MRI windows aren't leaded, that would do nothing for RF or magnetic interference. MRI windows contain copper mesh and are designed to integrate with the rest of the Faraday cage to both keep the RF generated by the machine in the room, and keep external radiation away from the sensors. Also keep the acoustic noise in the room.

This also doesn't do anything against the (static) magnetic field, which is really hard to block except with material like steel, which don't make very good windows. Newer machines have a counter-magnet to redirect the field to extend less far from the machine.