I. Introduction
Today, there are various kinds of techniques for the non-destructive and non-contact inspection of the objects by using different electro-magnetic waves, such as microwaves, infrared lights, and X-rays. Recently, terahertz (THz) imaging technologies, which use frequency bands from 100 GHz to 10 THz, have been widely applied for medical, pharmaceutical, security and other industrial applications as well as in material science [1], [2]. In particular, similar to the computed tomography (CT) using infrared lights and X-rays, three-dimensional (3D) THz imaging or THz tomographic imaging has attracted a great deal of interest, since the 3D structural information is very powerful to inspect and characterize the object more accurately [2]–[4].