Co1-xMgxFe2O4 (x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0) nanoparticles embedded in amorphous SiO2 with particle sizes of approximately 4.5 nm were prepared using an original wet chemical method. We performed spin-echo magnetic resonance imaging measurements for Co1-xMgxFe2O4 nanoparticles using a 0.3-T MRI system. The particles exhibited a significant T2 shortening effect compared with that of agarose, depending on the composition. All particles exhibited effective relaxivity, R2, and a significant contrast was observed in the phantom image. Furthermore, thiol group was modified to enable particles to bind specifically to maleimide proteins. These particles are expected to be potential theranostic agents.