Where is the ideal site for transseptal puncture? In the 40° RAO projection, a pigtail catheter placed in the non-coronary aortic cusp helps define the posterior border of the aortic root. The optimal puncture site (X) is typically located 1–3 cm inferior to the posterior boundary of the aortic root, within the fossa ovalis region. Accurate localization is critical to safely access the left atrium while minimizing the risk of aortic puncture and other complications. Reference: Croft CH, Lipscomb K. Modified technique of transseptal left heart catheterization. J Am Coll Cardiol. 1985;5(4):904–910.