Acute and chronic changes of the apparent diffusion coefficient in neurological disorders - biophysical mechanisms and possible underlying histopathology

Diffusion-weighted imaging (DWI) of the brain has become a valuable tool fu r the reliable detection and diagnosis of several neurological disorders. A lthough DWI is in wide use in daily practice, the underlying biophysical me chanisms that contribute: to changes in the apparent diffusion coefficient (ADC) are still under discussion. Alterations in the apparent water diffusi on rate reflect pathological changes in the brain tissue state, via changes in the diffusion characteristics of the intra- and extra-cellular water co mpartments including restricted diffusion, water exchange across permeable boundaries, the concept of the extra-cellular tortuosity and the intra- and extra-cellular volume fraction. A reduction of the ADC has been detected i n acute neurological diseases, while disease states associated with dominan t acute vasogenic edema formation or chronic tissue destruction usually sho w elevations of the ADC. Compromise of energy metabolism is likely to contr ibute to a reduction of the ADC while already minor structural disintegrati on may contribute to elevations of the ADC. (C) 2001 Elsevier Science B.V. All rights reserved.