A method for positioning a balloon, which is fitted ata distal end of a catheter, at an ostium of a blood vesselof a patient, includes attempting to achieve circumferentialphysical contact between the balloon and an entirecircumference of the ostium. One or more slices across theostium are imaged with magnetic resonance imaging (MRI), andthe extent of blood flow along the circumference of theostium is estimated using the MRI. Based on the estimatedextent of blood flow, an extent of the circumferentialphysical contact between the balloon and the ostium isindicated to a user.
