1. A medical image data processing method for determining output function data (S(k)), situated at grid points of a selected grid, from input function data (I.sub.c (r)), situated at grid points of a cartesian grid, the method comprising: determination of modified input function data (I.sub.c '(r)) by division of the input function data (I.sub.c (r)) by the Fourier transform (c(r)) of a convolution kernel (C(k)), determination of the Fourier transform (S.sub.c '(k)) of the modified input function data (I.sub.c '(r)), and determination of the output function data (S(k)) by convolution of the Fourier transform (S.sub.c '(k)) of the modified input function data with the convolution kernel (C(k)), wherein the convolution kernel (C(k)) is a weighting function for weighting the grid points of the cartesian grid in conformity with their distance from grid points of the selected grid.