  We demonstrate a method of imaging spatially varying magnetic fields using a
thin layer of nitrogen-vacancy (NV) centers at the surface of a diamond chip.
Fluorescence emitted by the two-dimensional NV ensemble is detected by a CCD
array, from which a vector magnetic field pattern is reconstructed. As a
demonstration, AC current is passed through wires placed on the diamond chip
surface, and the resulting AC magnetic field patterns are imaged using an
echo-based technique with sub-micron resolution over a 140 \mu m x 140 \mu m
field of view, giving single-pixel sensitivity ~100 nT/\sqrt{Hz}. We discuss
ongoing efforts to further improve sensitivity and potential bioimaging
applications such as real-time imaging of activity in functional, cultured
networks of neurons.
