**3.2 MRI**

MRI is a powerful noninvasive imaging modality that is utilized widely in clinical medicine. MRI is based on the property that hydrogen protons will align and process around an external alternating magnetic field. The subsequent process through which these protons return to their original state is referred to as the relaxation phenomenon. Two independent processes, longitudinal relaxation (T1-recovery) and transverse relaxation (T2-decay), are monitored to generate the MR image. Local variations in relaxation, corresponding to image contrast, arise from the proton density and the chemical and physical natures of the different tissues. Due to their ability to enhance proton relaxation and accumulation in specific tissues, MFMNPs have been actively investigated as contrast enhancement agents for MRI.
