How do chest CT findings differ between chronic obstructive pulmonary disease (COPD) and interstitial lung disease (ILD)?

CT Findings in COPD vs ILD: Key Distinguishing Features

CT chest is the preferred imaging modality for both COPD and ILD, with distinct patterns that allow differentiation: COPD demonstrates emphysema (low attenuation areas), bronchial wall thickening, and air trapping on expiratory imaging, while ILD shows reticular opacities, ground-glass attenuation, honeycombing, and traction bronchiectasis with predominantly basal and peripheral distribution. 1

COPD-Specific CT Findings

Primary Parenchymal Features

• Emphysema patterns are the hallmark finding, appearing as areas of low attenuation without visible walls, distributed as centrilobular (most common), panlobular, or paraseptal subtypes 1, 2
• Quantitative CT parameters correlate with pulmonary function tests and can track disease progression, with visual assessments being both accurate and reproducible 1
• Air trapping on expiratory CT imaging reflects airflow limitation and correlates with dyspnea severity, present in 31-35% of early COPD cases 1, 3

Airway Abnormalities

• Bronchial wall thickening is detected in 57-62% of COPD patients and represents chronic airway inflammation 3, 4
• CT identifies early COPD changes in asymptomatic smokers with normal spirometry, demonstrating superior sensitivity over chest radiography 1
• Bronchiectasis may coexist, particularly in patients with chronic bronchitis phenotype 1, 3

Prognostic Value

• CT-based phenotypes predict future hospitalization, symptomatic decline, and mortality in COPD patients 1
• The extent and distribution of emphysema guide therapeutic decisions, particularly for lung volume reduction surgery 5

ILD-Specific CT Findings

Characteristic Patterns

• Reticular opacities with basal and peripheral predominance are the classic finding in usual interstitial pneumonia (UIP) pattern 1
• Honeycombing (clustered cystic airspaces with thick walls) indicates advanced fibrosis and serves as an important prognostic variable 1
• Ground-glass opacities represent active inflammation or early fibrosis, often requiring prone imaging to distinguish from dependent atelectasis 1
• Traction bronchiectasis results from fibrotic distortion of airways and helps differentiate ILD from COPD-related bronchiectasis 1, 6

Diagnostic Accuracy

• HRCT has 95.7% sensitivity and 63.8% specificity for detecting ILD with ≥20% lung involvement 1
• CT findings often permit confident diagnosis of UIP pattern without biopsy, though diagnostic yield improves with multidisciplinary discussion 1
• HRCT demonstrates superior diagnostic accuracy compared to chest radiography, with first-choice diagnoses made with high confidence being significantly more accurate (p<0.001) 7

Protocol Considerations

• HRCT protocols should include thin-slice reconstruction (<1.5mm), inspiratory prone images, and supine end-expiratory imaging 1
• Prone imaging differentiates mild dependent atelectasis from early fibrosis 1
• Expiratory imaging assesses for air-trapping, which can occur in both COPD and some ILDs 1

Combined Pulmonary Fibrosis and Emphysema (CPFE)

Recognition and Significance

• CPFE presents with emphysema in upper lung fields and diffuse ILD in lower zones, representing coexistence of both diseases 8
• Patients with CPFE have higher mortality compared to COPD alone, with poor prognostic factors including exacerbations, lung cancer, and pulmonary hypertension 8
• This entity shares risk factors with both diseases: smoking, male sex, and advanced age 8

Critical Imaging Pitfalls to Avoid

Technical Considerations

• CT angiogram studies are inadequate for ILD assessment because incomplete inspiration produces atelectasis that can obscure, accentuate, or mimic ILD 1
• Standard chest CT with thin-slice reconstruction is often sufficient; contrast adds no diagnostic value for parenchymal lung disease 4
• Chest radiography has poor sensitivity (69-71%) for airway abnormalities and may be normal in early COPD 3, 4

Diagnostic Errors

• Don't assume bronchiectasis automatically indicates COPD—it occurs in 20% of asymptomatic elderly patients and is common in ILD with traction bronchiectasis 9, 6
• The "straight-edge" sign (sharp demarcation between normal and abnormal lung) has 94% specificity for connective tissue disease-associated ILD versus IPF 6
• Interstitial lung abnormalities on CT in COPD patients indicate worse prognosis and may represent early ILD requiring different management 8

Monitoring and Follow-up Imaging

ILD Surveillance

• For systemic autoimmune rheumatic disease-associated ILD, HRCT should be performed when clinically indicated rather than at routine intervals 1
• The extent of honeycombing and other fibrotic features on serial CT serves as prognostic markers 1

COPD Surveillance

• Routine follow-up CT is not indicated for stable COPD; repeat imaging is warranted only for acute exacerbations, suspected complications, or lung cancer screening 4
• Quantitative CT parameters can track disease progression in research settings but are not routinely used clinically 1, 5