  This paper aims at providing a small-volume expansion framework for the
scattering resonances of an open cavity perturbed by small particles. The
induced shift of the scattering frequencies by the small particles is derived
without neglecting the radiation effect. The formula holds for arbitrary-shaped
particles. It shows a strong enhancement in the frequency shift in the case of
plasmonic particles. The formula is used to image small particles located near
the boundary of an open resonator which admits whispering-gallery modes.
Numerical examples of interest for applications are presented.
