Pauli blocking in the nuclear mediumπNtransition matrix

We give an analytic expression for the in-medium nonstatic $\pi N$ transition matrix in nuclear matter. Pauli blocking and spreading effects are included explicitly. Spin-flip terms are retained. We discuss the modifications of the freespace amplitude as a function of energy and density and show that large dynamical isospin effects are expected in pion scattering on heavy nuclei.