Interstitial Lung Abnormalities: State of the Art

Abstract

The clinical importance of interstitial lung abnormality (ILA) is increasingly recognized. In July 2020, the Fleischner Society published a position paper about ILA. The purposes of this article are to summarize the definition, existing evidence, clinical management, and unresolved issues for ILA from a radiologic standpoint and to provide a practical guide for radiologists. ILA is a common incidental finding at CT and is often progressive and associated with worsened clinical outcomes. The hazard ratios for mortality range from 1.3 to 2.7 in large cohorts. Risk factors for ILA include age, smoking status, other inhalational exposures, and genetic factors (eg, gene encoding mucin 5B variant). Radiologists should systematically record the presence, morphologic characteristics, distribution, and subcategories of ILA (ie, nonsubpleural, subpleural nonfibrotic, and subpleural fibrotic), as these are informative for predicting progression and mortality. Clinically significant interstitial lung disease should not be considered ILA. Individuals with ILA are triaged into higher- and lower-risk groups depending on their risk factors for progression, and systematic follow-up, including CT, should be considered for the higher-risk group. Artificial intelligence-based automated analysis for ILA may be helpful, but further validation and improvement are needed. Radiologists have a central role in clinical management and research on ILA.

Graphical abstract

Figure 1:

Flowchart of definition and subcategories of interstitial lung abnormality (ILA). * = Fibrosis is characterized by the presence of architectural distortion with traction bronchiectasis or honeycombing (or both). (Adapted, with permission, from reference .)

Figure 2:

Thin-section CT scans show typical findings of interstitial lung abnormalities. (A) Ground-glass abnormality (arrows) is seen in peripheral lung zone. (B) Reticular abnormality with ground-glass opacity (arrows) is seen in subpleural area. (C) Lung distortion is suggested by volume loss and displacement of bronchi and vessels toward medial side (arrow). Distortion accompanies ground-glass abnormality and traction bronchiectasis. (D) Traction bronchiectasis (arrow) is conspicuous in left lower lobe. Ground-glass and reticular abnormalities are also seen in subpleural area. (E) Honeycombing is demonstrated as appearance of clustered cystic air spaces (arrow). (F) Nonemphysematous cysts can be seen as lucencies with well-defined walls (arrows). Surrounding ground-glass and reticular abnormalities are also seen.

Figure 3:

Distribution of interstitial lung abnormalities (ILAs). (A) ILA with subpleural predominance. Axial CT scan shows reticulation and ground-glass abnormality (arrowheads) in bilateral subpleural area. (B) ILA with central predominance. Axial CT scan shows patchy ground-glass abnormalities (arrowheads) in central area of left lower lobe. Nodular ground-glass abnormalities (arrow) are also seen in central area of right lower lobe. (C) ILA with upper-lobe predominance. Coronal CT scan shows subpleural reticulation and ground-glass abnormality (arrows) in both upper lobes. (D) ILA with lower-lobe, or basal, predominance. Coronal CT scan shows subpleural ground-glass abnormality (arrows) and nonemphysematous cyst in bilateral basal area.

Figure 4:

Typical interstitial lung abnormalities (ILAs). (A) Nonsubpleural and nonfibrotic ILA. CT scan shows multifocal ground-glass abnormalities (arrows) in central area of both lungs. (B) Subpleural nonfibrotic ILA. CT scan shows predominantly subpleural ground-glass and linear abnormalities (arrowheads) without evidence of fibrosis. (C) Subpleural fibrotic ILA. Besides subpleural ground-glass and linear abnormalities (arrowheads), CT scan shows traction bronchiectasis (arrow) and architectural distortion in left lower lobe, which suggests lung fibrosis.

Figure 5:

Pitfalls in diagnosis of interstitial lung abnormalities. (A) Centrilobular nodularity. CT scan shows numerous poorly defined ground-glass centrilobular nodules (arrows). (B) Apical cap or pleuroparenchymal fibroelastosis–like lesion. CT scan shows subpleural consolidative nodules with traction bronchiectasis (arrow) in right apical lung. Subpleural linear and nodular opacities (arrowheads) are also seen in left lung. (C) Osteophyte-related focal fibrosis. CT scan shows focal reticulation and ground-glass abnormality adjacent to osteophyte (circle). (D) Aspiration. CT scan shows ill-defined ground-glass abnormalities (arrowheads) in left lower lobe. Bronchial wall thickening (arrows) is also seen, which suggests that ground-glass abnormalities are related to aspiration.

Figure 6:

Dependent abnormality. L = left. (A) Supine CT scan shows faint ground-glass abnormality (arrowhead) in dependent portion of right (R) lower lobe. (B) Abnormality disappeared (arrowhead) in same area on prone CT scan, indicating abnormality on A was atelectasis and not interstitial lung abnormality.

Figure 7:

Progression of interstitial lung abnormalities (ILAs) in three individuals (A and B, C and D, and E and F). A, C, and, E are CT scans obtained 5 years before B, D, and F, respectively, in each individual. (A) Scan shows no evidence of ILA. (B) Scan shows newly developed ground-glass abnormalities (arrows) in subpleural area of bilateral lungs. (C) Scan shows slight ground-glass abnormality and nonemphysematous cysts (arrows) but no clear evidence of fibrosis. (D) Scan shows increased severity and extent of abnormalities (straight arrows) with new traction bronchiectasis indicating lung fibrosis (curved arrow). (E) Scan shows mild unilateral ground-glass abnormalities (arrows) suggesting ILAs. (F) Individual developed severe architectural distortion with traction bronchiectasis in both lower lobes (circles).

Figure 8:

Use of data-driven texture analysis to quantify progression of interstitial lung abnormality (ILA) in Genetic Epidemiology of Chronic Obstructive Pulmonary Disease Study patient. (A) Baseline CT scan shows minimal ILA. (B) CT scan with data-driven texture analysis overlay precisely identifies fibrotic abnormality, quantified as 1.4% of lung volume. (C) CT scan obtained 5 years later shows clear disease progression. (D) Follow-up CT scan with data-driven texture analysis overlay shows extent of ILA progression, quantified as 5.6% of lung volume. (Figure 8 courtesy of Stephen M. Humphries, PhD, Quantitative Imaging Laboratory, National Jewish Health.)

Figure 9:

Proposed triage rubric for interstitial lung abnormalities (ILAs) found at chest CT. Action items for radiologist are in blue, action items for treating physician or pulmonologist are in green, and action items for a pulmonologist, ideally with interstitial lung disease (ILD) experience, are in orange. * = Nontrivial abnormalities present in three or more lung zones (above bottom of aortic arch, between aortic arch and top of inferior pulmonary vein, and below inferior pulmonary vein). (Adapted, with permission, from reference .)