Acute multiple sclerosis lesion: conversion of restricted diffusion due to vasogenic edema.

It is widely accepted that acute demyelinating plaques in patients with multiple sclerosis (MS) demonstrate increased apparent diffusion coefficient (ADC) and increased diffusion weighted imaging (DWI) signals on MRI. These imaging characteristics in acute MS lesions have been postulated to be due to peripheral vasogenic edema that typically increases the ADC. This assumption is commonly used to differentiate stroke from MS lesions since acute and subacute stroke lesions demonstrate increased DWI signal with reduced ADC due to acute cytotoxic edema. We report a case of active relapsing-remitting MS with two new symptomatic contrast-enhancing lesions. The lesions had reduced diffusion on the ADC map in the early acute phase of MS exacerbation. The reduced ADC signal was subsequently "converted" to increased ADC signal that coincided with the development of profound peripheral vasogenic edema seen on T2-weighted images. To our knowledge, this is the first serial MRI study describing decreased ADC signal in the early acute phase of contrast-enhancing MS lesion. The implications of decreased diffusion in the acute phase of MS lesions for the disease pathogenesis are discussed.