This chapter describes the working principles, current trends, and applications of magnetic resonance imaging (MRI). It explores functional MRI (fMRI)—its need, design, working principles, and the contrast agents used. The chapter also describes various properties and fabrication methodologies for nanoparticles along with their numerous advantages and applications. It argues how nanoparticles are implemented in the contrast agents used for fMRI and the ways in which the enhances imaging. The physical phenomenon of nuclear magnetic resonance is based on the principle that charged particles or certain nuclei possess a characteristic spin associated with their magnetic moment, which produces various effects on the application of an external magnetic field. The difference in energy levels of the parallel and antiparallel protons produces a net magnetization, which is oriented along the direction of the magnetic field. This net magnetization is the “vector sum” of the magnetic moments of the protons under the magnetic field applied.