An implantable lead adapted to transmit electrical signals between a connector assembly on a proximal end of the lead and at least one electrode carried by a distal end of the lead comprises a helical fixation element extendable and retractable from the distal end of the lead, the header comprising (a) an inner header part comprising an electrically conductive material that is substantially transparent fluoroscopically, the inner header part having a distal end, (b) an outer header part comprising an electrically insulating material, and (c) a collar attached to the distal end of the inner header part. The collar comprises a material that is substantially opaque fluoroscopically. The collar may be electrically conductive, and electrically and mechanically connected, preferably by means of an overlap joint, to the distal end of the inner header part. The conductive collar thus may be electrically connected to an electrical contact on the connector assembly via the electrically conductive inner header part, whereby the collar may be used for mapping the electrical activity of local body tissue. Alternatively, the collar may be electrically isolated by, for example, covering the outer surface of the collar with an electrically insulating layer.