  This paper deals with the reconstruction of small-amplitude perturbations in
the electric properties (permittivity and conductivity) of a medium from
boundary measurements of the electric field at a fixed frequency. The
underlying model is the three-dimensional time-harmonic Maxwell equations in
the electric field. Sensitivity analysis with respect to the parameters is
performed, and explicit relations between the boundary measurements and the
characteristics of the perturbations are found from an appropriate integral
equation and extensive numerical simulations in 3D. The resulting non-iterative
algorithm allows to retrieve efficiently the centre and volume of the
perturbations in various situations from the simple sphere to a realistic model
of the human head.
