CT apparatus and an image processing method used by the same

A method for automatically determining the best effective reconstruction gap, the method including scanning a phantom to collect image data of the phantom, using a plurality of different gap values to reconstruct image of the phantom respectively, based on the image data, thus obtaining a plurality of images respectively associated with different gap values, selecting the best image from the plurality of images, and automatically determining the gap value associated with the best image, and save it as the best effective reconstruction gap.