*3.1. Patient-Based Analysis*

Among the 37 patients, no malignant signs were found in 10 patients, while 24 patients had lymph node metastasis, and 3 patients presented coexisted recurrence with lymph node metastases. PET/MR correctly determined the disease status of 36 patients (97.3%), while PET/CT identified 31 cases (83.8%). Of the three coexisting cases, two were correctly identified by PET/MR and one was correctly identified by PET/CT. A recurrent focus was missed by both modalities. Of 10 negative patients, 3 presented pulmonary metastases on whole-body PET/CT, 4 were pathologically confirmed as negative, and the remaining 3 patients were found to have decreased Tg during follow-up. Figure 2 displays a patient with recurrence and lymph node metastasis, where PET/MR successfully identified the recurrent lesion in the thyroid region, but PET/CT missed it. Lymph node metastases in these patients were all successfully identified by both modalities. In the 24 patients with lymph node metastases detected, all were correctly diagnosed by PET/MR, and only 20 patients were recognized by PET/CT. Figure 3 shows a DTC patient with lymph node metastases, which were identified by PET/MR but missed by PET/CT.

**Figure 2.** A 17-year-old post-operative DTC patient underwent 131I treatment three times with persistently elevated Tg up to 371.0 ug/L. 131I-WBS showed scattered iodine uptake foci in the right paratracheal area and lung lobes. 18F-FDG PET/CT was performed for whole-body systemic evaluation. 18F-FDG PET/CT showed no significant abnormalities in the thyroid region ((**b**,**d**), red arrows); additional PET/MR of the neck showed a long T2 signal nodule in the right thyroid region with a mild metabolic increase SUVmax 1.7 ((**a**,**c**), red arrows). In combination with medical history and 131I examination, a residual/recurrent thyroid cancer was diagnosed. The patient was subsequently reoperated, and residual/recurrent thyroid cancer was confirmed by histological pathology.

**Figure 3.** A 36-year-old patient with Tg 39.1 μg/L and negative 131I-WBS after comprehensive treatment was admitted to 18F-FDG PET/CT and underwent additional neck PET/MR. PET/MR clearly presented the morphological and metabolic changes of lymph nodes in the VB region ((**b**,**d**), red arrows). PET/CT showed no significant changes in the corresponding lesions ((**a**,**e**), red arrows). After reoperation, 1 lymph node in the left VB, 8 lymph nodes in the left level IIA, 7 lymph nodes in the left level III, and 10 lymph nodes in the left level IV were taken for pathomorphological examination, and only the lymph node in the VB was pathologically diagnosed to be a metastatic lymph node (**c**).
