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Microcomputed tomography is an important technique for distinguishing the vascular network from tissues with similar X-ray attenuation. Here, we describe a composite of barium sulfate (BaSO) nanoparticles, calcium carbonate (CaCO) nanoparticles, and alginate that provides improved performance over microscale BaSO particles, which are currently used clinically as X-ray contrast agents. BaSO and CaCO nanoparticles were synthesized using a polyol method with tetraethylene glycol as solvent and capping agent.