or the tubing may on the whole represent only a two layer field decoupling conductor the metallization may have a fine structure which is created by a lithographic process for example the carrier of the metallization may be a helix or may have some other fine structure the carrier should be very elastic due to its shape so as not to excessively increase the rigidity of the electrode the rigidity of a tubing can be varied e g by means of multiple recesses e g by incisions which greatly reduce the planar moment of inertia of the structure without greatly influencing the longitudinal rigidity fig9 a and 9b show an exemplary embodiment of such a tubular structure with mutually offset periodically recurring sections a tubular structure can be inserted easily with a guide wire into the internal lumen of the electrode even if it has a low intrinsic rigidity on the distal end a structure may be provided which together with another fitting structure in the interior of the electrode forms a retaining mechanism on insertion of the tubing with a guide wire the tubing is held in the tip and the guide wire can be removed without altering the position of the tubing the retaining mechanism should be releasable to allow removal of the field decoupling conductor as another modification of the embodiment illustrated in fig1 a to 1c.