Terahertz imaging techniques via time-domain spectroscopy in reflection mode is typically based on the time-domain analysis of the peak-to-peak amplitude of the signal or its energy. In the frequency domain, one usually exploits the absorption frequencies of the materials employed. In this work, we show how these techniques fail in imaging (in a spectroscopic approach) a multilayered structure consisting of a metallic pattern embedded between two nominally identical lossy dielectric layers. For this purpose, alternative imaging techniques are here proposed by exploiting a simple ray-optics analysis and a Fabry-Perot interpretation of the electromagnetic interaction between the transmitted THz beam and the metal-dielectric structure.