UIP Pattern HRCT Findings
The definitive UIP pattern on HRCT requires honeycombing with subpleural and basal predominance, while probable UIP shows reticular pattern with peripheral traction bronchiectasis/bronchiolectasis in the same distribution but without honeycombing. 1
Definite UIP Pattern
The classic UIP pattern demonstrates the following key features 1, 2:
Distribution
- Subpleural and basal predominant - this is the hallmark distribution
- Often heterogeneous with areas of normal lung interspersed with fibrosis
- May occasionally be diffuse or asymmetrical
Essential CT Features
- Honeycombing - clustered cystic airspaces with well-defined walls and variable diameters, seen in single or multiple layers 2
- Peripheral traction bronchiectasis or bronchiolectasis - may accompany honeycombing
- Reticular pattern with irregular thickening of interlobular septa
Permitted Additional Features
- Mild ground-glass opacity is acceptable and does not exclude UIP 2
- Pulmonary ossification
- Irregular pleural surfaces 3
Probable UIP Pattern
When honeycombing is absent but other features suggest UIP 1, 2:
- Subpleural and basal predominant distribution (same as definite UIP)
- Reticular pattern with peripheral traction bronchiectasis or bronchiolectasis
- Heterogeneous distribution of abnormalities
- May have mild ground-glass opacity
Clinical significance: Probable UIP patients may represent earlier disease with better prognosis compared to definite UIP, showing higher survival rates and better baseline pulmonary function 4, 5.
Features That Suggest Alternative Diagnosis
The following findings should raise suspicion for diagnoses other than IPF 1, 2:
CT Features Inconsistent with UIP
- Predominant ground-glass opacity (consider hypersensitivity pneumonitis, NSIP, drug toxicity)
- Profuse centrilobular micronodules (consider hypersensitivity pneumonitis or smoking-related disease)
- Consolidation (consider organizing pneumonia)
- Cysts (other than honeycombing)
- Marked mosaic attenuation
- Nodules (consider sarcoidosis)
Distribution Patterns Suggesting Alternatives
- Peribronchovascular predominance with subpleural sparing (consider NSIP)
- Perilymphatic distribution (consider sarcoidosis)
- Upper or mid-lung predominance
Ancillary Findings
- Pleural plaques (consider asbestosis)
- Dilated esophagus (consider connective tissue disease)
- Extensive lymph node enlargement
- Pleural effusions or thickening (consider connective tissue disease or drug toxicity)
Indeterminate for UIP
This category includes cases with 1, 2:
- Subpleural and basal fibrosis that doesn't meet UIP or probable UIP criteria
- Features that don't explicitly suggest an alternative diagnosis
- Very limited subpleural ground-glass opacification or reticulation without obvious fibrosis features
Critical pitfall: Always obtain prone inspiratory views to confirm that subpleural opacities are not simply dependent atelectasis 2.
Histologic Correlation
Important caveat: Honeycombing on HRCT correlates best with bronchiolectasis histologically, not just respiratory-lined cysts 6. The radiologic-pathologic correlation shows that approximately 29% of patients with histopathologic UIP do not fulfill HRCT criteria for definite UIP 2.
Acute Exacerbation Pattern
Bilateral ground-glass opacification with or without consolidation superimposed on a background UIP pattern is highly suggestive of acute exacerbation and can confirm underlying IPF diagnosis 2.