The magnetic lead field provides a relation between the current density distribution in the patient and the measurement of a SQUID magnetometer. This approach can be derived in a new and simple way by a derivation based on energy concepts. Using the lead field and conservation of charge conditions provides two linear, spatially invariant imaging equations relating the current density and flux measurements made by a planar array of SQUID magnetometers. These equations, plus an assumption that the current density is confined to a plane, are solved using Fourier techniques. The validity of the resulting equations is shown both analytically and with a computer model. The effects of not satisfying the planar assumption are described.