Brain imaging with 123I radionuclide remains essential to assess the dopamine transporter activity or cerebral blood flow in various cerebral disorders. However, imaging with 123I-labeled tracers suffers from down-scatter contaminations from the emission of a series of high-energy (>183 keV, ~3% abundance) gamma photons in addition to the primary photons (159 keV, 83% abundance). In this work, we investigated through simulation studies the effect of down-scatter contamination on image quality using multiple pinhole configurations and aperture sizes of AdaptiSPECT-C, which is a next-generation multi-pinhole system currently under construction. We simulated a brain phantom with source distribution for the perfusion imaging agent 123I-IMP as imaged 1h post injection. To enable comparison of imaging without down-scatter interactions, reconstructions were compared qualitatively and quantitively to the ones obtained from acquisition of similar activity distribution simulated for solely the 159-keV principal emission of 123I. In this initial study, we demonstrated through quantification and visual inspection of cerebral perfusion reconstruction incorporating down-scatter correction that the inclusion of down-scatter counts does not hamper the imaging performance of AdaptiSPECT-C even for the pinhole combination the most contaminated by such interactions. We have initiated a comparison of these findings against the ones obtained from a dual-head system employing parallel-hole collimator for which acquisition is considerably more impacted by down-scatter interactions.