Objective: To perform a comprehensive physical-level assessment of 13 contrast agents, including those with potential applications in CT imaging, focusing on their radiation shielding characteristics and transport behaviors—such as energy deposition, collision frequency, and attenuation performance—under low-energy X-ray conditions. Methods: A dual-method framework was adopted. Geant4 Monte Carlo simulations were used to construct an X-ray tube model and simulate contrast agent interactions in a breast-equivalent water phantom, enabling analysis of microscopic radiation transport parameters including energy deposition, track length, and collision frequency. In parallel, Phy-X/PSD software was used to calculate macroscopic attenuation indices, including the linear attenuation coefficient (LAC), mass attenuation coefficient (MAC), mean free path (MFP), half-value layer (HVL), and exposure buildup factor (EBF), over a wide photon energy range. Results: The study revealed a strong consistency between radiation shielding metrics and transport characteristics across the same energy ranges. For instance, iothalamate meglumine exhibited the highest energy deposition (0.085 60 ​MeV), shortest MFP (1.13 ​cm), and highest collision frequency (5.24 ​× ​108), indicating excellent attenuation potential in the low-energy CT range. Gadolinium- and iron-based agents, while traditionally used in MR imaging, showed distinctive and stable transport behavior at medium-to-high energies, suggesting promising utility in CT or dual-modality applications. Conclusions: These findings highlight the importance of integrating microscopic transport analysis with macroscopic shielding evaluation to fully characterize contrast agent performance. The study provides a validated theoretical foundation for contrast agent screening and optimization in X-ray imaging, and supports future research into clinical applicability and biological safety of emerging contrast materials.