  We present a theoretical analysis of the standing wave patterns in STM
images, which occur around surface point defects. We consider arbitrary
dispersion relations for the surface states and calculate the conductance for a
system containing a small-size tunnel contact and a surface impurity. We find
rigorous theoretical relations between the interference patterns in the
real-space STM images, their Fourier transforms and the Fermi contours of
two-dimensional electrons. We propose a new method for reconstructing Fermi
contours of surface electron states, directly from the real-space STM images
around isolated surface defects.
