Inorganic nanocrystals, such as metallic gold nanocrystals and also quantum dots, are the most popular contrast agents. Regarding the performance of these agents, the most important parameters are long circulation time and stability under in vivo conditions. This could be achieved by encapsulating these agents in polymeric supports, as the polymeric core shields the probe from external conditions. Once the particles are coated with biodegradable polymers such as polyethylene glycol (PEG), the systemic clearance of the particles is greatly reduced, thus increasing their lifetime in the bloodstream. In short, the concept of nanocarriers could overcome the main obstacle to the use of contrast agents .