

Zonal predominance

Upper vs lower

  • Upper
    Upper: CASSET P
    Cystic fibrosis
    Ankylosing spondylitis
    Eosinophilic granuloma
  • Lower: drug induced pneumonitis, asbestosis, interstitial lung diseases

Central vs peripheral

  • Central: sarcoidosis, pulmonary edema, PJP (batwing)
  • Peripheral: chronic eosinophilic pneumonia, COP, adenocarcinoma in situ, contusion and vasculitis

    Peripheral consolidation in chronic eosinophilic pneumonia









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