We present an analytical perturbation analysis for studying the sensit
ivity of diffusive photon flux to the addition of a small spherical de
fect object in multiple-scattering media such as human tissues. As a f
irst simple application of our perturbation method, we derive analytic
ally the photon migration path distributions and the shapes of the so-
called banana regions in which the photon migration paths are concentr
ated. We then derive analytically the sensitivity of detected photon f
lux densities to the inclusion of small spherical defects in the multi
ple-scattering medium for both single-source and two-source configurat
ions, at both steady-state (dc) and frequency-modulation conditions, a
nd compare the results with Monte Carlo simulations.