A kit of parts for centring an external element with respect to an implantable medical device, said kit of parts comprising: An implantable medical device for being implanted under the skin of a patient and comprising; (a) an internal housing characterized by a secondary axis, (b) a light source positioned such as to emit a light beam coaxial to the secondary axis, and of wavelength and intensity sufficient for being transmitted through the tissues and skin separating the implantable medical device from the exterior when implanted under the skin of a patient, and An external element comprising an external housing characterized by a primary axis, and containing an external electronic unit, the external element comprising: (a) A number, N > 2, of photodetectors (12a-12d) forming a polygon of N edges, normal to the primary axis, and which centroid belongs to the primary axis, and (b) An indicator indicating how the external element is to be displaced over the surface of the skin of a patient in order to position the external element with the primary axis, being coaxial with the secondary axis, of the implantable medical device, Characterized in that, the external element comprises a processor or an electronic logic control circuit configured to: (a) Measure the energy received by each of the photodetectors of the external element from the light source, (b) Calculate the mean energy, E m, defined as the average of the energies, E i, received by each of the N photodetectors, E m = ∑ E i / N, with i = 1 to N, (c) For each photodetector, determine whether the received energy, E i, falls within an optimal range comprised between (Em - ε) and (Em + ε), wherein ε is a predefined optimal deviation, (d) If the energy, Ei, received by each of the N photodetectors falls within the optimal range, indicate that the external element is optimally positioned with the primary axis being coaxial to the secondary axis within the optimal deviation, (e) If the energy, E i, of at least one of the N photodetectors falls out of the optimal range, set the indicator to indicate that the external element is not positioned optimally and to indicate the direction the external element must be displaced in order to bring the primary axis coaxially to the secondary axis of the implantable medical device. An implantable medical device for being implanted under the skin of a patient and comprising; (a) an internal housing characterized by a secondary axis, (b) a light source positioned such as to emit a light beam coaxial to the secondary axis, and of wavelength and intensity sufficient for being transmitted through the tissues and skin separating the implantable medical device from the exterior when implanted under the skin of a patient, and (a) an internal housing characterized by a secondary axis, (b) a light source positioned such as to emit a light beam coaxial to the secondary axis, and of wavelength and intensity sufficient for being transmitted through the tissues and skin separating the implantable medical device from the exterior when implanted under the skin of a patient, and An external element comprising an external housing characterized by a primary axis, and containing an external electronic unit, the external element comprising: (a) A number, N > 2, of photodetectors (12a-12d) forming a polygon of N edges, normal to the primary axis, and which centroid belongs to the primary axis, and (b) An indicator indicating how the external element is to be displaced over the surface of the skin of a patient in order to position the external element with the primary axis, being coaxial with the secondary axis, of the implantable medical device, (a) A number, N > 2, of photodetectors (12a-12d) forming a polygon of N edges, normal to the primary axis, and which centroid belongs to the primary axis, and (b) An indicator indicating how the external element is to be displaced over the surface of the skin of a patient in order to position the external element with the primary axis, being coaxial with the secondary axis, of the implantable medical device, (a) Measure the energy received by each of the photodetectors of the external element from the light source, (b) Calculate the mean energy, E m, defined as the average of the energies, E i, received by each of the N photodetectors, E m = ∑ E i / N, with i = 1 to N, (c) For each photodetector, determine whether the received energy, E i, falls within an optimal range comprised between (Em - ε) and (Em + ε), wherein ε is a predefined optimal deviation, (d) If the energy, Ei, received by each of the N photodetectors falls within the optimal range, indicate that the external element is optimally positioned with the primary axis being coaxial to the secondary axis within the optimal deviation, (e) If the energy, E i, of at least one of the N photodetectors falls out of the optimal range, set the indicator to indicate that the external element is not positioned optimally and to indicate the direction the external element must be displaced in order to bring the primary axis coaxially to the secondary axis of the implantable medical device.