  We study photoproduction off nuclei based on a self consistent and covariant
many body approach for the pion and isobar propagation in infinite nuclear
matter. For the first time the t-channel exchange of an in-medium pion is
evaluated in the presence of vertex correction effects consistently. In
particular the interference pattern with the s-channel in-medium nucleon and
isobar exchange contribution is considered. Electromagnetic gauge invariance is
kept as a consequence of various Ward identities obeyed by the computation.
Adjusting the set of Migdal parameters to the data set we predict an attractive
mass shift for the isobar of about 50 MeV at nuclear saturation density.
