**5. Conclusions**

This study compared 18F-FDG PET/CT and additional neck PET/MR scans head to head to analyze the detection ability and diagnostic efficacy of recurrent/residual DTC lesions and cervical lymph node metastases, as well as the characteristics of each parameter. The results showed that 18F-FDG PET/MR was more accurate in diagnosing recurrent and metastatic lesions, both from a patient-based and a lesion-based perspective. PET/MR detected more lesions and presented better image conspicuity and diagnostic performance due to the high soft tissue resolution and multi-sequence imaging available with MRI. The addition of neck PET/MR after whole-body PET/CT may be recommended to provide more precise diagnostic information and scope of surgical resection without additional ionizing radiation. Further prospective studies with expanded samples and economic benefit analysis are needed to support the conclusions.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/ 10.3390/cancers13143436/s1, Figure S1: SUVs, diameters of malignant and benign lymph nodes detected by both PET/MR and PET/CT, Figure S2: Bland–Altman analysis of SUVmax, SUVmean, and diameter measurements provided in two modalities, Figure S3: Correlation of SUVs and lymph node diameters measured on PET/MR and PET/CT.

**Author Contributions:** X.L., W.C. and F.L. designed the study; Y.S., F.L. and X.L. reviewed all the images, obtained the data, designed the statistical analysis, and finished the interpretation of the patient cohort; T.H., Z.G. and J.M. recruited the patients and provided the clinical data; W.R. and F.H. performed PET/CT and PET/MR examination; Y.S. wrote the manuscript; X.L., M.H.Y. and W.C. revised the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Key Project of National Nature Science Foundation of China (No. 82030052 and 81630049), Key Project of Hubei Province Technical Innovation (No. 2017ACA182), and University of Wisconsin–Madison.

**Institutional Review Board Statement:** The study was conducted according to the guidelines of the Declaration of Helsinki, and approved by the Institutional Review Board (or Ethics Committee) of Union Hospital, Tongji Medical College, Huazhong University of Science and Technology (2018-48).

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

**Data Availability Statement:** The data presented in this study are available in this article (and supplementary material).

**Conflicts of Interest:** Weibo Cai is a scientific advisor, stockholder, and grantee of Focus-X Therapeutics, Inc. All other authors declare no conflict of interest.
