Catheters, systems, and methods are provided for performing medical procedures, such as tissue ablation, adjacent the ostia of anatomical vessels, such as pulmonary veins. The catheter comprises an elongated flexible catheter body including a proximal shaft portion and a distal shaft portion, which has a proximal section pre-shaped to form a curve having an apex sized to be inserted into an anatomical vessel, such as a pulmonary vein, and a distal section configured to contact an ostium of the vessel when the curve apex is inserted within the vessel ostium. The catheter further comprises a steering mechanism configured for decreasing a radius of curvature of the curve.