*Limitations*

The most important limitation of this study is the small number of subjects included in this pilot study. Moreover, the lack of use of consistent US system settings and the involvement of several examiners could have affected our results. Additionally, since only patients in the subgroup of normal-weight donors showed a strong and significant correlation between MeanLin and DTPA clearance, a broad usage of CEUS is not applicable. Future studies are needed to analyze confounders and correlations in a larger population to enable a broad and standardized usage of this method.

## **5. Conclusions**

This pilot study, for the first time, demonstrated a significant relationship between kidney perfusion in CEUS and kidney function in living kidney donors. However, its significance is limited to patients of normal weight and BMI, and the US settings appear to affect renal perfusion quantified by TIC measurements on CEUS. This pilot study provides important new insights and supports the role of CEUS in assessing whole organ perfusion. Future research should analyze the relationships and potential confounders identified here in a larger population.

**Author Contributions:** Conceptualization, N.E.-B., F.F. and T.F.; methodology, N.E.-B. and M.H.L.; software, N.E.-B.; validation, F.F., T.F. and B.H.; formal analysis, N.E.-B.; investigation, N.E.-B., M.H.L. and T.F.; data curation, N.E.-B., L.L. and F.F.; writing—original draft preparation, N.E.-B.; writing— review and editing, all authors; visualization, R.P.; supervision, F.F., T.F., M.H.L., T.S., K.K., A.S., K.B., K.M. and A.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** "The study was conducted in accordance with the Declaration of Helsinki, and approved by the Institutional Ethics Committee) of Charité Universitätsmedizin Berlin (protocol code EA1/406/16, 26.01.2017)".

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Acknowledgments:** The authors thank Bettina Herwig for language editing of the manuscript, Annette Aigner for statistical advice and Christian Greis (Bracco Imaging) for scientific advice.

**Conflicts of Interest:** The authors declare no conflict of interest. Markus H. Lerchbaumer reports having received consultancy honoraria from Siemens Healthineers. Thomas Fischer reports having received consultancy honoraria from Siemens Healthineers, Bracco Imaging and Canon Medical Imaging.
