68-Ga-RM2 PET/MRI in Imaging Patients With Estrogen Receptor-Positive Breast Cancer

Inclusion Criteria: * ER+ breast cancer at initial diagnosis prior to surgery or at recurrence (at least one ER+ lesion based on results of biopsy). * Able to provide written consent * Karnofsky performance status of ≥ 50 (or Eastern Cooperative Oncology Group (ECOG)/World Health Organization (WHO) equivalent) Exclusion Criteria: * Less than 18 years-old at the time of radiotracer administration * Inability to lie still for the entire imaging time * Inability to complete the needed investigational and standard-of-care imaging examinations due to other reasons (severe claustrophobia, radiation phobia, etc.) * Any additional medical condition, serious intercurrent illness, or other extenuating circumstance that, in the opinion of the Investigator, may significantly interfere with study compliance * Renal function impairment preventing administration of MRI contrast * Metallic implants (contraindicated for MRI)