  It is shown that the study of the imaginary part and of the corresponding
dispersion relations of Feynman graph amplitudes within the differential
equations method can provide a powerful tool for the solution of the equations,
especially in the massive case. The main features of the approach are
illustrated by discussing the simple cases of the 1-loop self-mass and of a
particular vertex amplitude, and then used for the evaluation of the two-loop
massive sunrise and the QED kite graph (the problem studied by Sabry in 1962),
up to first order in the (d-4) expansion.
