  Grain growth in protoplanetary disks is the first step towards the formation
of the rocky cores of planets. Models predict that grains grow, migrate, and
fragment in the disk and predict varying dust properties as a function of
radius, age, and physical properties. High-angular resolution observations at
more than one (sub-)mm wavelength are the essential tool for constraining grain
growth and migration on the disk midplane. We developed a procedure to analyze
self-consistently multi wavelength (sub-)mm continuum interferometric
observations of protoplanetary disks to constrain the radial distribution of
dust properties. We apply this technique to existing multi frequency continuum
mm observations of the disk around CQ Tau, a A8 pre-main sequence star with a
well-studied disk. We demonstrate that our models can be used to simultaneously
constrain the disk and dust structure. In CQ Tau, the best-fitting model has a
radial dependence of the maximum grain size, which decreases from a few cm in
the inner disk (<40 AU) to a few mm at 80 AU. Nevertheless, the currently
available dataset does not allow us to exclude the possibility of a uniform
grain size distribution at a 3sigma level.
