Suspensions of iodinated polymer nanoparticles are evaluated as contrast agent for Computed Tomography (CT) and Spectral Photon Counting Computed Tomography (SPCCT). Iodine containing moieties are grafted to poly(vinyl alcohol) by means of a covalent ester bond up to high degree of substitution of 0.77 providing high iodine content of 71 wt%. Polymer nanoparticles of 150 nm diameter stabilized by the block copolymer poly(caprolactone)-b-poly(ethylene glycol) are highly stable in water and human serum. High coverage of nanoparticles by PEG chains in a dense brush conformation (0.30 molecules·nm-2) provides resistance against fast elimination by mononuclear phagocytes system. Iodine concentration is increased up to 100 mg(i)·mL-1 by a centrifugation/redispersion step, which sets radiopacity of the contrast agent in the right range for imaging cardiovascular system and biodistribution. SPCCT 'Material Decomposition' and 'K-edge reconstruction' methods allow accurate quantification of iodine, as well as specific discrimination of iodine and gadolinium in mixed phantom samples. Intravenous injection of iodinated polymer nanoparticles to rats provides a clear visualization of the cardiovascular system over several hours followed by progressive accumulation in liver and spleen. This material is a 'blood pool' contrast agent with very long residence time in the blood stream.