1. In computer tomography apparatus for determining a radiation attenuation distribution in a body which includes a radiation source, radiation source displacement means for irradiating a region of the body from a plurality of source positions, detection means for detecting attenuated radiation which has passed through the body in order to form groups of measurement data, filter means for determining groups of back-projection data from the groups of measurement data, back projection means which determine the attenuation distribution from the groups of back-projection data, and storage means for storing the attenuation distribution the improvement wherein the filter means comprise a combination of convolution means, and a recursive filter and a first adder which is connected to sum outputs from the convolution means and the recursive filter and has an output connected to the back-projection means.