## Most Common Site of PE on CTPA **Key Point:** The right lower lobe pulmonary artery is the most frequent site of pulmonary embolism, accounting for approximately 40–50% of emboli detected on CTPA. ### Anatomical Basis The right lower lobe artery is preferentially affected because: 1. **Larger caliber** — The right lower lobe artery has a larger diameter than other segmental arteries, making it a lower-resistance pathway for emboli. 2. **Straighter course** — The right lower lobe artery branches more directly from the right main pulmonary artery, allowing emboli to travel with less deflection. 3. **Gravity and flow dynamics** — Emboli preferentially lodge in vessels with higher blood flow and less acute angles. ### Distribution of PE Sites | Site | Frequency | Reason | | --- | --- | --- | | Right lower lobe | 40–50% | Largest caliber, straight course | | Right upper lobe | 20–25% | Moderate flow | | Left lower lobe | 15–20% | Smaller than right lower | | Left upper lobe | 10–15% | Smallest caliber | **High-Yield:** On CTPA, always scrutinize the right lower lobe pulmonary artery first when screening for PE — this is where the majority of emboli will be found. **Clinical Pearl:** Isolated subsegmental PE in the right lower lobe is common but may be clinically insignificant in hemodynamically stable patients without cardiopulmonary disease. ### CTPA Protocol Reminder CTPAs are acquired during the pulmonary arterial phase (13–15 seconds after bolus arrival in the right atrium) to maximize contrast opacification of the pulmonary arteries and detect thrombi as filling defects. [cite:Harrison 21e Ch 297]
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