Based on the successful feasibility demonstration in Phase I, Physical Optics Corporation (POC) proposes to continue the development of a novel Structural Integrity Inspection and Visualization System (SIRIUS), which addresses NASA's need for compact nondestructive evaluation (NDE) of the structural integrity of spacecraft components and structures. SIRIUS is based on one-sided three-dimensional (3D) structure imaging via collecting the information on density profiles in multiple object cross sections through hard X-ray Compton-scattered imaging. The SIRIUS Phase I prototype system demonstrated excellent potential for 3D localization of defects in various aerospace materials and structures, such as thermal protection system (TPS) ceramic foam tiles, micrometeoroid and orbital debris (MMOD) shielding, spacecraft pressure walls, inflatable habitat structures, composite overwrapped pressure vehicles (COPV), aluminum-rubber composites, and metal honeycomb materials. In Phase II, POC will develop a compact, standalone version of the SIRIUS system with improved performance (in terms of resolution and data acquisition speed), as well as an advanced graphical user interface (GUI) for straightforward data acquisition and visualization. At the end of Phase II, POC will perform a TRL-6 demonstration of SIRIUS at the NASA facilities. SIRIUS will provide NASA with an effective NDE tool for both in-space and on-ground applications.