Multi-modality imaging in clinical and pre-clinical settings promises to forge new vistas for better diagnostic interpretation compared to stand-alone imaging systems. For in vivo imaging studies, accurate interpretation of diseased conditions is possible if structural, functional and metabolic information of the region of interest is provided at the same time. At Brookhaven National Laboratory, a small animal positron emission tomograph (PET) detector was integrated into a Bruker Biospin 9.4 T microMRI scanner to perform simultaneous PET/MRI studies in rodents. The custom-built MRI coils were optimized for simultaneous PET/MRI studies. The feasibility of acquiring simultaneous PET/MRI data has been demonstrated that can provide structural soft-tissue contrast information from MRI complementing with physiological and metabolic information from PET at the same time. However, the technical challenges involved in integrating PET inside the high field MRI scanner have been predominant, because of the mutual electromagnetic interactions between the two systems that may compromise the overall performance of either system. The main aim of the dissertation is to identify and evaluate the electromagnetic interactions between the PET and MRI systems. The impact of radiofrequency pulses on the sensitive PET electronics was investigated by employing different shield configurations around the PET detector housing. In addition, the assessment of MR image quality was performed using phantoms in the presence of powered PET detector covered with different shields. With thin layers of copper shielding and good grounding practices, it has been shown that the effect of radio frequency pulses on PET readout electronics was minimized and good quality MR images were acquired. In vivo simultaneous PET/MRI data of a rat brain and a mouse heart were acquired with different PET radiotracers that illustrate one of the potential applications for performing simultaneous PET/MRI studies.