The invention relates to a rod-shaped body comprising a central section and a peripheral section, wherein the central section is arranged in the center of the rod-shaped body and is enclosed by the peripheral section. Both the central section and the peripheral section substantially extend along the entire length of the rod-shaped body. The central section comprises at least one non-metallic fiber bundle that is embedded in a non-ferromagnetic matrix material. The matrix material is doped with marker particles. The peripheral section comprises at least one undoped, non-ferromagnetic matrix material. The diameter of the central section is less than or equal to 0.2 mm, preferably less than or equal to 0.15 mm, and even more preferably, less than or equal to 0.1 mm, and in particular less than or equal to 0.08 mm.