1. A computer-implemented method for reducing image artifacts in X-ray image data, comprising: dividing pixels of X-ray image data into a plurality of pixel value regions based on a pixel value of each pixel, wherein each pixel value region has a different range of pixel values; generating calibrated X-ray image data for each pixel value region, wherein the respective calibrated X-ray image data for each pixel value region is generated using a different dose of radiation; calculating a gain slope for each pixel value region based on the calibrated X-ray image data; and calculating a pixel gain correction for the pixels of the X-ray image data based on at least one of the calculated gain slopes.