Indications
18F-fluorodeoxyglucose-PET/MR (18F-FDG-PET/MR) imaging is mainly applied for pediatric lymphomas [8,15,19] and sarcomas [14] at staging, restaging, and during long-term follow-ups [9]. Indeed, it allows an accurate assessment of the primary tumor and of eventual distant metastases in sarcomas (Fig. 1) as well as a precise investigation of all nodal and extra-nodal sites in patients with lymphomas (Fig. 2). Regarding this latter disease, 18F-FDG-PET/MR is highly valuable in monitoring the response to treatment, especially when pathological tissue is still detectable on morphological images and its viability has to be assessed [10]. In fact, in such cases, the decreased metabolic uptake is the main indicator of the efficacy of the treatment [11,12,20] (Fig. 3).
In our experience, 18F-FDG-PET/MR demonstrated to be very useful also for other rare tumors like pediatric pheochromocytoma and pancreatoblastoma.
Protocols
One of the main challenges in examining pediatric patients by PET/MR is represented by the compliance of the children. In fact, especially in very young children, the exam has to be performed under general anesthesia. In our center, using our 3T fully integrated PET/MR system (Biograph mMR; Siemens, Erlangen, Germany), in addition to the sequences for the attenuation correction, our general pediatric protocol includes axial T2weighted-Half fourier acquisition (HASTE), axial or coronal Short Tau Inversion Recovery (STIR) (Fig. 4). Contrast-enhanced imaging is mainly applied to assess the primary lesion due to sarcomas (Fig. 5) and/or the metastatic spread in abdominal organs.
Diffusion Weighted Imaging (DWI) is usually part of our protocol as whole-body or localized scan, according also to time-constraints and patient’s compliance (Fig. 6).
Since sarcomas usually metastasizes in the lungs, the most up-to-date sequences for pulmonary imaging like the ultrashort echo time (UTE) are included in our protocols. Nevertheless, chest CT is always performed, as recommended by the guidelines, in all these patients (Fig. 7). Furthermore, for soft tissue sarcomas, a multiplanar proton density fat saturated sequence and an axial T1w Turbo Spin Echo are applied on the primary lesion.
Among the additional sequences, the T1w in-phase and out-of-phase should be considered for adrenal gland tumors or to evaluate thymic rebound after chemotherapy, while MR cholangiography might be useful for hepatic or pancreatic primary tumors.