1. A method for operating a computed tomography apparatus for computerized post-processing of a reconstructed tomogram of a slice of a subject for removing artefacts, comprising the method steps of: a) obtaining an image matrix IBILD having N.sub.p .multidot.N.sub.p pixels, said pixels respectively having CT values, and masking bone and air portions therein by setting all CT values in the pixel matrix greater than a determined threshold SWO equal to SWO, and setting all CT values smaller than a determined threshold SWU equal to SWU, to obtain a new image matrix R1BILD results, also having N.sub.p .multidot.N.sub.p pixel values; b) conducting a median filtering procedure in R1BILD, along a plurality of straight processing lines, all proceeding through a center of rotation of the computed tomography apparatus and, which cover the image matrix R1BILD so that each pixel lies on one of said straight processing lines, by employing a median filter having 2M.sub.1 +1 support points with a spacing a.sub.R1, to obtain a median-filtered image MED1BILD having pixels with respective values P each resulting from a median value Med of the pixel values of the 2M.sub.1 +1 support points of the image matrix R1BILD; c) forming of a difference image DIFF1=R1BILD-MED1 BILD for all N.sub.p .multidot.N.sub.p pixel values, and conducting of a threshold operation with a artefact threshold S.sub.art in the image DIFF1, by setting all pixel values in DIFF1 having a magnitude greater than S.sub.art equal to +S.sub.art and setting all pixel values in DIFF1 having a magnitude less than S.sub.art to -S.sub.art, to obtain an image R2BILD having pixel values; d) low-pass filtering the pixel values of the image R2BILD along circular arcs around said center of rotation, by averaging, wherein, for each pixel P in the image R2BILD with coordinates xp, yp on the circular arc around the rotational center that proceeds through P, having a radius .sqroot.x.sup.2.sub.p +y.sup.2.sub.p , the 2N.sub.w +1 pixels with the angular spacing -N.sub.w .multidot.SPHI, -(N.sub.w +1).multidot.SPHI,. .. (N.sub.w -1).multidot.SPHI, N.sub.w .multidot.SPHI from P are sought, SPH1 being a selected angle increment, and the value of the pixel P is replaced by the average of the 2N.sub.w +1 pixels, to obtain an image matrix WBILD; and e) subtracting the image matrix WBILD from the image matrix IBILD, a final image in which the ring artefacts are removed.