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Extrapleural Fat Sign

Etymology

  • Derived from “extra-” (outside) and “pleural,” referring to the pleura, the membrane covering the lungs and chest wall. The term refers to fat deposition located outside the pleural space.

AKA

  • None.

What is it?

  • The extrapleural fat sign is a radiologic finding where a smoothly marginated, radiolucent or low-density area of fat is seen outside the parietal pleura on imaging, often adjacent to the chest wall.
  • It is most commonly a benign finding but can be associated with pleural thickening, extrapleural masses, or chest wall abnormalities.

Characterized by

  • A smooth, non-infiltrative appearance of fat located outside the pleura.
  • Adjacent pleural thickening may cause fat displacement or highlight its presence.
  • Most clearly visualized on CT as a low-density region.

Anatomically affecting

  • The extrapleural space, located between the parietal pleura and the inner surface of the chest wall.
  • Contains fat, connective tissue, blood vessels, and lymphatics.

Causes include

  • Most Common Causes:
    • Normal extrapleural fat deposition.
  • Other Causes include:
    • Infection: Empyema with pleural thickening.
    • Inflammation: Chronic pleuritis.
    • Neoplasm: Extrapleural lipomas, liposarcoma, or metastatic disease.
    • Mechanical: Chest wall trauma, rib fractures causing extrapleural hematoma.
    • Metabolic: Obesity-related fat deposition.
    • Idiopathic: Focal fat hypertrophy with no associated pathology.

Pathophysiology

  • The extrapleural space normally contains a variable amount of fat that can become more prominent due to:
    • Fat hypertrophy in obese patients. Additionally, brown fat may be located in the extrapleural space, particularly in younger individuals, and can exhibit increased metabolic activity on PET-CT, potentially mimicking pathological processes.
    • Displacement by adjacent processes like pleural thickening, masses, or hematomas.
  • It is distinct from pleural or intrathoracic abnormalities.

Histopathology

  • Normal fat tissue with or without associated fibrous or inflammatory changes depending on the underlying cause.

Imaging

Applied Anatomy
  • Parts: Extrapleural fat between the parietal pleura and chest wall.
  • Size: Variable; may range from small focal fat pads to prominent regions of fat hypertrophy.
  • Shape: Smoothly marginated, concave toward the lung.
  • Position: Adjacent to the inner surface of the chest wall, external to the pleural cavity.
  • Character: Radiolucent on X-ray; low attenuation (?10 to ?15 HU) on CT.
  • Time: Static or progressive depending on underlying pathology.
CXR
  • Appears as a smoothly marginated opacity adjacent to the chest wall.
  • May be seen as fat lucency on a lateral chest X-ray.
  • The sign becomes more evident when pleural thickening displaces the fat, forming a smooth, convex edge.
CT
  • Key Modality:
    • Visualized as a low-attenuation region (?10 to ?15 HU) adjacent to the chest wall.
    • Clearly demarcated from adjacent structures by the parietal pleura.
    • Pleural thickening, hematomas, or extrapleural masses can displace or distort the extrapleural fat.
    • Helps differentiate extrapleural fat from pleural fluid, pleural plaques, or soft-tissue masses.
    • In obese patients, prominent extrapleural fat may be seen diffusely along the chest wall.
MRI
  • Extrapleural fat demonstrates:
    • High signal intensity on T1-weighted images.
    • Suppression on fat-suppressed sequences, confirming its fatty nature.
PET-CT
  • Fat typically shows no metabolic activity unless associated with pathology such as liposarcoma or inflammatory changes.
  • However, brown fat may be located in the extrapleural space, particularly in younger individuals, and can exhibit increased metabolic activity on PET-CT, mimicking pathological processes.
Other
  • Ultrasound: May show echogenic regions corresponding to fat with no internal vascularity.

Differential Diagnosis

  • Pleural effusion: Uniform soft tissue attenuation without fat lucency.
  • Pleural plaques: Well-defined calcified or non-calcified plaques in the pleural space.
  • Extrapleural hematoma: Can mimic fat but has higher attenuation (30-70 HU on CT).
  • Soft-tissue tumors: Lipomas and liposarcomas may appear similar but require differentiation via MRI or biopsy.
  • Obesity-related fat deposition: A benign process.

Recommendations

  • Further Imaging:
    • Chest CT to confirm fat attenuation and differentiate from other chest wall or pleural pathologies.
    • MRI in ambiguous cases to confirm fatty composition.
  • Laboratory Correlation:
    • Not typically required unless associated with infection or malignancy.
  • Biopsy: Consider if imaging features suggest a neoplastic process (e.g., liposarcoma).

Key Points and Pearls

  • The extrapleural fat sign is a benign finding in most cases, often seen in obese individuals.
  • It is characterized by a smoothly marginated, low-density region external to the parietal pleura.
  • CT is the key imaging modality for identifying extrapleural fat and excluding other pleural or extrapleural abnormalities.
  • Differentiating extrapleural fat from pleural fluid, plaques, or masses is critical for accurate diagnosis.

Parallels with Human Endeavors

The extrapleural fat sign reflects the body’s adaptive cushioning mechanisms, akin to:

Architecture: The placement of insulation layers in structures to protect against external forces.

             Nature: Layers of fat in animals serving as insulation and protection, such as blubber in whales or fat pads in camels.

 

This radiologic sign is a reminder of how protective and adaptive layers serve both functional and aesthetic purposes in natural and human-made systems.