Normal Proximal Pulmonary Artery Diameter on CT and MRI
The proximal pulmonary arteries — the left and right main branches arising from the main pulmonary trunk — carry the full output of the right ventricle to the pulmonary circulation. Accurate measurement of their diameters on cross-sectional imaging is clinically important because dilatation is one of the earliest and most reliable imaging signs of pulmonary hypertension and chronic right heart strain.
Normal Reference Values
| Location | Measurement |
|---|---|
| Left | 21 ± 3.5 mm |
| Right | 18.7 ± 2.8 mm |
Clinical Significance
A right proximal pulmonary artery diameter exceeding approximately 28 mm or a left exceeding approximately 29 mm on CT has been associated with pulmonary hypertension in several validation studies. However, even mild dilatation beyond the normal ranges provided here warrants correlation with echocardiographic pressures and clinical findings, particularly in the context of dyspnea, hypoxia, or right ventricular remodeling.
Pulmonary artery enlargement can be caused by a range of conditions. Key pitfalls include overestimation due to cardiac motion artifact or oblique measurement planes, and underestimation in patients with significant mediastinal fibrosis restricting vessel distension.
- Pulmonary arterial hypertension (PAH) — primary or connective tissue disease–associated
- Chronic thromboembolic pulmonary hypertension (CTEPH)
- Left heart disease with elevated pulmonary venous pressure
- Congenital heart disease with left-to-right shunt (e.g., ASD, VSD)
- Pulmonary valve stenosis causing post-stenotic dilatation (typically left PA)
Reference: Kuriyama K, Gamsu G, Stern RG et al. CT-determined pulmonary artery diameters in predicting pulmonary hypertension. Invest Radiol. 19 (1): 16-22.
Imaging Notes
On CT, the proximal left and right pulmonary arteries should be measured on axial images at their widest point perpendicular to the vessel long axis, ideally on ECG-gated or breath-hold acquisitions to minimize pulsation artifact. Measurements are best performed at the level just distal to the bifurcation of the main pulmonary artery, avoiding inclusion of vessel wall calcification or adjacent soft tissue.
On MRI, phase-contrast or steady-state free precession (SSFP) sequences provide excellent delineation of the pulmonary arterial walls. Measurements should similarly be taken perpendicular to the vessel axis on reconstructed multiplanar reformats. MRI has the added advantage of providing concurrent flow and pressure estimation without ionizing radiation, making it particularly valuable for longitudinal follow-up of known pulmonary hypertension.