DWI and FLAIR imaging in herpes simplex encephalitis: a comparative and topographical analysis.

DWI has been described in some reports to be superior to FLAIR in early stage herpes simple virus encephalitis (HSE). Few data exist on detailed topographical MRI analysis in HSE. Our aim was to study DWI and FLAIR, and analyse topographically these sequences in non-neonatal HSE patients with MRI performed within 60 days. Eleven HSE patients were analysed retrospectively. For topographical analysis, we developed a radiological 50-point score (25 points for each hemisphere, with each point corresponding to a brain area). In patients with MRI performed within 2 weeks (n = 9), DWI detected 11% more areas involved than FLAIR. Thalamic involvement was frequent (67%) in the early phase on FLAIR, being the only brain substructure better visualized on FLAIR than on DWI. In areas involved on both sequences, DWI showed more extensive (especially cortical) abnormalities in 14% of the areas. In patients with late MRI (n = 2), FLAIR was superior to DWI (with essentially white matter involvement). From the mesial temporal area, brain signal changes followed a centripetal (i.e. towards anterior, posterior, and superior parts of the brain) gradient. The cut-off score before involving the contralateral hemisphere was 8-9/25 in the initially involved hemisphere. DWI is slightly superior to FLAIR in acute-subacute HSE, except for the thalamus with FLAIR signal changes more frequently seen than earlier reported. Knowledge of typical topographical MRI involvement can help to differentiate from other conditions mimicking HSE.