A three-dimensional image-generating device is an inspection system incorporating three components: a radiation source, a modulating unit, and a radiation detector. All three components of the inspection system may be stationary. The radiation source irradiates an external inspected object with mono-energetic gamma rays. The modulating unit, used in conjunction with the unique energy-angle characteristics of the Compton scattering process, by encoding the gamma rays, enables the identification of the spatial origin of single-scattered gamma fluxes as they pass through the inspected object enroute to the detector. The radiation detector and the computerized processor identify gamma fluxes scattered from various locations within the inspected object. The three-dimensional distribution of scattering locations within the inspected object that produce the detected single-scattered gamma fluxes and their intensity is converted to a three-dimensional mass distribution as an image within the inspected object.