A medical system comprises a first medical probe having indicia, and a second medical probe in a coaxial arrangement with the first medical probe. The second medical probe has a detector (e.g., an optical detector) configured for detecting a passage of the indicia as the first and second medical probes are moved relative to each other. In one embodiment, the first medical probe is an inner medical probe, and the second medical probe is an outer medical probe. The medical system further comprises an electromechanical driver coupled to the first and second medical probes, and a controller configured for directing the electromechanical driver to move the first and second medical probes relative to each other, and for receiving signals from the detector indicating the relative movement between the first and second medical probes.
