Patent US 3,936,636

In radiological apparatus, edge values derived by monitoring the radiation passed through a body along many co-planar paths are subjected to a transformation process, such as a convolution integral or Fourier transform process, in order to evaluate the absorption or transmission coefficients of the elements of a matrix of elements notionally delineated, in the plane of the paths, in the body. The values are inversely transformed to synthesize further edge values, and corresponding ones of the first mentioned and further edge values are compared to derive edge value error signals. The edge value error signals are indicative of errors in the evaluation of said coefficients and they are subjected to the aforementioned transformation process to derive therefrom individual correction signals for the various elements of the matrix.