Abstract : This talk aims at introducing a new theoretical multi-frequency method to reconstruct width defects in waveguides and plates. Using cut-off frequencies of the structure, we can provide a stable inverse method that proves very robust to noised data. I will start by describing this method in the simplest case of a scalar waveguide. Then, I will provide theoretical tools to extend this method to elastic plates using Lamb modes. I will then focus on ZGV frequencies and show numerical reconstructions of width defects obtained with this method.