*Article* **Head-to-Head Comparison of Neck 18F-FDG PET/MR and PET/CT in the Diagnosis of Differentiated Thyroid Carcinoma Patients after Comprehensive Treatment**

**Yangmeihui Song 1,2,† , Fang Liu 1,2,†, Weiwei Ruan 1,2, Fan Hu 1,2, Muhsin H. Younis 3,4, Zairong Gao 1,2, Jie Ming 5, Tao Huang <sup>5</sup> , Weibo Cai 3,4 and Xiaoli Lan 1,2,\***


**Simple Summary:** The most advanced positron emission tomography–magnetic resonance (PET/MR) combines the high soft tissue contrast of MRI with the high functional/metabolic sensitivity of PET and has the potential to achieve the highest level of diagnostic performance for refractory malignancies in differentiated thyroid cancer (DTC) patients. The utility of PET/MR in the postoperative follow-up of DTC patients has been relatively ambiguous. This retrospective study compared 18Ffluorodeoxyglucose neck PET/MR with PET/CT head-to-head, in order to evaluate the diagnostic efficacy of PET/MR in assessment malignancy in DTC patients after comprehensive treatment. We determined that PET/MR presented better detection rates, image conspicuity, and sensitivity than PET/CT in recurrent DTC lesions and cervical lymph node metastases. The addition of neck PET/MR scan after whole-body PET/CT may provide more favorable diagnostic information.

**Abstract:** We explored the clinical value of 18F-FDG PET/MR in a head-to-head comparison with PET/CT in loco-regional recurrent and metastatic cervical lymph nodes of differentiated thyroid carcinoma (DTC) patients after comprehensive treatment. 18F-FDG PET/CT and neck PET/MR scans that were performed in DTC patients with suspected recurrence or cervical lymph node metastasis after comprehensive treatment were retrospectively analyzed. Detection rates, diagnostic efficacy, image conspicuity, and measured parameters were compared between 18F-FDG PET/CT and PET/MR. The gold standard was histopathological diagnosis or clinical and imaging follow-up results for more than 6 months. Among the 37 patients enrolled, no suspicious signs of tumor were found in 10 patients, 24 patients had lymph node metastasis, and 3 patients had both recurrence and lymph node metastases. A total of 130 lesions were analyzed, including 3 malignant and 6 benign thyroid nodules, as well as 74 malignant and 47 benign cervical lymph nodes. Compared with PET/CT, PET/MR presented better detection rates (91.5% vs. 80.8%), image conspicuity (2.74 ± 0.60 vs. 1.9 ± 0.50, *p* < 0.001, especially in complex level II), and sensitivity (80.5% vs. 61.0%). SUVmax differed in benign and malignant lymph nodes in both imaging modalities (*p* < 0.05). For the same lesion, the SUVmax, SUVmean, and diameters measured by PET/MR and PET/CT were consistent and had significant correlation. In conclusion, compared with 18F-FDG PET/CT, PET/MR was more accurate in determining recurrent and metastatic lesions, both from a patient-based and from a lesion-based perspective. Adding local 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 scaling-up prospective studies and economic benefit analysis are expected.

**Citation:** Song, Y.; Liu, F.; Ruan, W.; Hu, F.; Younis, M.H.; Gao, Z.; Ming, J.; Huang, T.; Cai, W.; Lan, X. Head-to-Head Comparison of Neck 18F-FDG PET/MR and PET/CT in the Diagnosis of Differentiated Thyroid Carcinoma Patients after Comprehensive Treatment. *Cancers* **2021**, *13*, 3436. https://doi.org/ 10.3390/cancers13143436

Academic Editors: Fabio Medas and Pier Francesco Alesina

Received: 17 May 2021 Accepted: 28 June 2021 Published: 9 July 2021

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**Keywords:** thyroid neoplasms; fluorodeoxyglucose F18 (18F-FDG); positron emission tomography (PET); magnetic resonance imaging (MRI); recurrence; neoplasm metastasis
