Crossing and Radon tomography for generalized parton distributions

The crossing properties of the matrix elements of non-local operators, parameterized by generalized parton distribution, are considered. They are especially simple in terms of the double distributions which are common for the various kinematical regions. As a result, double distributions may be in principle extracted from the combined data in these regions by making use of the inverse Radon transform, known as a standard method in tomography. The ambiguities analogous to the ones for the vector potential in the two-dimensional magneto-statics are outlined. The possible generalizations are discussed.