Bio-magnetic imaging systems can be used to locate simple sources of magnetic fields in the human body. In this paper we consider characterization of bio-magnetic imaging systems. We formulate a generalization of the point spread function (PSF) and optical transfer function (OTF) for use in this characterization. The generalized PSF and OTF are vector quantities derived using two components factors: the Fourier transformed Maxwell equations the estimator used in converting magnetic fields in electric-current densities An example is chosen from co-planar imaging systems, i.e., systems where the magnetic field measurements and electric currents are all located in parallel planes. Imaging ambiguities of these systems are examined in terms of imaging artifacts.