1. A method for processing X-ray image data, the method comprising: receiving sum image data and difference image data, wherein the sum image data and the difference image data comprise intensity information of X-rays of two different energies passing through an object, the sum image data is based on a sum intensity of the two different energies and the difference image data is based on a difference intensity of the two different energies; partitioning the difference image data into a low frequency range and a high frequency range; and generating low noise image data by replacing the high frequency range of the difference image with a high frequency range based on the sum image data.