1. In a method for producing a cross sectional image of a body from the distribution of measured total absorption values of ionized rays which penetrate the body in the cross sectional plane and of which at least some rays intersect elements of the image, in which method the distribution of the attenuation coefficients in the body cross sectional plane is derived by a convolution method including the steps of: (a) correcting the measured values obtained by passing radiation through the body to take into account particular radiation in use to obtain a corrected measurement series; (b) convolving the corrected measurement series using a stored convolution kernel; and (c) re-projecting and additively superimposing the convolved measurement series, the improvement comprising the steps of; (d) permanently storing a plurality of convolution kernels; and (e) forming the convolution kernel to be used in the step of convolving by multiplying each of a plurality of the stored convolution kernels by a pre-selected constant and then performing an additive superimposition of the multiplication results.