  We address the problem of pion production in low energy $\pi$-nucleus
collisions. For the production mechanism we assume a simple model consisting of
a coherent sum of single pion exchange and the excitation---followed by the
decay into two pions and a nucleon---of the $N^*(1440)$ resonance. The
production amplitude is modified by the final state interaction between the
pions calculated using the chirally improved J\"ulich meson exchange model
including the polarization of the nuclear medium by the pions. The model
reproduces well the experimentally observed $\pi A \to \pi\pi A$ cross
sections, especially the enhancement with increasing $A$ of the $\pi^+\pi^-$
mass distribution in the threshold region.
