Prospective cohort study evaluating FDG PET in 56 patients with confirmed autoimmune encephalitis - based on 2016 Graus criteria, and 2021 paraneoplastic neurological syndromes criteria - at the acute phase, before immunomodulating treatment, or within 10 days of treatment initiation.

Autoimmune encephalitis is a diagnostic challenge, and requires early diagnosis for improved neurological outcomes. FDG PET has shown very high sensitivity, suggesting better performances than MRI, but almost exclusively in small sized retrospective studies. Brain FDG PET is therefore not included in current diagnostic criteria, conversely to brain MRI. This study will include 56 patients with confirmed - seropositive or seronegative - autoimmune encephalitis, based on 2016 Graus criteria for autoimmune encephalitis and 2021 criteria for paraneoplastic neurological syndromes. The main objective is to conduct a prospective evaluation of the diagnostic value of FDG PET performed in the acute phase before treatment initiation, or within 10 days of treatment initiation. A follow-up PET performed 3 months after treatment initiation will also be performed and analysed for all patients as part of secondary objective analyses.

Inclusion Criteria:

• Age ≥ 18 years old

• Newly diagnosed autoimmune encephalitis based on at least 1 of the 3 following criteria :

  1. " Definite limbic autoimmune encephalitis " according to 2016 Graus et al. criteria
  2. " Possible autoimmune encephalitis " according to 2016 Graus et al. criteria AND typical autoantibody detected in serum or CSF
  3. " Probable or certain paraneoplastic neurological syndrome " according to Graus et al. 2021 criteria (excluding peripheral neurological syndromes)

• Less than 6 months since first neurological symptoms imputable to autoimmune encephalitis

• Affiliated or entitled to a social security system (except AME)

• Obtaining free, written and informed consent (patient or legal representative or the close relative)

Exclusion criteria

• History of brain tumor, head trauma, infarction or cerebral hematoma likely to result in altered cerebral carbohydrate metabolism on PET
• Patients who hae been on immunotherapy (corticosteroid bolus, IVIg, plasma exchange, endoxan, rituximab or other immunotherapy) fr more than 10 days
• Pregnant or breast-feeding woman
• Ventilated intubated patient
• Absolute contraindication to MRI (Pacemaker, cochlear implant, etc.)
• Presence of cognitive disorders incompatible with goog cooperation with the PET scan
• Algic or agitated patient unable to remain immobile in supine position for 30 minutes
• Deprived of liberty or under a protective measure (guardianship or curatorship)
• Patient taking part in other interventional research involving radiopharmaceutical injections

The procedures carried out with the French data privacy authority (CNIL, Commission nationale de l'informatique et des libertés) do not provide for the transmission of the database, nor do the information and consent documents signed by the patients. Consultation by the editorial board or interested researchers of individual participant data that underlie the results reported in the article after deidentification may nevertheless be considered, subject to prior determination of the terms and conditions of such consultation and in respect for compliance with the applicable regulations.

Beginning 3 months and ending 3 years following article publication. Requests out of these time frame can also be submitted to the sponsor

Researchers who provide a methodologically sound proposal.