Study of 18F-FFNP Breast PET/MRI

Inclusion Criteria: * Postmenopausal status defined by either * prior bilateral oophorectomy * age greater than or equal to 60 years of age * age less than 60 years of age and amenorrheic for 12 or more months in the absence of prior chemotherapy, tamoxifen, toremifene or ovarian suppression and FSH and estradiol in the postmenopausal range per local normal range (Group 2 only) * Diagnosis of biopsy-proven invasive breast cancer measuring at least 1.0 cm in diameter by any imaging modality. Malignancy may be located within the breast, axilla (e.g. metastatic axillary lymph node), or both the breast and axilla * Biopsy-proven PR-positive invasive breast cancer * Definitive surgical excision of the primary tumor planned without neoadjuvant therapy; defined as therapy (chemotherapy, targeted therapy, radiation therapy or endocrine therapy) given to decrease the size of the tumor prior to planned surgery. (Group 2 only) Exclusion Criteria: * Inability or unwillingness to provide informed consent to the study * HER2-positive breast cancer, as defined by immunohistochemical staining 3+ OR positive by in situ hybridization (Group 2 only) * PR and Ki67 IHC slides or FFPE tissue blocks from clinical breast biopsy not available * Patients who have completed neoadjuvant chemotherapy, endocrine therapy, targeted therapy, surgical resection, or radiation for the current biopsy-proven malignancy (Group 2 only) * Patients who are planning to undergo anastrozole as standard of care neoadjuvant therapy * Patients who are currently taking aromatase inhibitors or ER antagonists (tamoxifen, raloxifene) * Patients with breast expanders * Patients who are pregnant or lactating * Patients with clinical contraindication for use of aromatase inhibitors (AI) while on study as determined by investigator (Group 2 only) * Patients with a contraindication to gadolinium-based contrast agents, including allergy or impaired renal function (per UW Health Guidelines) * Patients with a history of allergic reaction attributable to compounds of similar chemical or biologic composition to 18F-FFNP * Patients with history of allergic reaction to anastrozole (Group 2 only) * Patients in liver failure as judged by the patient's physician * Patients with standard contraindications to MRI (per UW Health Guidelines) * Patients requiring conscious sedation for imaging are not eligible; patients requiring mild, oral anxiolytics for the clinical MRI scan will be allowed to participate as long as the following criteria are met: * The patient has their own prescription for the medication * The informed consent process is conducted prior to the self-administration of the medication. * The patient comes to the research visit with a driver. * Patients unable to lie prone for 45 minutes for imaging