The primary objective of this study is to determine the correlation between the distribution of F-18 FES within ER+ breast tumors as seen on Positron Emission Mammography (PEM) images of the breast, and the distribution of cells stained ER+ within the tumor by immunohistochemistry (IHC) measurements at surgical pathology. The secondary aim is to determine if the correlation (or lack of) between F-18 FES uptake and F-18 FDG uptake as imaged by PEM, is an accurate representation of the heterogeneity of ER expression in the tumor.