  We derive a novel chiral power counting scheme for in-medium chiral
perturbation theory with explicit nucleonic and pionic degrees of freedom
coupled to external sources. It allows for a systematic expansion taking into
account local as well as pion-mediated inter-nucleon interactions. Based on
this power counting, one can identify classes of non-perturbative diagrams that
require a resummation. Within this scheme, the pion self-energy in asymmetric
nuclear matter is analyzed and calculated up-to-and-including next-to-leading
order (NLO). It is shown that the corrections involving in-medium
nucleon-nucleon interactions cancel between each other at NLO. As a result,
there are no corrections up to this order to the linear density approximation
for the in-medium pion self-energy.
