ASL MRI and 18F-FDG-PET in autoimmune limbic encephalitis: clues from two paradigmatic cases

Autoimmune limbic encephalitis (LE) is a neurological condition characterized by seizures and cognitive dysfunction. Fluorine-18 fluorodeoxyglucose (18F-FDG-PET) has recently proved to be an important diagnostic tool in this condition since it may highlight brain metabolism abnormalities in a very early stage of the disease. Two main 18F-FDG-PET patterns have been described: the mixed hypermetabolic/hypometabolic and the neurodegenerative one. Arterial spin labeling (ASL) is an MRI technique showing brain perfusion, rarely used in autoimmune neurological conditions. The aim of the present study was to study patients with LE with both techniques, in order to compare their results. Two patients with LE underwent to 18F-FDG-PET and ASL MRI scans using the pseudo-continuous arterial spin labeling (PCASL) technique. Areas of altered perfusion and metabolism were analyzed by visual inspection, and findings were compared between the two techniques. In the first patient, a relapsing LGI-1 LE, right hippocampal hypermetabolism was detected by 18F-FDG-PET (mixed hypermetabolic/hypometabolic pattern), while ASL MRI showed right hippocampal increased perfusion. In the second patient, a seronegative LE, 18F-FDG-PET scan detected a left hemispheric hypoperfusion (neurodegenerative pattern) and ASL MRI yielded similar results. The two 18F-FDG-PET patterns of altered metabolism were similarly detected by ASL imaging. ASL and 18F-FDG-PET findings are strongly concordant in LE. ASL imaging was able to detect the two main 18F-FDG-PET patterns previously described in patients with LE.