SPECT of epidemic encephalitis B demonstrates brain lesions at the acute stage with hypertransfusion and, at the subacute and chronic stages, hypoperfusion.

The lesions of epidemic encephalitis B are located at the central nervous system, which are especially prominent at the cerebral cortex, basal ganglia, thalamus, and midbrain, followed by the lesions at the cerebellum, pons, and medulla oblongata. Autopsy shows diffuse lesions of meningoencephalitis at the gray and white matters of the cerebral hemisphere, basal ganglia, brain stem, cerebellum, and thalamus. Histology reveals brain tissue edema, congestion, capillary hemorrhage, perivascular lymphocytes infiltration, and perivascular brain tissue necrosis. MR imaging demonstrates abnormal signal at the thalamus, midbrain, cerebral cortex, cerebellum, pons, basal ganglia, and even spinal cord. Nearly all cases are demonstrated with lesions at the bilateral thalamus and midbrain, with or without lesions in other brain parts. But there is also a report about case of epidemic encephalitis B with lesions only at the substantia nigra (black matter). The lesions are mostly demonstrated with low or equal T₁WI signal and high T₂WI signal. Occasionally, hemorrhage is demonstrated at the thalamus and basal ganglia. Contrast MR imaging demonstrates enhancement of the meninges, while usually no abnormal or slight enhancement of lesions at the brain parenchyma. DWI has higher sensitivity to changes in the acute stage than other radiological examinations, although its sensitivity decreases along with prolonged course of the disease. Both T₂WI and FLAIR have higher sensitivity in the acute and convalescence stages, especially at the convalescence stage (Fig. 14.2).