The purpose of this study is to evaluate the accuracy of Positron emission tomography, Computed Tomography perfusion and 3 Tesla MRI in the radiological down-staging of rectal cancer following neoadjuvant chemoradiation. The restaging result obtained by these imaging studies will be compared to standard imaging studies and the histopathology of the surgical specimen. We hypothesize that these 3 imaging modalities will detect lymph node metastases pretreatment better than standard pretreatment scans and will better determine the degree of response of the primary rectal tumor to preoperative chemoradiation.