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SPECT-CT confirmed metastasis in the left axillary SLNs of tumour mice. In vitro uptake of 68Ga-Sienna+ in macrophage cells (J774A.1) was significantly higher (12 ± 1%) than in cancer cells (2.0 ± 0.1% P < 0.001). 68Ga-Sienna+ was produced in high radiochemical purity (>93%) without the need for purification and was stable in vitro. Imaging results were confirmed by histology. 68Ga-Sienna+ was injected into front paws and followed by PET-MRI. Left axillary SLN metastasis was monitored by hNIS/SPECT-CT and compared to the healthy right axillary SLN. NSG-mice were inoculated with 3E.Δ.NT cells. In vitro cell uptake was compared between 3E.Δ.NT breast cancer cells, expressing the hNIS reporter gene, and macrophage cell lines (J774A.1 RAW264.7.GFP). Radiochemical stability was tested in human serum. A clinically approved SPIO for SLN localisation (Sienna+) was radiolabelled with 68Ga without a chelator. Here we evaluate this approach by synthesising a GMP-compatible 68Ga-SPIO ( 68Ga-Sienna+) followed by PET-MR imaging and histology studies in a metastatic breast cancer mouse model. In this context, radiolabelled SPIOs and PET-MRI could find applications to locate SLNs with high sensitivity at the whole-body level (using PET) and guide high-resolution MRI to evaluate their metastatic status. To improve SLNB, whole-body imaging could improve detection and potentially prevent unnecessary surgery by identifying healthy and metastatic SLNs. It involves excision and histology of sentinel lymph nodes (SLNs) and presents two main challenges: (i) sensitive whole-body localisation of SLNs, and (ii) lack of pre-operative knowledge of their metastatic status, resulting in a high number (>70%) of healthy SLN excisions. Sentinel lymph node biopsy (SLNB) is commonly performed in cancers that metastasise via the lymphatic system.