A method includes inserting into a patient body a catheter, which includes an insertion tube and at least an expandable distal-end device coupled to a distal end of the insertion tube. The distal-end device is expanded in a blood vessel, thereby forming an annular guiding trench between the distal-end device and a circumference of the blood vessel, the annular trench shaped to receive and guide a distal tip of a medical instrument. The distal tip of the medical instrument is guided in the annular guiding trench formed by the expanded distal-end device of the catheter, so as to approach multiple points on the circumference of the blood vessel.
