Patent Number: 
Section: claims

1. 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, so as to illuminate a point that moves over the longitudinal edges of a plurality of leaves of the array; anda detector for light reflected from the reflective elements. 2. The multi-leaf collimator according to claim 1 in which the scanning light source comprises a source of light that illuminates a mirror, the mirror being controllably adjustable so as to direct a reflected beam of light in a scan pattern. 3. The multi-leaf collimator according to claim 2 in which the mirror is part of a micro-electromechanical device. 4. The multi-leaf collimator according to claim 1 further comprising at least one mirror locatable in a radiation beam path to permit location of the scanning light source out of the beam path. 5. The multi-leaf collimator according to claim 1 in which the scanning pattern illuminates a point that moves along the longitudinal edges of a plurality of leaves of the array in succession. 6. The multi-leaf collimator according to claim 1 in which the scanning light source comprises a laser. 7. The multi-leaf collimator according to claim 1 in which the reflective element is retro-reflective. 8. The multi-leaf collimator according to claim 1 in which the leaves are mounted in a frame, on which is mounted at least one reflective object. 9. An apparatus for location-detection of an object according to claim 1, adapted to repeat the scanning pattern once a scan of the region has been completed. 10. A Radiotherapeutic apparatus comprising a multi-leaf collimator according to claim 1.