A method for positioning a balloon, which is fitted at a distal end of a catheter, at an ostium of a blood vessel of a patient, includes attempting to achieve circumferential physical contact between the balloon and an entire circumference of the ostium. One or more slices across the ostium are imaged with magnetic resonance imaging (MRI), and the extent of blood flow along the circumference of the ostium is estimated using the MRI. Based on the estimated extent of blood flow, an extent of the circumferential physical contact between the balloon and the ostium is indicated to a user.
