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 each of a plurality of pixels of the first radiation image and data of one pixel of the second radiation image, respectively; and acquiring an image of the subject based on the difference for each of the plurality of pixels of the first radiation image, wherein the first radiation image is of a high resolution, and the second radiation image is of the high resolution, and the calculating comprises: down-sampling the first radiation image of the high resolution and the second radiation image of the high resolution to acquire the first radiation image of a low resolution and the second radiation image of the low resolution; and calculating a difference between data of a pixel of the first radiation image of the low resolution and data of a pixel of the second radiation image of the low resolution, for each of pixels of the first radiation image of the low resolution.