Non Destructive Testing (NDT) has become the most used technique to inspect objects in order to find manufacturing defects (quality control), investigate contents (custom control), detect damages (structural control). However, the mono-energetic NDT can only discover a density variation in the analyzed sample, but in most cases no hypothesis can be done on its composition; a complete inspection of an object would require the recognition of the materials composing the analyzed sample, and this can be achieved by means of the dual energy x-ray radiography. In this context, the INFN - Gruppo Collegato di Messina is planning to implement the radio-tomographic system of the Dipartimento di Fisica, Universita di Messina, based on a 5 MeV electron linac, to the aim to provide dual energy x-ray beams for material recognition purposes. Preliminary caculations have been performed to evaluate the different electron energies obtainable acting on the linac parameters. Moreover, according to a theoretical study on the x-ray transmission for two chosen x-ray energy spectra, the material recognition should be possible by means of the developed radio-tomographic device.