Apparatus for collimating an X-ray beam, the apparatus comprising: a multi-slit rotatable collimator comprising: a semi-tubular structure extending coaxially along a longitudinal axis, the semi-tubular structure being formed out of an X-ray impermeable material; at least two slits (280A, 280B) formed in the semi-tubular structure, wherein the at least two slits (280A, 280B) extend parallel to the longitudinal axis of the semi-tubular structure, and further wherein one of the two slits (280A, 280B) has a width greater than the other of the two slits (280A, 280B); a mount (220, 225) 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, characterized in that the drive mechanism comprises a drive shaft and a drive wheel carrying a pin which rotates a driven wheel having slots, with the number and location of slots being coordinated with the number and location of slits and with the solid portions of the semi-tubular structure, such that rotation of the drive shaft can selectively align a particular slit or a solid portion of the semi-tubular structure with the X-ray beam to selectively (i) position a first of the two slits (280A, 280B) in the path of the X-ray beam so as to tailor the X-ray beam to the width of the first of the two slits, (ii) position the second of the two slits (280A, 280B) in the path of the X-ray beam so as to tailor the X-ray beam to the width of the second of the two slits (280A, 280B), and (iii) 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. a semi-tubular structure extending coaxially along a longitudinal axis, the semi-tubular structure being formed out of an X-ray impermeable material; at least two slits (280A, 280B) formed in the semi-tubular structure, wherein the at least two slits (280A, 280B) extend parallel to the longitudinal axis of the semi-tubular structure, and further wherein one of the two slits (280A, 280B) has a width greater than the other of the two slits (280A, 280B); a mount (220, 225) 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, characterized in that the drive mechanism comprises a drive shaft and a drive wheel carrying a pin which rotates a driven wheel having slots, with the number and location of slots being coordinated with the number and location of slits and with the solid portions of the semi-tubular structure, such that rotation of the drive shaft can selectively align a particular slit or a solid portion of the semi-tubular structure with the X-ray beam to selectively (i) position a first of the two slits (280A, 280B) in the path of the X-ray beam so as to tailor the X-ray beam to the width of the first of the two slits, (ii) position the second of the two slits (280A, 280B) in the path of the X-ray beam so as to tailor the X-ray beam to the width of the second of the two slits (280A, 280B), and (iii) 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.