Pulmonary fibrosis on CT

Diagnosis of usual interstitial pneumonia (UIP)

Usual interstitial pneumonia (UIP) is a central pattern most commonly associated with idiopathic pulmonary fibrosis (IPF). The diagnosis is established taking into account the patient's age (over 60 years) and smoking history, and a multidisciplinary discussion is mandatory. Different levels of diagnostic confidence are distinguished on HRCT:

1. Definite UIP: The changes are predominantly bilateral, basal, and subpleural in character. The main sign is parenchymal destruction in the form of «honeycombing» (honeycombing). As the disease progresses, fibrosis extends peripherally into the anterior parts of the upper lobes, a phenomenon known as the «propeller blade» sign (propeller blade distribution). The «ground-glass opacity» sign is not predominant; if present, it is usually accompanied by traction bronchiectasis, indicating an irreversible fibrotic substrate.
2. Probable UIP: An identical basal subpleural reticular pattern with peripheral traction bronchiectasis or bronchiolectasis, but strictly without signs of «honeycombing».

Indeterminate and alternative patterns

If the HRCT picture does not fit the classic criteria of definite or probable UIP, two more categories are distinguished. In such cases, a surgical biopsy is often required, although it may still ultimately reveal UIP.

• Indeterminate pattern: The distribution of fibrosis is heterogeneous or asymmetric. Only barely perceptible reticulation, mild ground-glass opacity, or architectural distortion is visualized, which may be regarded as an early disease pattern.
• Alternative diagnosis: A peribronchovascular or perilymphatic distribution is observed with a predominance of changes in the upper or middle zones. Findings uncharacteristic of UIP are present: thin-walled cysts, pronounced mosaic attenuation, a predominance of ground-glass opacity areas, abundant micronodules, or areas of consolidation.

Lung involvement in systemic diseases

Interstitial lung disease frequently accompanies connective tissue diseases. HRCT helps detect specific markers of systemic pathologies:

• Systemic sclerosis (scleroderma): Most commonly presents with a picture of fibrotic nonspecific interstitial pneumonia (NSIP). The lower lobes are predominantly affected: reticulation and ground-glass opacity with traction bronchiectasis are visible. «Honeycombing» is extremely rare. CT features include enlargement of the pulmonary artery and marked esophageal dilatation (frequently with a fluid-gas level). Pleural thickening is detected in only 10% of patients. It is important to remember that pulmonary hypertension can occur even in the absence of parenchymal fibrosis.
• Rheumatoid arthritis: Accompanied by mild cylindrical bronchiectasis and constrictive obliterative bronchiolitis, which appears as areas of decreased attenuation with a depleted vascular pattern. Erosions of the distal clavicles may also suggest this etiology.
• Sjögren's syndrome: Tomograms typically reveal NSIP or lymphoid interstitial pneumonia (LIP) patterns. The most specific sign is a combination of multiple solid nodules (often calcified due to amyloid deposits) with thin-walled isolated cysts. Against the background of LIP, there is a risk of developing a lymphoproliferative disorder.

Class and exam questions

What else to study on this topic

• Clinical and histological classification of interstitial pneumonias (NSIP, COP, AIP, DIP)
• Differential diagnosis of chronic hypersensitivity pneumonitis on expiratory HRCT
• Signs of drug-induced lung injury and patient management tactics
• HRCT patterns of smoking-related interstitial lung diseases
• Idiopathic pleuroparenchymal fibroelastosis (IPPFE) as a novel nosological entity

Other topics in the «Chest Radiology» section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.