  Several newly developing hybrid imaging methods (e.g., those combining
electrical impedance or optical imaging with acoustics) enable one to obtain
some auxiliary interior information (usually some combination of the electrical
conductivity and the current) about the parameters of the tissues. This
information, in turn, happens to stabilize the exponentially unstable and thus
low resolution optical and electrical impedance tomography.
  Various known instances of this effect have been studied individually. We
show that there is a simple general technique (covering all known cases) that
shows what kind of interior data stabilizes the reconstruction, and why.
Namely, we show when the linearized problem becomes elliptic
pseudo-differential one, and thus stable. Stability here is meant as the
problem being Fredholm, so the local uniqueness is not shown and probably does
not hold in such generality.
