What is the significance of mosaic attenuation of lungs on computed tomography (CT)?

Significance of Mosaic Attenuation of Lungs on CT

Mosaic attenuation on lung CT is a significant radiological finding that indicates one of three underlying pathological processes: small airway disease, pulmonary vascular disease, or infiltrative lung disease, with hypersensitivity pneumonitis being the most specific clinical diagnosis when the characteristic "three-density pattern" is present. 1

Definition and Appearance

• Mosaic attenuation refers to a patchwork of regions with varying lung density on inspiratory CT images, appearing as a combination of areas with low and high attenuation 1
• This pattern creates a geographic appearance with sharply defined borders corresponding to anatomic boundaries of secondary pulmonary lobules 2
• The term should only be used for findings on inspiratory CT images, not to be confused with air trapping which is an expiratory finding 1

Underlying Causes

1. Small Airway Disease

• Characterized by areas of decreased attenuation and decreased vascularity due to air trapping 1
• Common in hypersensitivity pneumonitis, which shows ill-defined centrilobular nodules (<5mm) on inspiratory images 1
• Air trapping is confirmed on expiratory CT images, where affected areas fail to increase in density 1

2. Pulmonary Vascular Disease

• Seen in chronic thromboembolic pulmonary hypertension (CTEPH) and other vascular disorders 1
• Shows inhomogeneous perfusion with hyperperfused (high attenuation) and hypoperfused (low attenuation) areas 1
• Vessels within lucent regions appear smaller than those in more opaque lung regions 3
• Present in 74% of patients with pulmonary hypertension due to vascular disease, compared to only 5% in pulmonary hypertension due to lung disease 4

3. Infiltrative Lung Disease

• Areas of ground-glass opacity (high attenuation) interspersed with normal lung 1
• Vessels appear more uniform in size throughout different regions of lung attenuation 3
• Often seen in hypersensitivity pneumonitis and other interstitial lung diseases 5

Diagnostic Significance

Hypersensitivity Pneumonitis (HP)

• The "three-density pattern" (previously called "headcheese sign") is highly specific for fibrotic HP 1
• This pattern shows three different lung densities: normal-appearing lung, ground-glass opacities (high attenuation), and lucent lung (decreased attenuation with decreased vascularity) 1
• In fibrotic HP, mosaic attenuation is often described as "extensive" and "marked" 1
• Five or more lobules of mosaic attenuation in each of three or more lobes bilaterally has high specificity for fibrotic HP and helps differentiate it from idiopathic pulmonary fibrosis 1

Chronic Thromboembolic Pulmonary Hypertension (CTEPH)

• CT scan in CTEPH typically shows inhomogeneous perfusion with a mosaic pattern 1
• This "ground glass" or mosaic pattern can also be seen in pulmonary venoocclusive disease, but in that condition, it's coupled with thickening of interlobular septa not usually seen in CTEPH 1

Differentiation Between Causes

• Expiratory CT scans are crucial for differentiating between vascular and airway diseases 3

  • In airway disease: attenuation differences are accentuated on expiration due to air trapping 1
  • In vascular disease: same gradient of attenuation between areas remains on expiration 1

• Vessel size assessment helps distinguish causes 3

  • In small airways and vascular diseases: vessels within lucent regions appear smaller 3
  • In infiltrative diseases: vessels are more uniform in size throughout different regions 3

• Distribution patterns provide additional clues 5

  • Fibrotic HP: typically most severe in mid or mid and lower lung zones with relative basal sparing 5
  • Non-fibrotic HP: typically diffuse without central or peripheral predominance 5

Clinical Implications

• Mosaic attenuation should prompt consideration of specific diagnoses, particularly hypersensitivity pneumonitis when the three-density pattern is present 5
• CT findings alone are not sufficient for definitive diagnosis and must be integrated with exposure history and clinical information 5
• Multidisciplinary discussion is recommended for definitive diagnosis, especially in cases of suspected HP 5
• The pattern helps guide appropriate management, including antigen avoidance in HP or anticoagulation in CTEPH 1, 5

Optimal CT Technique for Evaluation

• Non-contrast examination with volumetric acquisition using submillimetric collimation 1
• Both inspiratory and expiratory acquisitions are essential (expiratory to evaluate air trapping) 1
• Thin-section CT images (<1.5 mm) using a high-spatial-frequency algorithm 1
• Recommended radiation dose of 1-3 mSv ("reduced" dose) 1