A system for performing a medical intervention in a subjects body includes a medical device (10) having several segments (12, 15) to be navigated through the body lumens an external magnet and a control unit connected with the magnet. The magnet generates control signals for the external magnet to generate a magnetic field (B) having a direction (40) to orient a magnetically responsive element (25) in the distal end portion (20) of the medical device (10), especially to cause the distal end portion (20) of the medical device (10) to bend in a given direction (30) for improving navigation. The control unit is configured to limit the generation of control signals for the external magnet that the direction (40) of the magnetic field (B) is limited within a field limiting frame (60) around the longitudinal axis (30) of the distal end portion (20) of the medical device (10).
