1. A method of controlling an X-ray in a computed tomography (CT) apparatus, the method comprising: emitting X-rays toward an object while rotating an X-ray emitter around the object on a rotating frame of the CT apparatus; detecting, by second detector elements of a detector, the X-rays emitted from a plurality of points along a rotation path of the X-ray emitter, and generating projection data from the detected X-rays; obtaining scout images of the object; setting an imaging region of the object in the obtained scout images; determining an outline of the imaging region based on transverse axes lengths of the imaging region; controlling X-rays emitted toward the object by adjusting a distance between elements of a transverse collimator of the CT apparatus according to the determined outline; and reconstructing a cross-sectional X-ray image of the object based on X-ray projection data generated by detecting the controlled X-rays, wherein the scout images of the object comprise information about the transverse axes lengths of the imaging region at an angle at which the rotating frame rotates.