  The relativistic structure of the self-energy of a nucleon in nuclear matter
is investigated including the imaginary and real components which arise from
the terms of first and second order in the NN interaction. A parameterized form
of Brueckner $G$ matrix is used for the NN interaction. The effects of the
terms beyond the DBHF approximation on quasiparticle energies and the optical
potential for nucleon-nucleus scattering are discussed.
