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This work was part of the feasibility study of performing simultaneous PET and SPECT imaging with a microPET scanner. SPECT data were acquired with slit-slat collimation by inserting a slit-slat collimator into microPET's detector ring but were contaminated by 511 keV photons' down- scattered events from PET tracer. In this study, contribution of down-scattered events in Tc-99m energy window (135-145 keV) was characterized through Monte Carlo simulation and measurements. With relative ratio of Tc-99m and F-18 activity being 1:1, ratio of recorded events from Tc-99m and F-18 source was 1:18.31. Nevertheless, simulation results revealed that the majority of down-scattering effects occurred in the detector crystals and thus were less object dependent. We investigated two subtraction-based compensation methods. The first method used an auxiliary energy window (AEW) and the second method employed extra shielding measurement (ESM) to acquire down- scatter- only data. Simulated and experimental studies showed improved image quality after applying the correction methods. With the activity ratio of SPECT tracer to PET tracer being 5:1, good SPECT images were obtained after correction, which were comparable to those from pure SPECT imaging.