A combined magnetic resonance (MR) imaging and positron emission tomography (PET) system is provided. In one embodiment, a system is provided comprising a radiofrequency (RF) antenna and an RF screenspaced apart from one another to create an RF field reflux zone. The RF screen comprises a plurality of openings, and a plurality of a scintillator crystals are positioned in the plurality of openingsin the RF screen such that at least a portion of the plurality of scintillator crystals are positioned in the RF field reflux zone. Other embodiments are provided, and each of the embodiments described herein can be used alone or in combination with one another.