  This paper concerns the reconstruction of the scattering coefficient in a
two-dimensional transport equation from angularly averaged measurements when
the probing source is isotropic and time-harmonic. This is a practical setting
in the medical imaging modality called Optical Tomography. As the modulation
frequency of the source increases, we show that the reconstruction of the
scattering coefficient improves. More precisely, as the frequency $\omega$
increases, we show that all frequencies of the scattering coefficient lower
than $b$ are reconstructed stably with an accuracy that improves as $\omega$
increases and $b$ decreases. The proofs are based on an analysis of the single
scattering singularities of the transport equation and on careful analyses of
oscillatory integrals by stationary phase arguments.
