Monitoring Early Response to Targeted Therapy in Stage IV HER2+ Breast Cancer Patients With Advanced PET/MR Imaging

Inclusion Criteria: * Patients must be ≥ 18 years old and ≤ 75 years old * Patients with HER2+ metastatic breast cancer * HER2-positive breast cancer prospectively determined on the primary tumor by a local pathology laboratory and defined as immunohistochemistry (IHC) score of 3+ and/or positive by in situ hybridization (ISH) (defined by ISH ratio of ≥ 2.0 for the number of HER2 gene copies to the number of chromosome 17 copies). Only one positive result is required for eligibility * Estrogen/progesterone receptor positive OR negative disease allowed * Patients must have measurable disease in one metastatic lesion per RECIST v 1.1 * Stage IV HER2+ breast cancer patients eligible for a new therapeutic regimen that includes HER2-targeted treatment who are naïve to that regimen * Estimated life expectancy of greater than six months Exclusion Criteria: * Children, less than 18 years of age, will be excluded from this study * Metastatic breast cancer patients who are HER2 positive and have already started their current HER2-targeted therapy regimen for metastatic disease * Patients who have not recovered from grade 2 or higher toxicities of prior therapy to the point that they would be appropriate for re-dosing will be ineligible for study treatment * Patient is pregnant or breastfeeding, or expecting to conceive children within the projected duration of the study, starting with the screening visit through 7 months after the last dose of study treatment * Patient is considered a poor medical risk due to a serious, uncontrolled medical disorder, nonmalignant systemic disease or active infection that requires systemic therapy. Specific examples include, but are not limited to, active, non-infectious pneumonitis; uncontrolled major seizure disorder; unstable spinal cord compression; superior vena cava syndrome; or any psychiatric or substance abuse disorders that would interfere with cooperation with the requirements of the study (including obtaining informed consent). * Patients found to have constitutionally present non-magnetic resonance (MR) compatible ferromagnetic materials * Unable to lie still on the imaging table for one (1) hour * Inability to receive gadolinium-based contrast agent * Patients for whom a PET/MRI is technically not feasible (e.g. breast volume, obesity, \> body mass index (BMI) 36)