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. Included herein are methods for forming such agents, comprising forming an ensemble of core/shell nanoparticles, wherein the mean diameter of the ensemble of core/shell nanoparticles is selected so as to render the nanoparticles in the ensemble substantially clearable by a mammalian kidney.