A medical device with metal parts to be introduced into animal or human bodies is rendered visible during RMN imaging by covering the exterior surface of the metal parts with a layer comprising Nickel monoxide. When placed within the electromagnetic field generated by a NMR apparatus, the layer reduces disturbances induced in this field in the vicinity of the metal part. This improvement is chiefly related to endoprostheses for subjects or patients who will be submitted to NMR imaging. In particular, the invention is aimed at luminal endoprostheses comprising a metal frame, as stents.