1. A method of correcting CT numbers in image data, the method comprising: selecting at least one reference region in a CT image of an object, wherein the at least one reference region is a portion of the object associated with substantially same CT number when no artifacts are present in the CT image; estimating artifacts in at least one other region in the same CT image based at least in part on detected CT numbers in the at least one reference region, wherein the at least one other region is another portion of the same object; and correcting CT numbers in the at least one other region in the CT image based on the estimated artifacts.