We propose and evaluate the performance of an improved preclinical positron emission tomography (PET) scanner design, referred to as Polaroid-PET, consisting of a detector equipped with a layer of horizontal Polaroid to filter scintillation photons with vertical polarization. This makes it possible to improve the spatial resolution of PET scanners based on monolithic crystals. First, a detector module based on a lutetium-yttrium orthosilicate monolithic crystal with 10 mm thickness and silicon photomultipliers (SiPMs) was implemented in the GEANT4 Monte Carlo toolkit. Subsequently, a layer of Polaroid was inserted between the crystal and the SiPMs. Two preclinical PET scanners based on ten detector modules with and without Polaroid were simulated. The performance of the proposed detector modules and corresponding PET scanner for the two configurations (with and without Polaroid) was assessed using standard performance parameters, including spatial resolution, sensitivity, optical photon ratio detected for positioning, and image quality. The detector module fitted with Polaroid led to higher spatial resolution (1.05 mm FWHM) in comparison with a detector without Polaroid (1.30 mm FHWM) for a point source located at the center of the detector module. From 100% of optical photons produced in the scintillator crystal, 65% and 66% were used for positioning in the detectors without and with Polaroid, respectively. Polaroid-PET resulted in higher axial spatial resolution (0.83 mm FWHM) compared to the scanner without Polaroid (1.01 mm FWHM) for a point source at the center of the field of view (CFOV). The absolute sensitivity at the CFOV was 4.37% and 4.31% for regular and Polaroid-PET, respectively. Planar images of a grid phantom demonstrated the potential of the detector with a Polaroid in distinguishing point sources located at close distances. Our results indicated that Polaroid-PET may improve spatial resolution by filtering the reflected optical photons according to their polarization state, while retaining the high sensitivity expected with monolithic crystal detector blocks.