Study participants will be consented prior to undergoing the PEM studies. The order in which the PEM and MRI studies are performed will NOT be randomized, and will be dictated by patient schedule. Co-registration of the PEM images from F-18 FDG and F-18 FES will be used to assess the heterogeneity of uptake of the F-18 FES relative to that of F-18 FDG. Heterogeneity of ER expression in the tumor will be determined by immunohistochemistry on the pathologic tissue and findings will then be correlated with the ratio of activity in the co-registered images of F-18 FES and F-18 FDG. An integrated interpretation of the MRI and PEM images will be performed to match lesions seen on the three scans and will be made available to the ordering physician and surgeon.