CT Pulmonary Angiography (CTPA) is Required
To assess for a dilated pulmonary artery, you need a CT pulmonary angiography (CTPA) with intravenous contrast timed for optimal pulmonary artery opacification. This is the definitive imaging modality for evaluating pulmonary artery diameter and associated cardiovascular findings 1, 2.
Why CTPA is the Correct Examination
CTPA uses IV contrast specifically timed to achieve peak opacification of the pulmonary arteries, allowing precise measurement of vessel diameter and detection of dilation 1. The contrast timing is critical—adequate pulmonary artery opacification requires attenuation values of approximately 200-250 HU 2.
Key Diagnostic Measurements on CTPA
When evaluating for pulmonary artery dilation, CTPA allows measurement of:
- Main pulmonary artery (MPA) diameter: Dilation defined as ≥29 mm 1
- MPA to ascending aorta ratio: Abnormal when >1 1
- Right and left descending PA diameters: 16 mm and 21 mm respectively are upper limits 1
- Segmental PA to bronchus ratio: Abnormal when >1:1 1
These measurements have high diagnostic accuracy—MPA diameter on CTPA demonstrates 87% sensitivity, 89% specificity, and 97% positive predictive value for pulmonary hypertension 1.
Why Non-Contrast CT is Insufficient
A standard chest CT without contrast cannot adequately assess pulmonary artery diameter because:
- The vessel lumen is not optimally opacified
- Distinguishing the vessel wall from surrounding mediastinal structures becomes difficult
- Precise diameter measurements require clear delineation of the arterial lumen 3, 4
Additional Cardiovascular Assessment
CTPA simultaneously evaluates right ventricular function and other cardiovascular changes associated with pulmonary artery dilation, including:
- Right ventricular enlargement (RV/LV ratio ≥1) 1, 2
- Interventricular septal flattening or bowing 1
- RV free wall thickness ≥6 mm 1
- Pericardial effusion or thickening 1
- Bronchial artery enlargement >1.5 mm 1
Common Pitfall to Avoid
Do not order PET/CT to evaluate pulmonary artery dilation. PET/CT cannot visualize the pulmonary arteries adequately because their diameter falls below the detection threshold of this modality 5. CT angiography or MR angiography is required for pulmonary artery assessment 5.
Technical Considerations
For optimal diagnostic quality, ensure the scan protocol includes 2:
- Adequate contrast bolus with proper timing for arterial phase
- Scan delay allowing complete arterial opacification
- Thin slice thickness (<1 mm) for detailed vessel evaluation
- Both mediastinal and lung windows for comprehensive assessment
The examination should extend from the inferior vena cava through the lung apices to capture the entire pulmonary arterial tree and associated cardiac structures 3, 6.