Nanoparticles are frequently used as carriers for therapeutic drugs but can also be used as part of imaging compounds for various imaging modalities. In this talk, an overview will be given on the use of nanoparticles for imaging and theranostics. Most nanoparticles are characterized by either long blood half-lifes or by rapid uptake by the RES. In addition, they tend to selectively accumulate in tissues with high vessel leakiness due to EPR effects. Thus, they are favourably suited as drug carriers in oncology. The large surface of nanoparticles enables to generate different functionalities in the same probe and adding an imaging marker to such a therapeutic probe results in a theranostic agent that can be used for patient selection and for controlling probe accumulation. Another important indication for diagnostic nanoparticles is imaging of the RES as it is used clinically to detect tumors in liver and lymph nodes using iron oxide nanoparticles. Since nanoparticles are internalized strongly by cells they can also be used for cell labelling and in vivo cell tracking experiments. Alternatively, in the emerging field of tissue engineering nanoparticles can be used to label scaffolds in order to localize the transplants and to monitor their resorption and remodelling. For targeted imaging, however, except for very small renally cleared nanoparticles, nanoparticles are critical since their penetration into tissues is low and high background signal is generated due to EPR effects. If targeted nanoparticles are designed for intravascular imaging, however, one may consider designing them larger (up to micrometer size) in order to avoid the unspecific retention in the interstitial space.
In summary, although there are clear indications for the use of nanoparticles as diagnostic probes they are no magic bullets for imaging purposes and their use should be carefully considered taking into account alternative strategies such as the use of small molecules.