Conventional single photon emission computed tomography (SPECT) collimators are not suitable for 511 keV imaging. In order to use a SPECT system for single positron emission tomography (SPET), we need to design high-efficiency collimators capable of absorbing 511 keV photons. Slit slat (SS) collimators have been proposed for this purpose even though the optimization of such collimators has not yet been performed. In order to investigate the reliability of the system with SS collimator, it was decided to evaluate such a system. A gamma camera system with both parallel hole (PH) and SS collimators was simulated by Monte Carlo method. In these simulation parameters of the simulated system having 3/8 in NaI(Tl) crystal for 140 keV gamma-rays with PH collimator was compared with practical results and a good correlation was found between the simulated and practical results. In this way, the validity of our simulation code was confirmed. The crystal thickness for simulated gamma-camera system with SS collimator for detection of 511 keV photons was set to be 5/8 in which resulted in an intrinsic spatial resolution of 4mm. The simulated SS collimators consisted of 50 lead plates of various height, thickness and spacing. The imaging was performed by rotating the SS collimator from 0 degrees to 180 degrees in 73 steps of 2.5 degrees each. The physical parameters such as spatial resolution, efficiency and modulation transfer function (MTF) of this system for different SS collimator designs, (i.e. SS(64,3,3), SS(80,3,3), SS(64,4,5), SS(80,4,5), SS(85,4,5), where in SS(h,s,t) is used to indicate the septal height (h), septal separation (s), and septal thickness (t) in millimeters were evaluated. The value of these parameters were compared with the values obtained employing a 511 keV PH(85,4,2.5) collimator. Based on MTF values, our result showed that the SS(85,4,5) collimator has an optimum performance to the other SS collimators. The relative efficiency of the system with SS(85,4,5) collimator was about 20 times higher than the efficiency of the system with the PH(85,4,2.5) collimator whereas in this case the spatial resolution had deteriorated by almost 8% (i.e. from 14.1 to 15.0 mm). The result of this study shows that the SS collimators have great advantages over the conventional PH collimators for 511 keV imaging, providing much higher efficiency while maintaining a comparable spatial resolution. Employing this collimator makes it possible to use a SPECT system for metabolic imaging by SPET system.