Polysuccinimide (PSI) grafted with methylpoly(ethylene glycol) (MPEG) was synthesized and employed to encapsulate pre-synthesized magnetite using a co-precipitation method. This encapsulation was stabilized by the chemical bonding between PSI and magnetite $via$ the introduction of a bridging compound, 3-aminopropylethoxysilane (APS). The morphology and properties of the magnetite-encapsulated polymer particles were widely characterized to investigate their applications as potential magnetic resonance (MR) imaging probes. The Fourier transform infrared and Fourier transform nuclear magnetic resonance spectroscopy confirmed the successful synthesis of the MPEG-$g$-PSI- and APS-coated magnetites in addition to X-ray diffraction of their crystalline structure. The size and distribution of the pure and polymer-coated magnetite aggregates were examined using transmission electron microscopy and light scattering analysis. The magnetite-encapsulated aggregates prepared in this study showed excellent superparamagnetic and biocompatible properties that enabled their successful use as MR imaging probes.