What Are the CT Imaging Features of Pulmonary Fibrosis?
Pulmonary fibrosis is a chronic and progressive lung condition characterized by the thickening and scarring of lung tissue, primarily affecting the pulmonary interstitium—the supportive framework surrounding the air sacs (alveoli). To fully understand the CT imaging features of this disease, it's essential to differentiate between the pulmonary parenchyma and the pulmonary interstitium. The parenchyma refers to the functional, air-containing regions of the lungs responsible for gas exchange, while the interstitium includes connective tissues, blood vessels, lymphatics, and structural septa that support the lung architecture. When fibrosis develops, it predominantly targets the interstitial space, leading to irreversible damage visible on high-resolution computed tomography (HRCT) scans.
Key CT Signs of Early Interstitial Changes
One of the earliest detectable signs of interstitial lung disease on CT imaging is thickening of the interlobular septa. These are thin walls that define the boundaries of pulmonary lobules, which are small functional units of the lung measuring approximately 1.5 to 2 cm in diameter. In healthy lungs, these septa are barely visible, but when inflammation or fibrosis occurs, they become more prominent, appearing as fine linear opacities radiating from the pleura. This creates a polygonal pattern across lung fields, often described as "reticular" changes. A hallmark feature is the presence of peripheral lines perpendicular to the pleural surface, known as septal thickening—a key early indicator of interstitial involvement.
Irregular Interfaces: Vascular, Pleural, and Bronchial Clues
Vascular-Pleural Interface Irregularity
As pulmonary fibrosis progresses, structural distortion becomes more evident. One such sign is the vessel-lung interface irregularity, where blood vessels lose their smooth contours due to surrounding fibrotic tissue. Instead of blending seamlessly into the lung parenchyma, vessel margins appear jagged or blurred, reflecting underlying architectural disruption caused by scar formation.
Pleural-Lung Interface Abnormalities
Similarly, the normally smooth boundary between the lung surface and the inner chest wall (pleura) becomes uneven. This is referred to as the pleura-lung interface sign and appears as an irregular, wavy, or nodular pleural margin on CT scans. It indicates advanced interstitial remodeling and is commonly seen in idiopathic pulmonary fibrosis (IPF).
Peribronchial Fibrosis and Airway Distortion
Fibrotic changes also affect the areas around the bronchi, resulting in what's known as the bronchus-lung interface sign. Here, the walls of the bronchi may appear thickened or irregular due to collagen deposition and scarring. This can lead to traction bronchiectasis—abnormal dilation of airways pulled open by surrounding fibrosis—and impaired mucus clearance, increasing infection risk.
Advanced Fibrotic Changes: Honeycombing and Structural Collapse
In the later stages of pulmonary fibrosis, extensive scarring leads to significant loss of normal lung architecture. A defining radiological feature at this point is the development of honeycombing—multiple clustered cystic spaces, typically 3–10 mm in size, located predominantly in the peripheral and basal regions of the lungs. These honeycomb-like cavities represent end-stage lung remodeling, where normal alveoli have been replaced by fibrous tissue and dilated airspaces. Once present, honeycombing strongly suggests irreversible lung damage and is a critical diagnostic criterion for usual interstitial pneumonia (UIP), the pathological pattern most commonly associated with IPF.
Additional Imaging Patterns in Interstitial Lung Disease
Beyond the classic signs, several other CT findings support the diagnosis of pulmonary fibrosis:
- Subpleural curvilinear lines: Arc-shaped opacities running parallel to the chest wall, representing localized fibrosis tethering adjacent structures.
- Ground-glass opacities (GGO): Hazy areas of increased lung density that don't obscure underlying bronchial or vascular structures. While not specific to fibrosis, GGOs often coexist with reticular patterns and may indicate active inflammation or early fibrotic change.
- Traction bronchiectasis and bronchiolectasis: Abnormal widening of central and peripheral airways due to pulling forces from surrounding scar tissue.
It's important to note that no single imaging feature alone confirms pulmonary fibrosis. Radiologists interpret these findings in combination, assessing their distribution (e.g., basal and peripheral predominance), symmetry, and progression over time. Moreover, correlation with clinical history, pulmonary function tests, and sometimes biopsy results is essential to determine the underlying cause—whether idiopathic, autoimmune-related, occupational, or drug-induced.
Clinical Implications and Diagnostic Integration
High-resolution CT plays a pivotal role in the non-invasive evaluation of suspected interstitial lung disease. Recognizing characteristic patterns like septal thickening, irregular interfaces, and honeycombing allows for earlier diagnosis and intervention. However, accurate interpretation requires expertise and integration with patient symptoms such as progressive dyspnea, dry cough, and inspiratory crackles on auscultation. Multidisciplinary discussion involving pulmonologists, radiologists, and rheumatologists often enhances diagnostic precision and guides appropriate management strategies, including antifibrotic therapy, oxygen supplementation, or lung transplantation in severe cases.