The homogeneity of the magnetic field can be affected as large ferromagnetic objects moving close to the scanner, "pulling" the magnetic field lines as they pass. Site design considerations such as proximity to lift shafts are important. Large vehicles on a road running under a hospital may also have to be considered.
This is the case in the Leeds General Infirmary, Leeds, UK. See map (note orange arrow marking location of MRI department). The effect on images in this case is not significant.