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.