**2. Conclusions and Perspectives**

Molecular imaging and multimodal imaging are current topics extensively investigated by researchers and scientists. Since each diagnostic modality presents advantages and drawbacks, no single technique is able to provide a comprehensive overview of morphological, functional, and metabolic processes underlying tumors. Thus, a deep analysis was conducted on hybrid imaging, and several multimodal scanners are now routinely used in clinical practice among which PET/CT and PET/MRI are the most popular. The complementary information simultaneously obtained by PET and MRI offers new insights into disease diagnosis and treatment. As an example, dynamic contrast-enhanced MRI and PET with perfusion tracers are used to assess the tumor perfusion. A dual CA in a single probe allows a really simultaneous acquisition, and the co-localization of the two CAs guarantees a temporal and spatial correlation of the two imaging modalities. NPs can be used in PET/MRI as CAs for cancer imaging and, in order to gather anatomical and pathological information, their features must be properly adjusted: size, shape, charge, coating, and multivalency. Nevertheless, active targeting opened new pathways through the possibility of NP accumulation at the pathological site and, therefore, quantification is possible also on low-sensitivity techniques such as MRI. The impact of this approach can also be huge in the theranostic field since cancer imaging, diagnosis, and characterization can be used to gather important information about drug release, efficient therapy, and monitoring of response to treatment. Ideal candidates for a specific treatment could be so individuated, and personalized medicine can offer better results and faster healing. Even though, in the last few years, a variety of multimodal probes were produced, only few of them are approved for clinical use. Many challenges must be solved to promote NP clinical translation for both diagnostic and theranostic purposes. A multidisciplinary approach is necessary in order to focus on diagnostic applications and understand biomolecular processes at the basis of several pathologies. We expect that prevention, early diagnosis, patient management, and treatment could be improved such that a single performance can provide a comprehensive examination from which all essential parameters can be derived. In this perspective, multimodality plays a key role, and NPs can display their potential.

**Author Contributions:** E.F., D.F., A.C.d.P., E.T., C.C., and M.A. wrote the manuscript with the support of P.A.N., M.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by "Ricerca Corrente" Grant from Italian Ministry of Health (IRCCS SDN). **Conflicts of Interest:** The authors declare no conflict of interest.
