Spatial encoding is achieved by superposition of a static non-uniform selection field providing a single field-free point (FFP) and high field strength in its vicinity. The FFP is steered through the object of interest by means of the aforementioned drive field. In this way, data for reconstructing the particle distribution can be acquired with a recording coil. Here, it is exploited that only particles in the direct neighborhood of the FFP contribute to the signal, whereas afar particles stay in saturation.
 B. Gleich, J. Weizenecker: Tomographic imaging using the nonlinear response of magnetic particles, Nature, 2005, 435, p. 1214-1217
 J. Weizenecker, B. Gleich, J. Rahmer, H. Dahnke, J. Borgert: Three-dimensional real-time in vivo magnetic particle imaging, Physics in Medicine and Biology, 2009, 54, p.L1-L10