1. A method of controlling a radiography apparatus, the method comprising: acquiring a first radiation image of a subject before a contrast reagent is injected into the subject; acquiring a second radiation image of the subject after the contrast reagent is injected into the subject; calculating a difference between data of a pixel of the first radiation image and data of a pixel of the second radiation image, for each of pixels of the first radiation image; and acquiring an image of the subject based on the difference for each of pixels of the first radiation image.