This pilot study will recruit up to 20 patients with newly diagnosed ER+ breast cancer who have recently undergone or are scheduled to undergo a clinically-indicated breast MRI examination and are scheduled for surgical excision of the breast cancer. All patients will undergo two PEM studies, one with F-18 FDG and the second with F-18 FES. As F-18 FES is not an FDA-approved radiopharmaceutical, this FES study will be performed under an investigator IND cross-referenced to the National Cancer Institute IND 79,005 for F-18 FES. A time interval of 6-8 hours will be required between performance of the F-18 FDG study and the F-18 FES study. All patients will have the 2 PEM studies within 30 business days of the clinically indicated MRI. See Study Schema shown below. Subjects must meet the following eligibility criteria: