18F-FDG Brain PET/CT Metabolic Imaging in Patients with Suspected Autoimmune Encephalitis (AE) in the Early Stage: Can the Procedure Play a Complementary Diagnostic Role?

Background/Objectives: Recent studies reported that 18F-Fluorodeoxyglucose (FDG) positron –emission tomography/computed tomography (PET/CT), even in comparison with other traditional methods, can play a role in diagnosing AE and supporting early treatment. In the present study, we further investigated whether 18F-FDG PET/CT may be a complementary diagnostic tool to conventional procedures in patients with acute symptoms of suspected AE in the early phase. Methods: Eleven consecutive patients with recent acute symptoms suggestive of AE were retrospectively enrolled and underwent brain PET/CT after receiving an intravenous injection of 3.7 MBq/kg of 18F-FDG. Results: PET/CT showed abnormal FDG uptake in 9/11 patients classified as AE, while it was negative in the remaining 2/11 cases with vascular lesions. Magnetic resonance imaging (MRI), conducted in only 10/11 cases—one patient was a pacemaker wearer—identified suspected AE areas in 3/10 cases, ischemic lesions in another 3/10, and nonspecific data in the remaining 4/10 cases. Cerebrospinal fluid (CSF) tests revealed autoantibody delayed occurrence only in three patients (anti-GAD65, anti-Ma2, and anti-LGI1). After first-line treatment, 3/9 patients showed clinical improvement. Another 3/9 patients experienced partial improvement but with recurrence and new AE brain areas identified by PET/CT, which also detected favorable responses to second-line treatment in 2/3 cases. The remaining 3/9 patients, who were not responsive to treatment, ultimately died. Conclusions: In this study, PET/CT was effective in early identification of AE and enabling rapid therapy, even with inconclusive MRI and persistently negative or delayed positive CSF tests. PET/CT may aid in evaluating treatment response and detecting relapse. Notably, a negative PET/CT was associated with AE absence.