95% energy for brain is obtained from glucose metabolism and is directly related to neuronal activity. This characteristic is basis of utility of 18F-FDG PET-CT in dementia, movement disorders and tumors.
In a normal 18F-FDG brain PET, homogenous symmetrical distribution is noted in brain with grey matter having 2.5-4 fold higher activity than white matter. Visual Cortex, Frontal eye fields and posterior cingulate cortices have higher uptake than other cortices whereas mesial temporal lobe has lower uptake as compared to other cortices. Basal ganglia show higher activity as compared to Neocortex.
Alzheimer’s Dementia, Mesial temporal sclerosis and CJD lead to hypometabolism in involved cortices.
Autopsy findings in Autoimmune Encephalitis have revealed Perivascular T cell infiltrates in messiah temporal lobes and basal ganglia. These can lead to higher FDG uptake in areas of active inflammation.
Diagnosis of Autoimmune encephalitis (AIE) is established when these 4 Criteria are met: 1. Subacute onset (rapid progression in <3 months) of memory deficits, seizures or psychiatric symptoms suggesting limbic system involvement. 2. Bilateral brain abnormalities on T2-W FLAIR MRI highly restricted to medial temporal lobes. 3. CSF pleocytosis (WBC >5 cells/mm3) or EEG with epileptic / slow wave activity involving temporal lobes. 4. Reasonable exclusion of alternate causes. Detection of antibodies can establish the diagnosis if one of first 3 criteria are not met. 18F-FDG PET-CT can be used to fulfil the second criterion.
Common reported 18F-FDG PET findings in AIE include Temporal, Orbito-frontal and basal ganglia hypermetabolism. Hypometabolism in Parietal and Occipital cortices has also been reported. Likely cause of hypometabolism are direct blockade of receptors or ion channels by autoantibodies which may lead to a decrease in neuronal and synaptic activity. 18F-FDG PET has been shown to be more sensitive and can detect inflammation even in cases with normal MRI.