Diffusion-weighted imaging and apparent diffusion coefficient evaluation of herpes simplex encephalitis and Japanese encephalitis.

PURPOSE: The aim of the study was to evaluate (a) the role of diffusion-weighted imaging (DWI) and apparent diffusion coefficient (ADC) values in differentiating necrotising herpes simplex encephalitis (HSE) and non-necrotising Japanese encephalitis (JE) and (b) to correlate the ADC values with the duration of illness.

MATERIALS AND METHODS: Forty-five confirmed cases of encephalitis (38 patients with JE and 7 patients with HSE) underwent MR imaging. IgM antibody capture enzyme-linked immunosorbent assay (IgM MAC-ELISA) and polymerase chain reaction (PCR) tests were performed in cerebral spinal fluid (CSF) sample to confirm the diagnosis of JE and HSE respectively. MRI findings were recorded in terms of site of involvement, extent of lesions, visibility of each lesion on T2W, DWI and FLAIR sequences and ADC calculations. To observe the changes in ADC with duration of illness, patients with JE and HSE were regrouped on the basis of time since clinical presentation. Mean of the ADC value in each patient was noted and subjected for statistical analysis.

RESULTS: In HSE lesions there was a significant restricted diffusion with low average ADC values observed in acute stage and facilitated diffusion with high average ADC values observed in chronic stage. Whereas JE lesions did not show restricted diffusion and significant low ADC values in acute stage, though facilitated diffusion and high ADC values were observed in chronic stage.

CONCLUSION: The diffusion abnormality and conspicuity of lesions on DWI may be different in various acute encephalitis (HSE and JE). The ADC values are different in the acute stages of HSE and JE reflecting the difference in the degree of diffusability of water molecule. These observations may suggest that there may be an abundance of cytotoxic oedema in HSE and paucity of cytotoxic oedema in JE, in acute stage.