  We investigate the source imaging for a granular pion-emitting source model
in high energy heavy ion collisions. The two-pion source functions of the
granular sources exhibit a two-tiered structure. Using a parametrized formula
of granular two-pion source function, we examine the two-tiered structure of
the source functions for the imaging data of Au+Au collisions at Alternating
Gradient Synchrotron (AGS) and Relativistic Heavy Ion Collider (RHIC). We find
that the imaging technique introduced by Brown and Danielewicz is suitable for
probing the granular structure of the sources. Our data-fitting results
indicate that there is not visible granularity for the sources at AGS energies.
However, the data for the RHIC collisions with the selections of $40 < {\rm
centrality} < 90%$ and $0.20<k_{\rm T}<0.36$ GeV/c are better described by the
model with granular emission than that of one Gaussian. The model with granular
source has more parameters than the simple Gaussian, hence can describe more
complicated shapes.
