TSPO and myeloid sarcoma: Future studies with distinct cellular markers able to distinguish the brain resident microglia from monocyte-derived macrophages or further characterization of the cellular source of in vivo TSPO-PET and SPIO-MRI signals will be instrumental to better understand the specific contribution of each of these innovative methods to the in vivo detection and monitoring of the spatiotemporal changes of these cells and their role in initiating and promoting disease progression and degeneration in view of new MS therapeutic strategies.