A method for reconstructing 3D shape of a longitudinal device using an optical fiber with optical shape sensing (OSS) properties, e.g. Bragg gratings. By attaching the optical fiber to the longitudinal device, such that the optical fiber follows its 3D shape upon bending, known OSS techniques can be applied to reconstruct 3D shape of the optical fiber, and thus also the longitudinal device, e.g. a medical catheter. E.g. the optical fiber, e.g. placed in a guide wire, can be inserted in a lumen of the longitudinal device. Hereby, one OSS system can be used for 3D tracking a plurality of non-shape sensed catheters or other longitudinal devices. In case the longitudinal device is longer than the optical fiber, the position and shape of the remaining part of the longitudinal device may be estimated and visualized to a user, e.g. based on a known length of the longitudinal device, and based on an orientation of an end point of the optical fiber, e.g. using knowledge about the stiffness or other properties of the longitudinal device.