An overview of main results concerning THz detection related to plasma nonlinearities in nanometer field effect transistors is presented [1,8]. In particular the physical limits of the responsivity, speed and the dynamic range of these detectors are discussed. As a conclusion, we will present applications of the FET THz detectors for construction of focal plane arrays.  These arrays, together with in purpose developed diffractive 3D printed optics lead to construction of the demonstrators of the fast postal security imagers and nondestructive industrial quality control systems. We will show also first results of FET based imaging that uses for contrast not only usual THz radiation amplitude, but also the degree of its circular polarization - see Fig.1. The dark bands on the figure indicates no uniformity of distribution of glass fibers in the sample. The contrast is obtained by measurement of the degree of circular polarization of 300GHz radiation that is transmitted through the sample. This degree of polarization can be measured thanks to specific detection properties of plasma field effect transistors.