The development of superconducting joining technology for reacted magnesium diboride (MgB₂) conductors remains a critical challenge for the advancement of cryogen-free MgB₂-based magnets for magnetic resonance imaging (MRI). Herein, the fabrication of superconducting joints using reacted carbon-doped multifilament MgB₂ wires for MRI magnets is reported. To achieve successful superconducting joints, the powder-in-mold method was employed, which involved tuning the filament protection mechanism, the powder compaction pressure, and the heat treatment condition. The fabricated joints demonstrated clear superconducting-to-normal transitions in self-field, with effective magnetic field screening up to 0.5 T at 20 K. To evaluate the interface between one of the MgB₂ filaments and the MgB₂ bulk within the joint, serial sectioning was conducted for the first time in this type of superconducting joint. The serial sectioning revealed space formation at the interface, potentially caused by the volume shrinkage associated with the MgB₂ formation or the combined effect of the volume shrinkage and the different thermal expansion coefficients of the MgB₂ bulk, the filament, the mold, and the sealing material. These findings are expected to be pivotal in developing MgB₂ superconducting joining technology for MRI magnet applications through interface engineering.