The present disclosure provides an adjustable collimator for collimating a beam of energy emitted from a focal spot of a beam source. The collimator is particularly intended for collimating an x-ray beam of a computed tomography scanner after the x-ray beam has passed through a patient being scanned. The collimator includes two elongated parallel plates arranged side by side to define a collimating slit between the plates. At least one of the plates is movably relative to the other plate for varying a width of the collimating slit. The collimator also includes a movable cam operatively arranged with respect to the at least one movable plate such that movement of the cam in a first direction causes the width of the collimating slit to increase, while movement of the cam in a second direction causes the width of the collimating slit to decrease.