Abstract: Terahertz (THz) frequency range provides information which are generally absent in microwave and optical images, so there are unique advantages with THz imaging for applications, both in science and beyond, such as military, security and safety screening, biological and medical analysis, non-contact materials testing, etc. However, compared with visible light, X-ray, infrared, laser, ultrasonic, etc, THz imaging is the latest development in the imaging field. Moreover, the terahertz electromagnetic scattering model and terahertz inverse scattering imaging method are mostly from well-studied fields: the Geometrical Theory of Diffraction (GTD) and Inverse Synthesis Aperture Radar (ISAR), Born/Rytov Approximation and Computer Tomography (CT), for example. Recent studies show that the mechanism of THz inverse scattering imaging cannot be accurately described by radar or tomography respectively. It may be said that there is a terahertz gap in inverse scattering imaging. In this paper, the relationship between radar imaging and tomography are briefly presented. The radar images have a response in which a few bright points dominate the image, because of isotropic point scattering mechanism. In contrast, tomography contains significant diffuse scattering components, providing fill to the images. In order to get more of a diffuse-like image that is more readily recognized and interpreted by humans, the improved method based on Range-Doppler(RD) algorithm is investigated. Finally, the 0.14 THz radar initial results are presented.