## 1 Introduction  
Over the past two decades, terahertz (THz) and gigahertz (GHz) technologies have grown in importance and visibility as emerging techniques to be applied in a variety of fields. This has been possible thanks to recent advances in combining optical and electronic methods, which have enabled the generation and detection of very high signal-to-noise ratio and high data-acquisition rates--in particular in the spectral range between 0.1 and 3 THz [1; 2; 3; 4; 5; 6], often referred to as the \"Terahertz gap\", because these frequencies lie in a region where two means of generation: electronic, for low frequencies, and optical, for high frequencies, reach their technological limits [7; 8]. For this reason, a small number of studies about the interactions between radiation and matter has been carried out at these frequencies, due to the difficulty in generating and detecting THz radiation. Experimental results from various research fields have suggested that spectra in the THz spectral region mainly depend on molecular and intermolecular behavior such as hydrogen bond and phonon absorptions [9], although not all the absorption bands occurring in the THz region are unambiguously assigned [10; 11]. However, as compared with IR spectroscopic techniques, THz-TD spectroscopy has the advantage that it measures the transient electric field, and not simply its intensity [8]. Coherent detection allows the direct determination of both the amplitude and the phase of each of the spectral components making up the pulse. This means that it is possible to compute the absorption coefficient and refractive index of the sample from the amplitude and phase. In this respect, THz spectroscopy provides a convenient method for determining the complex permittivity [12; 13]. In the GHz region considered in the present paper (20-150 GHz), the presence of inhomogeneity or material discontinuities in the subsurface can be detected non-invasively by recording the dielectric contrast in composite materials by means of a measurement of the resonance frequency (F maps) and quality factor (Q maps) [14; 15; 16]. The \"National Agency for New Technologies, Energy and Sustainable Economic Development\" (ENEA) has a long-term experience in powerful sources operating in this spectral range. Indeed, several free electron devices, ranging from conventional Free Electron Lasers (FELs) [17; 18; 19] to Cherenkov [20] and grating [21; 22] FELs, were developed at the ENEA Center of Frascati between the 1980's and the 1990's. Most of these sources were able to produce radiation in the sub-mm region using powers that exceeded the kW level. It was then natural to make an investigation as to the possibility of developing imaging devices in the THz region. The development of FEL sources and applications in the THz region continued for more than two decades [23; 24; 25] and led to the development of sources based upon innovative interaction schemes, also in the region between 0.4 and 0.8 THz. Despite all the aspects that have already been presented and discussed in several scientific papers and books, it is evident that more research work will have to be conducted in order to optimize the results obtained with THz-GHz techniques. This is why the authors have been involved in research projects between different international academic and museum-research groups already engaged in exploring new opportunities for using GHz and THz methodologies in the art conservation field.