Apparatus for location-detection of an object within a region can comprise a reflective element mountable on the object, a scanning light source adapted to issue a beam of light in a scanning pattern illuminating a point that moves over the region, and a detector for light reflected from the reflective element. This can be used for leaf position detection in a multi-leaf collimator comprising an array of laterally spaced elongate leaves each having a longitudinal edge on which is mounted a, reflective element, a scanning light source adapted to issue a beam of light in a scanning pattern, illuminating a point that moves over the longitudinal edges of a plurality of leaves of the array, and a detector for light reflected from the reflective elements. A multi-leaf collimator of this type can be incorporated into a radiotherapeutic apparatus.
