Autoimmune Encephalitis other imaging findings
Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Julinka Auta Fernandes
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Overview
In autoimmune encephalitis, additional imaging may support the neurological assessment or identify an associated tumor. Brain fluorine-18 fluorodeoxyglucose positron emission tomography (18F-FDG PET) can be considered in selected diagnostically uncertain cases, including when magnetic resonance imaging (MRI) is normal or cannot be performed. An abnormal PET scan is not independently diagnostic of autoimmune encephalitis.[1]
Dedicated brain PET and whole-body PET answer different questions: the former assesses cerebral glucose metabolism, while the latter searches for an associated neoplasm.[2]
Key PET Findings in Autoimmune Encephalitis
Brain FDG-PET
In autoimmune encephalitis, brain FDG-PET may show regional hypometabolism, hypermetabolism, or a mixture of both. These describe relatively decreased or increased glucose utilization and do not directly demonstrate neuronal antibodies.
Reported patterns include:
- Temporal hypermetabolism in a limbic presentation.
- Cortical hypometabolism, sometimes involving several regions.
- Mixed cortical and subcortical metabolic abnormalities.
In a retrospective inpatient study, metabolic abnormalities were detected more often than abnormalities on the initial conventional investigations. Such findings support potential sensitivity, but do not establish diagnostic specificity in an unselected population with suspected encephalitis.[3]
N-methyl-D-aspartate receptor (NMDAR) antibody encephalitis can show a relative frontotemporal-to-occipital metabolic gradient, with anterior hypermetabolism and posterior hypometabolism. This pattern is not present in every patient and cannot identify the antibody on its own.[2]
Interpretation and Limitations
In autoimmune encephalitis, PET findings require interpretation alongside symptoms, seizure timing, medications, and other investigations. The 2024 Canadian consensus emphasizes that the specificity of brain PET abnormalities remains uncertain and that PET alone should not establish the diagnosis.[2]
Important technical and clinical considerations include:
- Blood glucose concentration and appropriate preparation before tracer administration.
- Seizures during or near the tracer-uptake period, which can alter regional metabolism.
- Sedatives and other medications that affect brain activity.
- Patient movement and image-processing artifacts.
The nuclear medicine team should document relevant medications, sedation timing, glucose measurements, and seizure information. These issues are addressed in the joint Society of Nuclear Medicine and Molecular Imaging and European Association of Nuclear Medicine brain FDG-PET procedure guideline.[5]
Abnormal tracer uptake is not specific for autoimmune inflammation: neuronal activity, inflammatory processes, and tumors can all affect glucose metabolism. A metabolic abnormality must therefore be interpreted in its clinical context.[5]
Whole-Body FDG-PET for Tumor Detection
In autoimmune encephalitis, whole-body FDG-PET may be considered when initial tumor investigations are unrevealing and suspicion remains high, particularly with intermediate- or high-risk antibody associations. Its selection should reflect the patient's phenotype and cancer risk rather than be automatic for every patient.[2]
PET has limited sensitivity for ovarian teratoma, so a negative whole-body study does not complete the tumor investigation in a patient with a strong teratoma-associated syndrome. The need for further or repeated screening remains individualized.[2]
Other Imaging Modalities
Single-Photon Emission Computed Tomography
In autoimmune encephalitis, single-photon emission computed tomography (SPECT) is not an established routine diagnostic test. A 2026 practical review of adult encephalitis describes SPECT as a research technique rather than routine clinical investigation. It should not be treated as a validated substitute for the standard encephalitis assessment.[6]
Mammography
In autoimmune encephalitis, mammography may be part of directed breast cancer investigation when appropriate, including after an unrevealing initial body screen. Selection depends on the patient's clinical and antibody-associated tumor risk and breast screening history. Neither an abnormal mammogram nor a breast tumor independently establishes the neurological diagnosis.[1]
References
- ↑ 1.0 1.1 Abboud H, Probasco JC, Irani S, et al. Autoimmune encephalitis: proposed best practice recommendations for diagnosis and acute management. Journal of Neurology, Neurosurgery & Psychiatry. 2021;92(7):757–768. doi:10.1136/jnnp-2020-325300. PMID:33649022.
- ↑ 2.0 2.1 2.2 2.3 2.4 Hahn C, Budhram A, Alikhani K, et al. Canadian Consensus Guidelines for the Diagnosis and Treatment of Autoimmune Encephalitis in Adults. Canadian Journal of Neurological Sciences. 2024;51(6):734–754. doi:10.1017/cjn.2024.16. PMID:38312020.
- ↑ Probasco JC, Solnes L, Nalluri A, et al. Abnormal brain metabolism on FDG-PET/CT is a common early finding in autoimmune encephalitis. Neurology: Neuroimmunology & Neuroinflammation. 2017;4(3):e352. doi:10.1212/NXI.0000000000000352. PMID:28567435.
- ↑ Stuckey S. Radiopaedia case rID: 30363. Radiopaedia.org.
- ↑ 5.0 5.1 Arbizu J, Morbelli S, Minoshima S, et al. SNMMI Procedure Standard/EANM Practice Guideline for Brain 18F-FDG PET Imaging, Version 2.0. Journal of Nuclear Medicine. 2025;66(Suppl 2):S45–S60. doi:10.2967/jnumed.124.268754. PMID:39419552.
- ↑ Suspected encephalitis in adults. Practical Neurology. 2026;26:426–439. doi:10.1136/pn-2024-004299. PMID:42055803.