Radiographic Findings of BOOP (Bronchiolitis Obliterans Organizing Pneumonia)
The characteristic radiographic appearance of BOOP consists of bilateral, diffuse alveolar opacities with normal lung volumes, often showing a distinctive peripheral distribution, with HRCT revealing patchy airspace consolidation, ground-glass opacities, small nodular opacities, and bronchial wall thickening predominantly in the lower lung zones and periphery. 1
Chest Radiograph Findings
Plain chest radiography demonstrates:
- Bilateral diffuse alveolar opacities with preserved lung volumes as the hallmark finding 1
- Peripheral distribution of opacities, similar to the pattern seen in chronic eosinophilic pneumonia 1
- Patchy areas of consolidation that are typically bilateral and peripheral 1, 2
- Migratory and recurrent pulmonary opacities are common features 1
- Rarely, unilateral alveolar opacities may occur 1, 3
- Irregular linear or nodular interstitial infiltrates are uncommon at initial presentation 1, 4
- Honeycombing is rarely seen at diagnosis 1
Important caveat: Chest X-ray may appear normal in early disease stages, making it an insensitive tool for early diagnosis 4. The plain radiograph often underestimates the extent of disease compared to CT imaging 1, 3.
High-Resolution CT Findings
HRCT is far superior to chest radiography and reveals:
Primary Parenchymal Patterns
- Patchy airspace consolidation occurring more frequently in the periphery and lower lung zones 1
- Ground-glass opacities distributed in a patchy fashion 1, 3, 5
- Small nodular opacities representing different degrees of bronchiolar and alveolar inflammation 1, 3, 5
- Peribronchovascular and/or peripheral distribution of consolidation 1
- Reversed halo sign (atoll sign) may be demonstrated 1
Airway and Distribution Features
- Bronchial wall thickening and dilation (bronchiectasis present in 60% of cases) 1, 6
- Predominantly peripheral, bilateral, and nonsegmental distribution 6
- Lower and middle lobes affected more than upper lobes (right lower lobe involved in 60%, left lower lobe in 53%) 6
- Subpleural distribution apparent in approximately 40% of cases on CT 3, 6
Additional CT Characteristics
- "Feeding vessel" or "bronchus sign" - a pulmonary vessel leading to a nodular opacity or air bronchogram entering a nodular opacity, seen in 80% of nodular BOOP cases 5
- Interstitial thickening in 66% of patients (axial, septal, or peripheral) 6
- Small pleural effusions in approximately 30% of cases 3
Spectrum of CT Presentations
BOOP demonstrates variable CT patterns: 5
- Focal nodular or mass-like opacities (42% of cases) 5
- Areas of consolidation resembling pneumonia (33% of cases) 5
- Peripheral subpleural reticular opacities (25% of cases) 5
- Patchy ground-glass infiltrates (8% of cases) 5
- Mixed patterns combining nodular opacities with pneumonic consolidation 5
- Diffuse nodularity as an unusual but recognized manifestation 7
Critical Diagnostic Considerations
CT reveals much more extensive disease than expected from plain chest radiography 1, making HRCT the preferred imaging modality when BOOP is suspected 4.
Key distinguishing features from other interstitial lung diseases:
- Normal lung volumes distinguish BOOP from restrictive fibrotic diseases 1
- Absence of severe fibrotic changes (honeycombing) at diagnosis differentiates BOOP from usual interstitial pneumonia 1, 2
- The patchy, peripheral distribution with lower lobe predominance is characteristic 1, 6
Poor prognostic radiographic indicators include: 2