Quantitative assessment of diffusion abnormalities in posterior reversibleencephalopathy syndrome

BACKGROUND AND PURPOSE: Previous studies have shown that lesions in posteri or reversible encephalopathy syndrome are often isointense on diffusion-wei ghted MR images. We hypothesized that 1) apparent diffusion coefficient (AD C) maps using various thresholds would show larger abnormalities in posteri or white matter (WM) and 2) isointense appearance of lesions on isotropic d iffusion-weighted images results from a balance of T2 prolongation effects and diffusibility effects. METHODS: T2-weighted MR images from 11 patients were reviewed. Hyperintense lesions were located in both anterior and posterior WM in eight patients a nd solely in posterior WM in three patients. The ADC maps were produced by use of ADC values greater than or equal to 3 SD and greater than or equal t o 10 SD above the mean value of normal WM. Lesions on diffusion-weighted im ages were classified as isointense or hypointense. ADC values within lesion s (ADC(L)) were compared with those of normal WM (ADC(N)), and compared for isointense lesions and hypointense lesions. RESULTS: The distribution of lesions with ADC values greater than or equal to 3 SD was essentially identical to that on T2-weighted images. Regions wi th ADC values greater than or equal to 10 SD were found in both anterior WM and posterior WM in two patients and solely in posterior WM in nine patien ts. On diffusion-weighted images, lesions appeared isointense in seven pati ents and hypointense in four patients. Mean ADC(L)/ADC(N) for all lesions w as 1.81; for hypointense lesions, 2.30. CONCLUSION. Vasogenic edema was more severe in posterior WM. Isointense les ions result from a balance of T2 effects and increased water diffusibility. Hypointense lesions have higher ADC values, which are not balanced by T2 e ffects.