Breast cancer is the most common type of cancer in Norwegian women, with 4,224 new cases in 2022. More precise diagnosis are expected to result in more accurate assessment of treatment effect, and to contribute to both providing better treatment and reducing overtreatment. Fibroblast activating protein (FAP) is expressed in the tumor stroma of 90% of all epithelial-based tumors, including breast tumors. Inhibitors for this protein (FAPI) has been developed for use as radioactive tracers. Breast tumors of various histopathological types, and local metastatic lymph nodes, have shown high uptake for such tracers. The main aim of this project is to establish 18F-FAPI PET/MRI for use in breast cancer in a wide range of disease stages. We will evaluate how FAPI PET/MRI correlates with histopathological assessment, the method's ability to grade tumors - including assessment of treatment - compared to CT, scintigraphy and FDG PET/CT, and ability to detect local recurrence.