In brain tumor studies, SPION size can be optimized to passively target and accumulate in a brain tumor because the tumor is hypervascularized with leaky vessels while the blood-brain barrier blocks access to healthy brain tissue. Active tumor targeting can also be achieved via surface chemistry modifications or the use of magnetic fields. For example, it was shown that lactoferrin-conjugated nanoparticles can be used to target brain glioma cells in MPI.By means of a human glioblastoma mouse model, fluorescent magnetic nanoparticles could be magnetically retained in the neovasculature as well as tissue of the tumor, using a magnetic micromesh.