A method of controlling an X-ray in a computed tomography apparatus including a rotating frame configured to rotate around an object, the method comprising: obtaining scout images (510, 530) of the object; setting an imaging region (32, 512, 532) of the object in the obtained scout images; determining an outline of the imaging region (32, 512, 532) based on transverse axes lengths (36, 410, 430, 514, 534) of the imaging region (32, 512, 532), wherein a transverse axis length is the length of the object in a width direction of said object set in the said scout images and wherein the outline contains information about an outline of the transverse axes lengths (36, 410, 430, 514, 534) of the imaging region (32, 512, 532) at an angle at which the rotating frame rotates; 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 reconstructing the cross-sectional X-ray image of the object comprises: determining first detector elements which are to receive the controlled X-rays corresponding to the determined outline, of a detector of the CT apparatus; determining second detector elements, of the detector, that actually detect the controlled X-rays; comparing the first detector elements with the second detector elements; and changing a position of a focal point of the X-rays emitted toward the object based on a comparison result. obtaining scout images (510, 530) of the object; setting an imaging region (32, 512, 532) of the object in the obtained scout images; determining an outline of the imaging region (32, 512, 532) based on transverse axes lengths (36, 410, 430, 514, 534) of the imaging region (32, 512, 532), wherein a transverse axis length is the length of the object in a width direction of said object set in the said scout images and wherein the outline contains information about an outline of the transverse axes lengths (36, 410, 430, 514, 534) of the imaging region (32, 512, 532) at an angle at which the rotating frame rotates; 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 reconstructing the cross-sectional X-ray image of the object comprises: determining first detector elements which are to receive the controlled X-rays corresponding to the determined outline, of a detector of the CT apparatus; determining second detector elements, of the detector, that actually detect the controlled X-rays; comparing the first detector elements with the second detector elements; and changing a position of a focal point of the X-rays emitted toward the object based on a comparison result. determining first detector elements which are to receive the controlled X-rays corresponding to the determined outline, of a detector of the CT apparatus; determining second detector elements, of the detector, that actually detect the controlled X-rays; comparing the first detector elements with the second detector elements; and changing a position of a focal point of the X-rays emitted toward the object based on a comparison result.