  The multiple passages of light through an absorption inhomogeneity of finite
size deep within a turbid medium is analyzed for optical imaging using the
``self-energy'' diagram. The nonlinear correction becomes more important for an
inhomogeneity of a larger size and with greater contrast in absorption with
respect to the host background. The nonlinear correction factor agrees well
with that from Monte Carlo simulations for CW light. The correction is about
$50%-75%$ in near infrared for an absorption inhomogeneity with the typical
optical properties found in tissues and of size of five times the transport
mean free path.
