Pulmonary Web on CT Chest
A pulmonary web on chest CT most commonly represents chronic thromboembolic disease (CTEPH) and requires immediate evaluation with ventilation-perfusion (V/Q) scanning and right heart catheterization to assess for pulmonary hypertension. 1
What is a Pulmonary Web?
A pulmonary web (also called a pulmonary artery web or flap) is a thin, linear filling defect within a pulmonary artery that forms an obtuse angle with the vessel wall. This finding is pathognomonic for chronic pulmonary embolism rather than acute PE 1.
Key Distinguishing Features:
Chronic PE characteristics on CT angiography:
- Webs or flaps within the vessel lumen
- Partial filling defects forming obtuse angles with the vessel wall (vs. acute PE which forms acute angles)
- Complete occlusive disease in vessels that are smaller than adjacent patent vessels
- Evidence of recanalization
- May see calcification within the vessel wall in end-stage disease 1
Acute PE characteristics (for comparison):
- Partial filling defects that are centrally located or form acute angles with vessel wall
- Complete arterial occlusion with enlarged affected artery
- Sharp interface with intravascular contrast 1
Recommended Evaluation
Step 1: Assess for Pulmonary Hypertension
Look for these CT findings suggesting PH 2:
- Main pulmonary artery (MPA) diameter ≥29 mm (87% sensitive, 89% specific)
- MPA to ascending aorta ratio >1 (96% positive predictive value)
- Right and left descending PA diameters ≥16 mm and ≥21 mm respectively
- Segmental PA to bronchus ratio >1:1
- Right ventricular enlargement (RV/LV lumen ratio ≥1)
- RV free wall thickness ≥6 mm
- Interventricular septal straightening
- Pericardial effusion or thickening
Step 2: Obtain V/Q Scan
V/Q scanning is the gold standard screening test for CTEPH 2. A normal or low-probability V/Q scan essentially excludes CTEPH, though it may be normal in other causes of pulmonary hypertension. This is critical because CTEPH is potentially curable with pulmonary thromboendarterectomy.
Step 3: Confirm with Right Heart Catheterization
RHC remains necessary to confirm pulmonary hypertension detected on imaging before initiating therapy 2.
Management Implications
Critical distinction: The presence of webs indicates chronic organized thrombus, not acute PE. This fundamentally changes management:
- Do NOT anticoagulate as you would for acute PE without further workup
- Patient may be a candidate for pulmonary thromboendarterectomy (PTE surgery) if CTEPH is confirmed
- If not a surgical candidate, may require pulmonary hypertension-specific medical therapy
- Lifelong anticoagulation will be needed regardless
Common Pitfalls to Avoid
Mistaking chronic for acute PE: Webs form obtuse angles with vessel walls; acute clots form acute angles 1
Missing associated pulmonary hypertension signs: Always measure PA dimensions and assess RV size when you see webs 2
Ordering CT instead of V/Q scan: While CT shows the web, V/Q scanning is more sensitive for detecting the perfusion defects of CTEPH 2
Technical factors causing misdiagnosis: Poor contrast timing, motion artifact, or inadequate vessel opacification can simulate or obscure webs 1
Additional Considerations
Pulmonary vascular pruning (decreased distal vessel volume) may be present and is associated with increased mortality in smokers, even without COPD 3. When evaluating webs, assess the entire pulmonary vasculature for:
- Distal vessel pruning (aBV5/aTBV ratio)
- Enlarged bronchial arteries (>1.5 mm diameter)
- Mosaic attenuation pattern in lung parenchyma
The finding of a pulmonary web is never incidental and always warrants complete evaluation for chronic thromboembolic disease and pulmonary hypertension.