Abstract: The monitoring, detection, and precise localization of radioactive sources are of paramount importance for safeguarding nuclear safety, public health, and national security. In scenarios involving a lost or leaking radioactive source, using a gamma camera to rapidly and accurately pinpoint the source’s location can minimize harm to both the public and search personnel. To enhance the omnidirectional angular resolution of gamma-ray imaging systems, this research innovatively proposes a 4π omnidirectional gamma imaging system that incorporates an externally mounted, staggered shielding structure. The integration of this staggered shield was designed to fundamentally augment the system’s spatial resolution by amplifying the disparity in energy deposition counts between adjacent spatial pixels. To achieve a full spatial field-of-view imaging capability, it is essential to proactively generate the detector system’s response matrix by simulating its response for every pixel within the imaging space. A Fibonacci network sampling strategy was employed to distribute 1 500 pixel points in an approximately uniform pattern across the spherical space. For each spatial pixel location, the Geant4 Monte Carlo simulation toolkit was used to model the energy deposition of a mono-energetic 662 keV parallel gamma-ray beam (from a ¹³⁷Cs source) within the detector. This process enabled the construction of a comprehensive system response matrix. Based on this matrix, the maximum likelihood expectation maximization (MLEM) algorithm was applied to iteratively reconstruct the distribution of radioactive sources within the imaging space. When evaluating the imaging performance of far-field nuclear radiation detector systems, angular resolution, localization accuracy, and imaging sensitivity are critical performance metrics. This study adopted two standard definitions for angular resolution: the full width at half maximum (FWHM) derived from a single point source image and the minimum separation angle required to distinguish two point sources. Accordingly, both single and double point source tests were conducted to calculate the respective angular resolutions. To assess the impact of the staggered shield on the system’s imaging performance, a comparative analysis was performed between two configurations: one equipped with the external shield and one without. The imaging sensitivity of the shielded configuration was evaluated by analyzing dual-source imaging results and the corresponding localization accuracy across varying imaging durations. The findings demonstrate that while the introduction of the shielding structure reduces the system sensitivity, it substantially improves the response contrast between pixels, consequently leading to a remarkable enhancement in angular resolution. Without the external shield, the gamma-ray imaging system struggles to differentiate the signals from two distinct sources. In contrast, when equipped with the shield, the system can clearly resolve the positions of the dual sources. Imaging experiments with a single ¹³⁷Cs point source reveal that the shielded gamma-ray imaging system achieves an angular resolution of less than 10°, with a localization accuracy of better than 6°. In dual point source tests, the unshielded system fails to separate the two sources due to insufficient resolution. However, with the shield, the imaging quality improves significantly, enabling clear distinction between two ¹³⁷Cs point sources separated by an angular distance of 20° (corresponding to an actual angle of approximately 25°). The localization accuracy for the reconstructed sources in this dual-source scenario is better than 9°. Furthermore, the dual-source resolution capability of the shielded gamma-ray imaging system strengthens with increased imaging time, achieving clear resolution of the two radioactive source positions after approximately five minutes. These collective results conclusively prove that the proposed design modification of incorporating a staggered shield significantly enhances the system’s overall imaging performance.