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.
PULMONARY HEMORRHAGE – WEGENER”S GRANULOMATOSIS aka GRANULOMATOSIS WITH POLYANGIITIS, GPA 19 year old male previously well with history of hemoptysis, sweating, fevers, myalgias, arthritis over 3 weeks. CT scan scout (above ) shows diffuse bilateral lobar infiltrates with subpleural sparing Coronal CT shows bilateral symmetrical lobar nodular consolidations involving the upper and lower lobes. The upper lobes are more consolidative and the lower lobes have an acinar pattern. These finding are consistent with acute pulmonary hemorrhage Lab shows ANCA positivity, acute renal failure (creatinine 6) and renal biopsy showing crescentic glomerulonephritis. Treated with cyclophosphamide Ashley Davidoff MD TheCommonVein.net 139195c PULMONARY HEMORRHAGE – WEGENER”S GRANULOMATOSIS aka GRANULOMATOSIS WITH POLYANGIITIS, GPA 19 year old male previously well with history of hemoptysis, sweating, fevers, myalgias, arthritis over 3 weeks. CT scan in the axial projection shows diffuse bilateral nodular consolidations (acinar pattern) with subpleural sparing consistent with pulmonary hemorrhage Lab shows ANCA positivity, acute renal failure (creatinine 6) and renal biopsy showing crescentic glomerulonephritis. Treated with cyclophosphamide Ashley Davidoff MD TheCommonVein.net 139193c PULMONARY HEMORRHAGE – WEGENER”S GRANULOMATOSIS aka GRANULOMATOSIS WITH POLYANGIITIS, GPA 19 year old male previously well with history of hemoptysis, sweating, fevers, myalgias, arthritis over 3 weeks. CT scan in the axial projection shows diffuse bilateral nodular consolidations (acinar pattern ringed in red) with subpleural sparing consistent with pulmonary hemorrhage Lab shows ANCA positivity, acute renal failure (creatinine 6) and renal biopsy showing crescentic glomerulonephritis. These finding are consistent with a diagnosis of GPA. He was treated with cyclophosphamide Ashley Davidoff MD TheCommonVein.net 139193cL
Acinar Nodules Post Trauma Coronal CT following trauma and resuscitative attempts in a 37 year old female shows 2-5mm solid and ground glass nodules in both the upper and lower lobes with confluence to form subsegmental foci of consolidation in the right lower lobe and right middle lobe. There is evidence of subpleural sparing with a more central distribution. These findings are consistent with hemorrhagic foci of acinar shadows or acinar nodules following trauma Ashley Davidoff MD TheCommonVein.net 137270 key words .lungs GGO ground glass opacities acinar shadows hemorrhage contusion post resuscitation 37F
Radiological Features
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.
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.
Characteristic Patterns on Imaging
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).
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.
Differential Diagnosis
Acinar nodules or shadows can result from diseases affecting the alveoli, airspaces, or small airways. Common causes include:
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.
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.
Vascular Disorders:
Pulmonary Hemorrhage:
Ill-defined acinar nodules due to alveolar filling with blood.
Septic Emboli:
Randomly distributed nodules, sometimes cavitating.
Neoplastic:
Lymphangitic Carcinomatosis:
Ill-defined nodular opacities along interlobular septa and acinar regions.
Other Causes:
Pulmonary Edema:
Diffuse ill-defined acinar nodules in a perihilar distribution.
Pulmonary Alveolar Proteinosis:
Ground-glass opacities with superimposed acinar nodules.
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
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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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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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