**3. Conclusions**

Imaging techniques have proven to be indispensable to the advancements in the field of TE and regenerative medicine. In vivo imaging and tracking methods provide vital information about different aspects of engineered tissue constructs post implantation. These features include the 3D geometrical microstructure; the interaction between biological molecules and the scaffold; and the

cellular behavior, interaction, and viability within the constructs. Acquiring this information is important to provide feedback to improve the design and fabrication of sca ffold systems for di fferent clinical applications, as well as to enhance the understanding of certain cellular processes for cell-based therapies. The individual techniques outlined in this study o ffer di fferent advantages for the in vivo monitoring of molecules and cells. Di fferent methods have distinct capabilities in tracking di fferent properties of 3D sca ffolds. These techniques also su ffer from distinct disadvantages which can limit their application in clinical trials. Thus, it is important for the researchers to choose the appropriate imaging modality for specific in vivo studies. The emergence of multimodal imaging has provided an alternative to overcome the shortcomings of the individual imaging techniques, thus enabling a more comprehensive visualization at di fferent levels. Furthermore, progresses in developing various contrast agents for di fferent imaging modalities have enhanced the imaging resolution as well as the ability to combine multifunctional contrast agents as both diagnostics and therapeutics. Finally, advanced imaging techniques can also be combined with new fabrication techniques, such as 3D bioprinting, thus allowing for patient-specific therapeutic applications.

**Author Contributions:** Conceptualization, V.S. and C.J.G.; Writing-Original Draft Preparation, C.J.G., M.L.T., A.S.T., and A.C.; Writing-Review & Editing, C.J.G., M.M., and V.S.; Project Administration, V.S.

**Funding:** This research was funded by the National Institute of Health (NIH) gran<sup>t</sup> number R00HL127295 and Emory University School of Medicine (Pediatric Research Alliance Pilot Grant and the Dean's Imagine, Innovate and Impact (I3) Research Award). Carmen J. Gil is supported by the National Science Foundation (NSF) Graduate Research Fellowship under Grant No. DGE-1650044.

**Conflicts of Interest:** The authors declare no conflict of interest.
