What is the clinical significance and management of mosaic attenuation on a computed tomography (CT) scan?

Mosaic Attenuation on CT: Clinical Significance and Management

Definition and Recognition

Mosaic attenuation is a sharply defined geographic patchwork of regions with differing lung density on inspiratory CT images that requires systematic evaluation to determine the underlying pathophysiology—small airway disease, pulmonary vascular disease, or infiltrative lung disease. 1

The term "mosaic attenuation" should be reserved exclusively for findings on inspiratory CT scans, not expiratory images where air trapping is the appropriate descriptor. 1

Diagnostic Approach: Distinguishing the Three Main Causes

Step 1: Assess Vessel Size in Lucent Areas

• Small airway disease and vascular disease: Pulmonary vessels within lucent regions are smaller relative to vessels in more opaque lung areas 2, 3
• Infiltrative disease: Vessels remain uniform in size throughout different attenuation regions 2, 3

Step 2: Obtain Expiratory CT Images

• Small airway disease: Lucent areas fail to increase in density on expiration, confirming air trapping 1, 4
• Vascular disease: Attenuation pattern remains relatively unchanged on expiration 5
• Infiltrative disease: Ground-glass opacities may become more apparent 5

Step 3: Look for Disease-Specific Patterns

The Three-Density Sign (Highly Specific Finding)

The three-density pattern—combining normal-appearing lung, ground-glass opacities, and lucent areas with decreased vessel size—is highly specific for hypersensitivity pneumonitis and should prompt immediate investigation for antigen exposure. 1, 4

• Five or more lobules of mosaic attenuation in each of three or more lobes bilaterally has high specificity for fibrotic hypersensitivity pneumonitis, distinguishing it from idiopathic pulmonary fibrosis 1, 4
• This pattern reflects the combination of inflammatory ground-glass opacities, air trapping from bronchiolar involvement, and preserved normal lung 1

Clinical Significance by Underlying Etiology

Small Airway Disease

• Bronchiolar obstruction causes air trapping distal to the obstruction, exaggerated on expiratory imaging 4
• Common causes include obliterative bronchiolitis (post-transplant), constrictive bronchiolitis, and cystic fibrosis 1
• In post-HSCT patients, mosaic attenuation correlates with obstructive pulmonary function tests characteristic of bronchiolitis obliterans syndrome 1

Pulmonary Vascular Disease

• Chronic thromboembolic pulmonary hypertension (CTEPH) shows inhomogeneous perfusion with hyperperfused and hypoperfused areas 4
• Mosaic attenuation is seen in 74% of patients with pulmonary artery hypertension due to vascular disease, compared to only 5% with lung disease and 12% with cardiac disease 6
• Areas of increased attenuation may not be visible using default CT lung windows; a window level of -800 HU with width of 200 HU optimizes detection 7

Infiltrative Lung Disease

• Hypersensitivity pneumonitis presents with patchy or diffuse ground-glass opacities interspersed with normal lung 1, 4
• Nonfibrotic HP shows diffusely distributed ground-glass opacities and mosaic attenuation with ill-defined centrilobular nodules 4
• Fibrotic HP demonstrates coexisting lung fibrosis with the characteristic three-density pattern 4

Management Algorithm

When Mosaic Attenuation is Detected:

1. Confirm with expiratory images to differentiate air trapping (small airway disease) from perfusion abnormalities (vascular disease) 1, 4

2. If three-density pattern present: Investigate for hypersensitivity pneumonitis

   • Obtain detailed exposure history for potential antigens 1
   • Consider multidisciplinary discussion for definitive diagnosis 4
   • Trial of antigen avoidance may be diagnostic, particularly in early nonfibrotic disease 1

3. If air trapping confirmed without three-density pattern: Consider small airway disease

   • In post-HSCT patients: Evaluate for bronchiolitis obliterans syndrome with pulmonary function tests 1
   • In cystic fibrosis: Correlate with lung clearance index measurements 1
   • Consider bronchoscopy with biopsy if diagnosis remains uncertain 1

4. If vascular pattern suspected: Pursue evaluation for pulmonary hypertension

   • Obtain echocardiography and right heart catheterization 6
   • Consider ventilation-perfusion scanning or SPECT/CT to assess perfusion defects 7

Critical Technical Considerations

Optimal CT technique requires volumetric acquisition with submillimetric collimation, both inspiratory and expiratory acquisitions, and thin-section images using high-spatial-frequency reconstruction algorithms. 4

• Young patients unable to comply with breath-holding may require general anesthesia for adequate expiratory imaging 1
• Use of optimized low-dose protocols can reduce radiation exposure by up to 78% while maintaining diagnostic quality 1

Common Pitfalls to Avoid

• Do not use the term "mosaic perfusion" as it implies vascular pathology when the pattern may have multiple etiologies 3
• Do not diagnose based on inspiratory images alone—expiratory images are essential for distinguishing air trapping from perfusion abnormalities 1, 4
• Do not assume consolidation or atelectasis indicates bacterial infection—these findings are common in viral bronchiolitis and do not mandate antibiotics 8
• Vascular disease is frequently misinterpreted as infiltrative lung disease or airway disease (correct identification in only 27-36% of cases in one study) 5