The development of magnetic iron oxide nanoparticles as magnetic resonance (MR) imaging agents for sensitive and accurate diagnosis through inorganic materials design and their synergistic integration with other imaging modalities is reviewed. Modulation of the size, composition, and shape allows magnetic nanoparticles to fit with their intended purposes, such as MRI imaging, thermal ablation, hyperthermia, combination therapy, and drug deliver. Specifically, magnetic nanoparticles that are synergistically combined with other useful materials have been explored as nanocarriers to regulate the delivery of drugs or genes into the target area as well as specific probes for a variety of imaging modalities, such as MRI, PET, ultrasound, and optics. Furthermore, when incorporated with magnetic fields, hybrid magnetic nanoparticles can preferentially migrate to the target area and induce thermal damage or control drug release, and can even precisely initiate cell signaling for therapeutic purpose. These external stimuli-responsive nanosystems have provided the clear advantages of controlled therapeutic performances in a remotely modulated fashion.
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