Apparatus for collimating an X-ray beam, the apparatus including a multi-slit rotatable collimator including a semi-tubular structure extending coaxially along a longitudinal axis and being formed out of an X-ray impermeable material, with at least two slits formed in the semi-tubular structure, wherein the at least two slits extend parallel to the longitudinal axis of the semi-tubular structure, a mount for rotatably supporting the semi-tubular structure in the path of an X-ray beam, and a drive mechanism for selectively rotating the semi-tubular structure about the longitudinal axis of the semi-tubular structure, whereby to selectively (i) position one slit in the path of the X-ray beam so as to tailor the X-ray beam to the width of that slit, and (ii) position a solid portion of the semi-tubular structure in the path of an X-ray beam so as to block an X-ray beam.
