An advanced, dual-particle, imaging system is being developed for standoff, passive detection of special nuclear material. This system consists of three planes of detectors and will be capable of imaging photons and fast neutrons simultaneously. The first two planes of the system consist of EJ-309 liquid scintillators, while the third plane consists of NaI detectors. The EJ-309 detectors have excellent pulse shape discrimination capabilities, which allows the imaging system to distinguish incident photons from neutrons. Preliminary investigations of the dual-particle imager focused primarily on the use of simple-backprojection methods for image reconstruction. However, efforts have been made toward implementing the use of a maximum-likelihood expectation-maximization algorithm for image reconstruction. MCNPX-PoliMi has been used to simulate system response matrices for the dual-particle imager. Results have been promising for image reconstruction using measurement-specific system response matrices. However, field use requires the ability to detect and locate unknown sources in unknown shielding configurations. This has resulted in the development of an algorithm capable of generating a measurement-specific system response matrix on the fly.