18F-FDG-PET and MRI in autoimmune encephalitis: a systematic review of brain findings

In the diagnostic assessment of autoimmune encephalitis (AE) associated with antineural antibody (Ab) imaging with F-18-fluorodeoxyglucose (FDG), positron emission tomography (PET) was initially only used to screen for occult malignancies in paraneoplastic cases. Today accumulating evidence also supports the use of PET imaging for the objective assessment of metabolic changes in the brain of patients with AE. On the other hand, magnetic resonance imaging (MRI) of the brain reveals a variable picture depending on the specific syndrome and associated antibody, and may be normal in a sizable proportion of patients. Among all the articles retrieved, we selected only studies and case series describing F-18-PET/CT, or multimodal imaging assessments (with both PET/CT and MRI performed in the same patient) on at least five patients with AE confirmed by the presence of antineural Ab. Studies describing MRI but not PET/CT findings on at least 20 cases were also included to strengthen the morphological considerations. We summarize the findings in the literature, commenting on studies involving the use of FDG-PET/CT in patients with AE, focusing on the added value and unsolved issues of using FDG-PET as opposed to MRI, and discussing them in the context of the available diagnostic criteria for AE. We also describe neuroimaging (PET and MRI) differences between AE with Ab against surface antigens vs AE with Ab against intracellular antigens. The timing of neuroimaging after the onset of symptoms is also considered. From a systematic review of the literature, it seems that some specific metabolic patterns correlate with the presence of specific Ab, such as a cerebral posterior hypometabolism in anti-NMDAR encephalitis, and a mesiotemporal hypermetabolism (associated with hyperintensities and swollen structures on MRI T2) in encephalitis with LGI1 and onconeural Ab. To ascertain the prognostic value of FDG-PET and its role in driving therapy, larger (preferably longitudinal) studies are needed on age-matched, untreated patients with the same Ab status, who undergo imaging at a similar time after the onset of their symptoms. This would enable a systematic correlation between MRI and FDG-PET findings, and help to clarify a number of unsolved clinical and technical issues.