About 20 to 30% of patients treated for cancer will have brain metastases. These brain metastases are found more frequently in patients with lung cancer, breast cancer or melanoma. The prognosis of these patients is unfavorable but prolonged survival can be obtained with the local and systemic treatments currently available. Brain MRI is the gold standard for evaluating brain metastases but has limitations in therapeutic evaluation, partially offset by PET imaging of amino acid metabolism. Our work aims to compare the performance of PET-DOPA with standard MRI for the detection of brain metastases (≥ 5mm) in lung cancer, breast cancer and melanoma; and to characterize these lesions using dynamic acquisitions obtained with a digital PET camera with high spatial resolution. Having better knowledge of the metabolic characteristics of newly discovered brain metastases, the objective of subsequent studies will be to better assess the per- or post-therapeutic efficacy of radiotherapy and the various systemic therapies available (chemotherapy, targeted therapy, immunotherapy).