We calculate dilepton pair production in pion-nucleon and pion-nucleus  collisions. We derive the elementary cross sections in an effective field theory approach. We use these cross sections in a transport model to study $\pi$-nucleus reactions.  We investigate especially what is the effect of the of the interference term (which is destructive in $\pi^+$n and constructive in $\pi^-$p collisions) of the $\rho$ and $\omega$ mesons on the dilepton spectra in $\pi^+$A and $\pi^-$A collisions. Our preliminary results predict a substantial, experimentally measurable change of the cross sections ratios of $\pi^+$ and $\pi^-$ induced reactions when we go from nucleon targets to heavy ones. This would open the possibility to study decoherence in strongly interacting matter.