Many of the modern systems for terahertz vision use the active location method, when an object is illuminated with a strong terahertz laser pulse, and either reflected or transmitted signal is analyzed. Passive systems which would sense terahertz radiation emitted by the object itself are practically absent. One of the main reasons for that is relatively poor sensitivity of detectors of terahertz radiation that are used. At the same time, using this sort of terahertz photodetecting systems would definitely open new possibilities in many fields including medical applications, security and many other areas. Estimates show that NEP of 10-15 W/Hz1/2 is enough for the passive terahertz photodetecting systems. In this paper, we report on a possibility for construction of such a system based on a doped narrowgap semiconductor – Pb1-xSnxTe(In). Doping of certain lead telluride-based alloys with some of the group III impurities results in appearance of strong and unusual effects that are not characteristic for undoped materials. These effects include the Fermi level pinning and persistent photoconductivity under the action of terahertz radiation at low temperatures. Appearance of these effects is due to unusual impurity states appearing in lead telluride-based narrow-gap semiconductors upon doping. The ground impurity state provides pinning of the Fermi level in a definite position of the energy spectrum depending only on the alloy composition. Metastable excited local states are responsible for appearance of persistent photoconductivity under the action of weak terahertz radiation and many other effects. We have performed direct experiments using terahertz laser excitation of Pb0.75Sn0.25Te(In). The experiment results have demonstrated that the metastable impurity states are not linked to any definite location in the energy spectrum, but to the quasiFermi level position which may be tuned by photoexcitation. The binding energy of these states may be less than 3 meV providing the red cut-off wavelength of the photoresponse to be more than 500 μm. One of the ways to increase the signal-to-noise ratio of photodetectors is integration of the incident signal. The persistent photoconductivity effect provides “internal” integration of the incident signal since the photoexcited free electrons are long-lived at low temperatures. This advantage is valuable only if there exist a way to quench quickly the persistent photoconductivity. In the case of Pb1xSnxTe(In), this quenching may be reached by application of strong radiofrequency pulses to sample contacts, the photoexcited free electrons may be localized for less than 1 μs. Therefore it is possible to operate in the regime of periodical accumulation and successive fast quenching of the photoresponse. Using this technique, we have measured the Pb0.75Sn0.25Te(In) performance at the frequency of 350 μm. Preliminary tests have demonstrated that the NEP value may be as low as 3*10-17 W/Hz1/2 at T = 4.2 K. However a number of other experiments are needed to confirm this figure. Nevertheless the estimated NEP value is more than enough for construction of a system for passive terahertz vision of room-temperature objects. We report on the architecture of a respective device using a single photodetector operating at 5.5 K in the regime of scanning optics. A frame of 256*256 pixels is expected to be formed for 40 seconds.