• What is it:
  • Acinar nodules are small, round opacities (5–10 mm in diameter) representing filling of the terminal respiratory unit (acini) with fluid, cells, or other material.
  • These nodules can result from infectious, inflammatory, hemorrhagic, or neoplastic processes affecting the alveoli and surrounding airways.
• Etymology:
  • Derived from the Latin word acinus, meaning “berry” or “cluster,” referring to the terminal units of the lung where gas exchange occurs.
• AKA:
  • clustered nodules.
  • acinar shadows
• Abbreviation:
  • AN (Acinar Nodules).
• How does it appear on each relevant imaging modality:
  • Chest CT (preferred):
    • Parts: Multiple, small nodules in a cluster-like or lobular pattern.
    • Size: 5–10 mm in diameter, often confluent.
    • Shape: Round or oval with variable margins depending on the cause.
    • Position: Frequently distributed along bronchovascular bundles or centrilobular regions; may also involve dependent lung regions in cases of hemorrhage.
    • Character:
      • Ground-glass opacity, consolidation, or ill-defined nodules.
      • Hemorrhagic nodules lack air bronchograms but may coalesce into larger opacities.
  • Chest X-ray:
    • Appears as poorly defined opacities; subtle and harder to detect compared to CT.
• Differential diagnosis (starting with the most likely causes):
  • Infection:
    • Bacterial pneumonia: The most common cause of acinar nodules, often associated with consolidation and air bronchograms.
    • Tuberculosis: Involves clustered nodules in active or subacute infection.
    • Fungal infections: Includes histoplasmosis, pneumocystis pneumonia, or invasive aspergillosis.
    • Viral infections: May produce diffuse or patchy acinar nodules (e.g., influenza, COVID-19).
  • Inflammation:
    • Hypersensitivity pneumonitis: Diffuse or scattered acinar nodules, often upper or mid-lung distribution.
    • Sarcoidosis: Rarely appears with inflammatory acinar involvement in early stages.
  • Pulmonary Hemorrhage:
    • Diffuse alveolar hemorrhage (DAH): Associated with vasculitis (e.g., granulomatosis with polyangiitis, microscopic polyangiitis) or connective tissue diseases (e.g., systemic lupus erythematosus).
    • Heart failure: Capillary rupture leading to hemorrhagic nodules, often in dependent lung regions.
    • Anticoagulation: Iatrogenic causes of hemorrhage.
  • Neoplasm:
    • Lymphangitic carcinomatosis: Disseminated small nodules along bronchovascular bundles.
  • Idiopathic:
    • Cryptogenic organizing pneumonia (COP): Acinar nodules from subacute inflammatory processes.
• Recommendations:
  • Further evaluation:
    • High-resolution CT (HRCT) to confirm the presence and distribution of acinar nodules.
    • Bronchoscopy with lavage for cases suspicious for hemorrhage, infection, or malignancy.
    • Expiratory CT for evaluating air trapping in inflammatory causes.
  • Laboratory workup:
    • Autoimmune panel (e.g., ANCA, ANA) for vasculitis or connective tissue disorders.
    • Microbial cultures for suspected infections.
    • Coagulation profile if hemorrhage is suspected from anticoagulation or iatrogenic causes.
  • Clinical correlation:
    • Evaluate symptoms such as fever, hemoptysis, anemia, or systemic autoimmune signs.
    • Consider exposure to environmental triggers for hypersensitivity pneumonitis.
• Key considerations and pearls:
  • Acinar nodules are a radiological pattern, not a diagnosis, and the underlying cause must be identified through clinical and imaging correlation.
  • Hemorrhagic nodules may resolve quickly, distinguishing them from infectious or inflammatory processes.
  • Persistent or progressive acinar nodules raise suspicion for neoplastic or idiopathic interstitial lung disease.
  • A multidisciplinary approach is often needed for accurate diagnosis and management.

  • Feature	Acinar Nodule	Acinar Shadow
    Definition	Localized, measurable nodule in the acini.	Diffuse, ill-defined opacity in the acini.
    Imaging Modality	Primarily described on CT.	Historically used on chest X-rays.
    Appearance	Well-defined nodules (5–10 mm).	Diffuse, fluffy, or cloud-like opacities.
    Resolution	High resolution allows discrete detection.	Lower resolution; cannot resolve individual nodules.
    Pathological Implications	Often specific and focal.	More diffuse, suggesting widespread disease.

 

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Radiological Features

1. Chest X-Ray (CXR):
   • Appearance:
     • Small, ill-defined nodular opacities (2-5 mm).
     • May appear clustered or confluent, forming larger areas of opacity.
   • Distribution:
     • Often diffuse or localized to specific regions depending on the underlying cause.
     • Commonly seen in the perihilar, middle, or lower lung zones.
   • Airspace Features:
     • May resemble consolidation if confluent, with indistinct borders.
2. CT (High-Resolution CT – HRCT):
   • Appearance:
     • Better delineation of acinar nodules as discrete, small round or polygonal opacities.
     • Often associated with ground-glass opacities or consolidation.
   • Distribution:
     • Centrilobular: Nodules centered around the bronchioles.
     • Random: Scattered without a specific pattern.
   • Air Bronchograms:
     • Visible airways within opacified regions if extensive acinar involvement.
   • Clustered Nodules:
     • Nodules may coalesce, giving a patchy appearance.

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Characteristic Patterns on Imaging

1. CXR:
   • “Bat-Wing” Pattern:
     • Seen in pulmonary edema where acinar shadows cluster around the hilar regions.
   • “Cotton Wool” Appearance:
     • Describes fluffy, ill-defined nodules (e.g., in infections or organizing pneumonia).
2. CT:
   • Tree-in-Bud Pattern:
     • Small acinar nodules connected by branching structures, often seen in infections like tuberculosis or bronchopneumonia.
   • Ground-Glass Halo:
     • Acinar nodules surrounded by ground-glass opacity, suggestive of hemorrhage or fungal infections.

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Differential Diagnosis

Acinar nodules or shadows can result from diseases affecting the alveoli, airspaces, or small airways. Common causes include:

1. Infectious Diseases:
   • Bacterial Pneumonia:
     • Patchy acinar consolidation or nodules in a lobar or segmental distribution.
   • Tuberculosis:
     • Acinar nodules with a centrilobular distribution.
   • Fungal Infections:
     • Nodules with surrounding ground-glass halo (e.g., invasive aspergillosis).
   • Viral Pneumonia:
     • Diffuse ground-glass opacities with acinar nodules.
2. Inflammatory/Immune Disorders:
   • Organizing Pneumonia:
     • Patchy, subpleural or peribronchial acinar nodules with surrounding ground-glass opacities.
   • Hypersensitivity Pneumonitis:
     • Centrilobular acinar nodules with ground-glass opacities.
3. Vascular Disorders:
   • Pulmonary Hemorrhage:
     • Ill-defined acinar nodules due to alveolar filling with blood.
   • Septic Emboli:
     • Randomly distributed nodules, sometimes cavitating.
4. Neoplastic:
   • Lymphangitic Carcinomatosis:
     • Ill-defined nodular opacities along interlobular septa and acinar regions.
5. Other Causes:
   • Pulmonary Edema:
     • Diffuse ill-defined acinar nodules in a perihilar distribution.
   • Pulmonary Alveolar Proteinosis:
     • Ground-glass opacities with superimposed acinar nodules.

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Clinical Correlation

The diagnosis of acinar nodules requires correlation with clinical features:

• Acute symptoms: Suggest infection or edema.
• Chronic symptoms: May indicate neoplastic or inflammatory conditions.
• Systemic signs: Help differentiate vascular or autoimmune causes