A device for positioning a catheter at an ostium. The device includes an attachment mechanism for direct attachment to the catheter at a location proximal to a treatment device positioned on the catheter, the attachment mechanism having a proximal end, a distal end, and a longitudinal axis extending from the proximal end to the distal end. The device also including a stopper element extending from the distal end of the attachment mechanism and projecting away from the longitudinal axis, the stopper element being bendable in a direction toward the proximal end of the attachment mechanism.