## 1 Introduction  
In recent years, the terahertz (THz) range of the electromagnetic spectrum (0.1-10 THz) has become very attractive, due to its applications in basic science and technology [1, 2, 3, 4, 5, 6]. THz photons (1 THz is equivalent to 4 meV, 300 \(\upmu\)m and 33 cm\({}^{-1}\)) due to their non-ionizing nature, can be exploited for many nondestructive applications in biomedicine, metrology and quality control of industrial productions, spectroscopic investigations, environmental monitoring, art conservation, particle-accelerator physics [7, 8, 9, 10, 11, 12, 13, 14, 15], etc. In addition, THz waves can propagate through inflammable liquids without causing combustion, resulting in their being suitable for security and gas sensing purposes [16, 17, 18, 19]. Moreover, many materials exhibit their spectral fingerprints in this frequency range. THz radiation is very sensitive to polar molecules like water whilst remaining rather transparent to non-polar molecules like plastics, ceramics, paper, cloths, fats, etc. These distinct optical responses are often the basis for both materials' characterization [20] and spectroscopic imaging [7]. In particular, THz time-domain spectroscopy (THz-TDS) is based on the synchronous and coherent detection of a THz pulse: both its amplitude and phase are recorded as a function of time, allowing the measurement of sample optical properties, such as the refractive index and the absorption coefficient, without the need of Kramers-Kronig relations. Actually, THz radiation can be produced and detected from a variety of designed THz emitters and receivers, both in continuous wave (CW) and in pulsed regime. AmongTHz pulsed radiation, some schemes use photoconductive antennas (PCAs) both for THz emission and detection [21; 22], some others exploit THz production using inorganic and organic crystals [23; 24], or by plasma in air [25] and electro-optic sampling (EOS) [26] for detection, etc. Despite a vast variety of applications, THz imaging is a challenging research field. THz imaging has the ability to look into and through transparent objects such as many material packages like PET, for food inspection; or can be used for tumor margins delineation both on in-vivo and ex-vivo biological tissues [7]. As THz imaging is rather new field, theories and mathematical models [27] for assessing THz imaging systems [28] are not completely matured yet. [PERSON] and coauthors [27; 29; 30] mathematically modeled transmission point spread function (PSF) for wideband THz imaging systems, using it to simulate and enhance the resolution of the THz imaging systems. Some efforts have been made to characterize the THz beam emitted from PCA [31], crystals [32] and plasma in air [33; 34], especially using THz cameras and microbolometric arrays. Conversely, 3D beam profile of THz radiation emitted from a PCA, along its propagation axis, without the using of THz cameras or profilers is lacking. As all imaging systems, also THz-based ones are limited in their performances by various factors, such as spatial resolution, contrast and noise. They, in turn, are affected by THz source, system configuration, measurements setting and data acquisition scheme. In particular, THz imaging poses a challenge to the spatial resolution that can be achieved. This is strictly related to the diffraction limit and to the PSF dependence on the wavelength [35]. Despite some algorithm and optics-based procedures for enhancing the resolution being proposed, the image quality assessment is necessary for the comparison of different imaging systems, particularly for emerging THz ones [36; 37; 38; 39; 40]. This paper presents a study of the performances of our home-made THz pulsed imaging (TPI) system, in terms of PSF, spatial resolution and image formation. In addition, the 3D broadband THz beam spatial distribution, emitted from a PCA, is characterized and visualized. In the first section of the paper, we discuss our home-made imaging system, based on a THz-TDS scheme.