1. A method of measuring a pulmonary nodule volume using a dual-energy subtraction image, the method comprising: (a) obtaining a dual-energy subtraction image of a chest; (b) setting a specific region comprising a pulmonary nodule region in the obtained dual-energy subtraction image; (c) interpolating a gray level trend of a region surrounding the pulmonary nodule region into the pulmonary nodule region using a fitting function; and (d) measuring the pulmonary nodule volume by subtracting the interpolation result from gray level data of the pulmonary nodule region, wherein (c) comprises: (c1) finding borders of the pulmonary nodule region in the specific region; (c2) selecting the fitting function and fitting the gray level data to interpolate the gray level trend of the region surrounding the pulmonary nodule region into the pulmonary nodule region; and (c3) correcting a deviation of the fitting result at the border from the gray level, wherein in (c3), when the deviation of the fitting result at the bordering point from the gray level is greater than a first predetermined value, the bordering point moves away from the center of the pulmonary nodule by one or more pixels, and when the deviation is less than a second predetermined value, the bordering point moves toward the center of the pulmonary nodule by one or more pixels.