1. A method of obtaining an X-ray image, the method comprising: obtaining a first image of an object; after obtaining the first image, receiving a determination whether the first image includes an entirety of a region of interest (ROI); and obtaining a second image of the object to include a portion of the ROI which is absent in the first image, in response to the receiving the determination that the portion of the ROI is absent in the first image, wherein a width of an X-ray beam of an X-ray source is made narrower for the obtaining the second image than that for the obtaining the first image, by adjusting at least one of a size and a position of a collimator of the X-ray source.