Understanding the CT Imaging Features of Pulmonary Interstitial Fibrosis

Pulmonary interstitial fibrosis is a progressive lung disorder characterized by the thickening and scarring of lung tissue, primarily affecting the interstitium—the network of tissue that supports the air sacs (alveoli). This condition often leads to impaired gas exchange and reduced lung function. High-resolution computed tomography (HRCT) plays a crucial role in diagnosing and monitoring the disease, offering detailed visualization of structural changes within the lungs. Radiologists and pulmonologists rely on specific imaging patterns to identify the extent and stage of fibrosis.

Key CT Findings in Pulmonary Interstitial Fibrosis

1. Presence of Small Nodular Opacities

One of the early signs observed on CT scans is the appearance of small nodular opacities scattered throughout the lung fields. These nodules typically represent localized areas of inflammation or fibrotic infiltration. They are often associated with thickened alveolar walls or partial obstruction within the alveolar spaces. While not unique to interstitial fibrosis, their distribution and pattern—especially when combined with other findings—can strongly suggest underlying interstitial disease.

2. Ground-Glass Opacities: A Common Early Indicator

Ground-glass opacities (GGOs) are among the most frequently seen radiological features in patients with pulmonary interstitial fibrosis. On CT imaging, these appear as hazy areas of increased density where the underlying bronchial and vascular structures remain visible. This finding indicates partial filling of the airspaces or thickening of the alveolar septa, often due to inflammation, edema, or early fibrotic changes. GGOs are typically more diffuse and bilateral, predominantly affecting the lower lobes and peripheral regions of the lungs.

3. Reticular or Mesh-Like Patterns

As the disease progresses, reticular opacities become more prominent on imaging. These appear as fine, linear, or net-like densities that reflect thickening of the interlobular septa or intralobular interstitium. This pattern is closely linked to the development of pulmonary fibrosis and often coexists with ground-glass opacities, forming what is known as a "reticuloground-glass" pattern. The presence of extensive reticulation usually correlates with more advanced stages of fibrosis and may precede the formation of honeycombing.

4. Traction Bronchiectasis: Structural Distortion Due to Scarring

Traction bronchiectasis is a hallmark of established pulmonary fibrosis. It occurs when chronic scarring pulls on the walls of the bronchi, causing abnormal and irreversible dilation of the airways, particularly in the peripheral lung zones. Unlike infectious bronchiectasis, this form is not due to airway wall weakness but rather mechanical distortion from surrounding fibrotic tissue. On CT scans, it appears as irregularly dilated bronchi that do not taper normally, often located in areas of significant parenchymal distortion.

5. Honeycombing: A Sign of End-Stage Lung Damage

Honeycombing represents the most severe and irreversible stage of pulmonary interstitial fibrosis. It appears on CT scans as clustered cystic air spaces, typically 3–10 mm in size, with thick, well-defined walls arranged in multiple layers. These changes result from the destruction of normal lung architecture and the formation of fibrotic cysts, primarily in the subpleural regions of the lower lobes. The presence of honeycombing is a strong diagnostic indicator of usual interstitial pneumonia (UIP) pattern and is associated with poor prognosis and limited treatment response.

Clinical Implications and Diagnostic Value of CT Imaging

High-resolution CT is indispensable in the evaluation of interstitial lung diseases. It allows clinicians to differentiate between various types of fibrosis, assess disease progression, and guide biopsy decisions when needed. Recognizing the combination of ground-glass opacities, reticular patterns, traction bronchiectasis, and honeycombing helps in establishing a confident diagnosis without invasive procedures. Moreover, serial CT scans can monitor therapeutic efficacy and help predict patient outcomes.

In conclusion, understanding the characteristic CT manifestations of pulmonary interstitial fibrosis enables earlier detection and better management of this debilitating condition. Radiological patterns serve not only as diagnostic tools but also as critical markers for staging and prognostication in clinical practice.