**1. Introduction**

Thyroid carcinoma is the most common endocrine malignant tumor worldwide, accounting for 2% of all cancers, after a two-fold increase over the last 25 years [1–3]. More than 90% of thyroid carcinomas are differentiated thyroid carcinoma (DTC), which includes papillary carcinoma (PTC) (85%) and follicular carcinoma (FTC) (12%). The prognosis of DTC is generally favorable after comprehensive treatment including surgery, radioactive iodine, and thyroid-stimulating hormone (TSH) suppression [4]. Nevertheless, up to 30% of patients may experience local recurrence and/or metastasis within several decades, which indicates a poor prognosis and a drop of the five-year survival rate from higher than 90% to 35–85% [5,6]. Postoperative recurrence appears most frequently (60–75%) in cervical lymph nodes (LNs) [7]. As a result, strict postoperative follow-up and advances in early detection are essential for a timely intervention in case of relapse and metastatic disease. Conventional 131I whole-body scan (131I-WBS), in association with periodic evaluation of serum thyroglobulin (Tg) and neck ultrasound, have been employed as the routine diagnostic procedure in the protocol of patients thyroidectomized for DTC [8,9]. However, 10–15% of follow-up DTC patients appear with abnormal thyroglobulin levels and negative findings on 131I-WBS [10–12]. In guidelines of different countries, the clinical indication of 18F-fluorodeoxyglucose positron emission tomography–computed tomography (18F-FDG PET/CT) has been widely accepted for postoperative DTC patients who present with the aforementioned discordant findings, as well as for the systemic assessment of patients with suspected metastases [8–10,13–15]. Nevertheless, according to previous reports, the sensitivity and specificity of 18F-FDG PET/CT in detecting DTC recurrence or metastasis are 46–100% and 66–100% respectively, which is considered inadequate [16,17].

Magnetic resonance imaging (MRI) is a useful technique for the diagnosis of thyroid nodules and metastatic cervical lymph nodes, which benefits from excellent soft-tissue contrast, superior spatial resolution, and the ability to functionally characterize tissues by utilizing non-contrast- or contrast-enhanced techniques such as diffusion-weighted imaging (DWI) and apparent diffusion coefficient (ADC) [18–20]. The hybrid PET/MR is a promising imaging modality that combines the high soft tissue contrast of MRI with the high functional/metabolic sensitivity of PET without additional ionizing radiation. For head and neck oncologic imaging, it has the potential to achieve the highest level of diagnostic performance. However, to date, the usefulness of PET/MR in head and neck malignancy has not been fully elucidated [21–23]. It is questionable whether simultaneous PET/MR can provide better diagnostic ability than CT, MRI, and PET/CT in loco-regional recurrent and metastatic cervical lymph nodes [16,24–26].

The purpose of the present study was therefore to evaluate the diagnostic ability of simultaneous neck PET/MRI in a head-to-head comparison with PET/CT for the assessment of malignancy in postoperative differentiated thyroid carcinoma (DTC) patients.
