The present invention is generally directed to core/shell nanoparticles, wherein such core/shell nanoparticles comprise a nanoparticle core and a nanoshell disposed about the nanoparticle core such that, in the aggregate, they form a core/shell nanoparticle that is operable for use as an imaging agent in X-ray/computed tomography (CT). Typically, such core/shell nanoparticle-based X-ray CT imaging agents further comprise a targeting species for targeting the imaging agent to diseased sites.