  We study the dynamic structure factor $S(\vec{q},\omega)$ of superfluid 4He
at zero temperature in the roton momentum region and beyond using
field-theoretical Green's function techniques. We start from the
Gavoret-Nozi\`{e}res two-particle propagator and introduce the concept of
quasiparticles. We treat the residual (weak) interaction between quasiparticles
as being local in coordinate space and weakly energy dependent. Our
quasiparticle model explicitly incorporates the Bose-Einstein condensate. A
complete formula for the dynamic susceptibility, which is related to $S
(\vec{q},\omega)$, is derived. The structure factor is numerically calculated
in a self-consistent way in the special case of a momentum independent
interaction between quasiparticles. Results are compared with experiment and
other theoretical approaches.
