000 Mosaic Attenuation Pattern

Mosaic attenuation is an
- imaging pattern
  - variable lung attenuation
  - results in a heterogeneous appearance of the parenchyma.
  - sometimes it is caused by air trapping
  - sometimes by perfusion abnormalities
  - sometimes normal

Nutshell Buzz

patchwork

differing pulmonary attenuation

Etymology

Derived from the Greek word mosaic, meaning a pattern made of diverse or variegated components, and attenuation, referring to the reduction in the intensity of X-ray beams as they pass through tissues.

AKA

None commonly used.

What is it?

Mosaic attenuation is a radiologic pattern observed on high-resolution computed tomography (HRCT) of the chest. It describes regions of differing lung attenuation that form a patchwork appearance, with areas of increased attenuation (darker) and decreased attenuation (lighter).

Characterized by:

Radiologic pattern:
Decreased attenuation areas: Represent hypoperfused or hypoventilated lung tissue.
Differentiating features:
- Sharp demarcation between high and low attenuation regions.
- Associated findings, such as air trapping on expiratory scans, are crucial for interpretation.
Head cheese sign:
- Refers to a combination of mosaic attenuation, ground-glass opacities, and areas of normal lung, commonly seen in hypersensitivity pneumonitis.
  - Radiologic pattern:
    - Increased attenuation areas: Typically indicate abnormal lung parenchyma due to alveolar filling, interstitial thickening, or vascular engorgement.
    - Decreased attenuation areas: Represent hypoperfused or hypoventilated lung tissue.
Differentiating features:
- Sharp demarcation between high and low attenuation regions.
- Associated findings, such as air trapping on expiratory scans, are crucial for interpretation.

Caused by:

Most common causes:
- Small airway diseases (e.g., bronchiolitis obliterans): Air trapping due to obstruction of small airways.
- Pulmonary vascular diseases: Patchy hypoperfusion secondary to vascular abnormalities (e.g., chronic thromboembolic pulmonary hypertension, vasculitis, or congestive heart failure (CHF)).
- Infections: Fungal or atypical pneumonias.

Congenital: Swyer-James syndrome (post-infectious bronchiolitis with unilateral air trapping).

Resulting in:

Abnormal lung function due to localized or diffuse parenchymal or vascular abnormalities.
Hypoxemia in severe cases due to ventilation-perfusion mismatch.

Structural changes:

Alternating areas of normal, hyperinflated, or consolidated lung parenchyma.
Redistribution of blood flow leading to patchy perfusion.

Pathophysiology:

Small airway disease:
- Air trapping leads to overinflation of specific lung regions.
- Hypoventilated areas appear as decreased attenuation zones during inspiration and persist during expiration.
- Inspiratory-expiratory HRCT distinction: Persistent air trapping in decreased attenuation areas confirms small airway disease.
Pulmonary vascular disease:
- Segmental or subsegmental hypoperfusion creates areas of low attenuation, particularly in chronic thromboembolic pulmonary hypertension.
- Inspiratory-expiratory HRCT distinction: Mosaic attenuation disappears on expiration as perfusion equilibrates.
- Small airway disease:
  - Air trapping leads to overinflation of specific lung regions.
  - Hypoventilated areas appear as decreased attenuation zones during inspiration and persist during expiration.
Pulmonary vascular disease:
- Segmental or subsegmental hypoperfusion creates areas of low attenuation, particularly in chronic thromboembolic pulmonary hypertension CHF.

Pathology:

Histopathological findings depend on the underlying etiology:
- Small airway diseases: Fibrosis or inflammation of terminal and respiratory bronchioles.
- Vascular diseases: Occlusion or stenosis of pulmonary arteries CHF.

Diagnosis:

Clinical correlation:
- Symptoms: Dyspnea, cough, or signs of chronic lung disease.
- History of occupational exposure, recurrent infections, or autoimmune disorders.
Imaging:
- HRCT is the gold standard:
  - Inspiratory and expiratory imaging is essential to evaluate for air trapping.
  - Heterogeneous attenuation pattern with well-demarcated regions.
- Ventilation-perfusion (V/Q) scan:
  - Patchy perfusion defects support pulmonary vascular involvement.
Pulmonary function tests (PFTs):
- May show obstructive or restrictive patterns, depending on the etiology.

Management:

Underlying cause treatment:
- Small airway disease: Bronchodilators, corticosteroids, or immunosuppressants.
- Pulmonary vascular disease: Anticoagulation or pulmonary vasodilators.
Supportive therapy:
- Oxygen therapy for hypoxemia.
- Pulmonary rehabilitation for functional improvement.

Radiology Detail:

CXR:
- Poor sensitivity; may show non-specific patchy opacities.
CT:
- Parts: Heterogeneous lung parenchyma with alternating areas of attenuation.
- Size: Patchy, segmental, or diffuse involvement.
- Shape: Sharp demarcations between affected and normal lung regions.
- Position: Random distribution or segmental pattern.
- Character:
  - Low attenuation: Air trapping (small airways or vascular hypoperfusion).
  - High attenuation: Consolidation, interstitial thickening, or alveolar filling.
- Time: Persistent pattern, though can evolve with treatment.
- Associated Findings: Air trapping on expiratory CT, traction bronchiectasis in ILDs.
Other relevant Imaging Modalities:
- V/Q scans: Helpful for pulmonary vascular disease.
- MRI: Rarely used but can assess pulmonary perfusion.

Pulmonary Function Tests (PFTs):

Obstructive pattern in small airway disease.
Restrictive pattern in interstitial lung disease.

Recommendations:

Perform HRCT with inspiratory and expiratory phases to confirm air trapping.
Correlate imaging findings with clinical history and PFTs.
Consider multidisciplinary evaluation for complex cases (pulmonologist, radiologist, and pathologist).

Key Points and Pearls:

Mosaic attenuation is a descriptive HRCT finding, not a diagnosis.
Differentiation between small airway, vascular, and interstitial causes requires clinical and radiologic correlation.
Expiratory imaging is critical for identifying air trapping in small airway diseases, where mosaic attenuation persists during expiration.
In pulmonary vascular diseases, mosaic attenuation reflects patchy perfusion defects that disappear on expiration.
The head cheese sign is a hallmark finding in certain interstitial lung diseases, such as hypersensitivity pneumonitis.
Mosaic attenuation is a descriptive HRCT finding, not a diagnosis.
Differentiation between small airway, vascular, and interstitial causes requires clinical and radiologic correlation.
Expiratory imaging is critical for identifying air trapping in small airway diseases.
In pulmonary vascular diseases, mosaic attenuation reflects patchy perfusion defects.