  Within the next-to-leading order (NLO) perturbative QCD (pQCD) parton model,
suppression of away-side hadron spectra associated with a high $p_{T}$ photon
due to parton energy loss is shown to provide a complete tomographic picture of
the dense matter formed in high-energy heavy-ion collisions at RHIC. Dictated
by the shape of the $\gamma$-triggered jet spectrum in NLO pQCD, hadron spectra
at large $z_T=p_T^{h}/p_T^{\gamma} \stackrel{>}{\sim} 1$ are more susceptible
to parton energy loss and therefore are dominated by surface emission of
$\gamma$-triggered jets, whereas small $z_{T}$ hadrons mainly come from
fragmentation of jets with reduced energy from volume emission. These lead to
different centrality dependence of the hadron suppression in different regions
of $z_{T}$.
