The present disclosure relates to an insert system for performing positron emission tomography (PET) imaging. The insert system can be reversibly installed to an existing system, such that PET functionality can be introduced into the existing system without the need to significantly modify the existing system. The present disclosure also relates to a multi-modality imaging system capable for conducting both PET imaging and magnetic resonance imaging (MRI). The PET and MRI imaging can be performed simultaneously or sequentially, while the performance of neither imaging modality is compromised for the operation of the other imaging modality.