HRCT Findings of Nonspecific Interstitial Pneumonia (NSIP)
The most characteristic HRCT finding in NSIP is bilateral ground-glass opacity, typically with subpleural sparing, which distinguishes it from usual interstitial pneumonia (UIP). 1
Primary HRCT Features
Ground-Glass Opacity
- Bilateral ground-glass opacity is the most common HRCT abnormality in NSIP, present in the vast majority of cases 1
- Ground-glass opacity may occur alone or mixed with reticular opacities 1, 2
- This pattern shows a spatially uniform distribution, contrasting with the patchy heterogeneity of UIP 2
Reticular Opacities and Traction Changes
- Irregular reticular opacities occur in approximately 75% of NSIP cases 1
- Traction bronchiectasis and bronchiolectasis are present in approximately 75% of cases 1
- Intralobular interstitial thickening is a common finding 3
- These fibrotic features can improve with corticosteroid therapy, unlike in UIP 3
Distribution Patterns
Spatial Distribution
- Bilateral, symmetric involvement with basal predominance 1, 4
- Subpleural or peribronchovascular distribution patterns may occur 5, 4
- Subpleural sparing is a helpful distinguishing feature from UIP 1
Key Distinguishing Features from UIP
Honeycombing
- Honeycombing is sparse or absent at presentation in NSIP 1, 4
- When honeycombing is the predominant feature, it has 90% predictive value for UIP rather than NSIP 2
- Honeycombing may increase in prevalence during follow-up in some NSIP cases 1
Pattern Characteristics
- NSIP shows spatially uniform distribution, while UIP demonstrates patchy, heterogeneous involvement 2
- UIP is more likely to be subpleural and patchy compared to NSIP 2
- The presence of predominant ground-glass and reticular opacity with minimal honeycombing has 96% correlation with NSIP 2
Additional HRCT Findings
Other Parenchymal Features
- Consolidation may be present, reflecting an organizing pneumonia component, which may suggest connective tissue disease 1
- Peribronchovascular interstitial thickening occurs in a subset of patients 3
- Parenchymal bands may be visible 3
- Thickening of bronchovascular bundles is seen in approximately 65% of cases 5
Volume Changes
- Reduced lung volumes or lobar volume loss may be present 4
Diagnostic Accuracy and Clinical Implications
HRCT Diagnostic Performance
- A confident HRCT diagnosis of NSIP is correct in 73% of cases 2
- The distinction between NSIP and UIP on HRCT has substantial agreement (kappa value 0.72) 2
- Extensive ground-glass opacification with subpleural sparing on HRCT suggests an alternative diagnosis to IPF and should prompt consideration of fibrotic NSIP 1
Common Pitfalls
- A subset of UIP patients may present with predominant ground-glass and reticular opacity, potentially requiring biopsy for definitive differentiation from NSIP 2
- When HRCT patterns are atypical or mixed, multidisciplinary discussion integrating clinical, radiological, and pathological findings is essential 1
- Semiquantitative HRCT fibrosis scoring shows lower median scores in NSIP (median 3) compared to UIP (median 5) 6