DWI exploits the random motion of water molecules. The extent of tissue cellularity and the presence of intact cell membrane help determine the impedance of water molecule diffusion. This impedance of water molecules diffusion can be quantitatively assessed using the apparent diffusion coefficient (ADC) value.
ADC values are calculated automatically by the software and then displayed as a parametric mapping that reflects the degree of diffusion of water molecules through different tissues. Then, by use of a dedicated workstation, ADC measurements are recorded for a given region by drawing regions of interest (ROIs) on the ADC map.
An ADC of a tissue is expressed in units of mm2/s. There is no unanimity regarding the boundaries of the range of normal diffusion, but ADC values less than 1.0 to 1.1 x 10-3 mm2/s (or 1000-1100 x 10-6 mm2/s) are generally acknowledged in adults as indicating restriction. However, this is entirely dependent on the organ being imaged and the pathology.
Some rough useful values (10-6 mm2/s) 8-10:
white matter: 670-800
cortical grey matter: 800-1000
deep grey matter: 700-850
Gd II: 1273 ± 293
Gd III: 1067 ± 276
Gd IV: 745 ± 135
Diffusion changes are correlated to the clinical deficit and are potentially useful for early diagnosis and longitudinal evaluation, especially in the context of pharmacological trials.