  Medium modification of scattering properties in interacting Bose systems are
considered by solving the Bethe-Salpeter equation. An equation of state for the
normal phase (generalized Beth-Uhlenbeck formula) is given using the in-medium
phase shifts to include two-particle correlations. Conclusions are drawn for
systems of bosonic atoms with repulsive interaction such as sodium $^{23}$Na
and rubidium $^{87}$Rb. It is shown that the in-medium scattering length and
the absolute value of the in-medium scattering phase shift for low scattering
energies increase with density.
