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, wherein calculating the gain slope for each pixel value region comprises: determining a lower pixel value boundary for the pixel value region, the lower pixel value boundary defining a lower limit of the range of pixel values in the respective pixel value region; determining a median value of an average gain calibration image for the respective pixel value region; multiplying the median value by a scaling factor to generate a scaled median value; and dividing the scaled median value by a difference between a median pixel value and the lower pixel value boundary; wherein the median pixel value corresponds with the median value of the average gain calibration image; 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.