To address NASA's need for advanced nondestructive evaluation (NDE) of complex built-up spacecraft structures, Physical Optics Corporation (POC) proposes to develop a new Penetrating Backscatter X-ray Imaging (PRAXI) system for in situ single-sided, three-dimensional (3D) NDE of the integrity of spacecraft components and structures. The PRAXI system is based on a novel approach for 3D Compton-based structural imaging, which requires only a small number of images for 3D data reconstruction. These new features enable PRAXI to achieve 10x faster operating speed, smaller form factor, and smaller weight, compared to Compton imaging tomography (CIT), previously developed by POC. The proposed PRAXI system will allow noncontact, single-sided inspection of various spacecraft structures (such as micrometeoroid and orbital debris (MMOD) shields, pressure vessels, inflatable habitats, and thermal protection systems), either for in-space NDE or for on-ground material development and quality control. In Phase I POC will demonstrate the feasibility of using the PRAXI system for NDE of spacecraft components by fabricating and testing a TRL-4 prototype, with the goal of achieving technology readiness level (TRL)-6 by the end of Phase II and delivering the prototype to NASA.