  Recent scanning tunneling experiments on $Ca_{2-x}Na_xCuO_2Cl_2$ by Hanaguri
et al. observe field-dependent quasiparticle interference effects which are
sensitive to the sign of the d-wave order parameter. Their analysis of spatial
fluctuations in the local density of states shows that there is a selective
enhancement of quasiparticle scattering events that preserve the gap sign, and
a selective depression of the quasiparticle scattering events that reverse the
gap sign. We introduce a model which accounts for this phenomenon as a
consequence of vortex pinning to impurities. Each pinned vortex embeds several
impurities in its core. The observations of recent experiments can be accounted
for by assuming that the scattering potentials of the impurities inside the
vortex cores acquire an additional resonant or Andreev scattering component,
both of which induce gap sign preserving scattering events.
