ADC is calculated by comparing the signal at b value 0 with the signal acquired at another b value with a diffusion gradient (between 500-1000 sec/mm2). ADC is measured in mm2/s. Of note, at a b value of 0, the signal intensity is based on T2 weighting (which is why this is another valuable opportunity to look for T2 hyperintense lesions).
DWI relies on the normal random motion of water molecules. If there are fewer cells and more water, water molecules will diffuse freely. As the molecules diffuse freely, they will not receive the full rephrasing gradient to capture all of their signal. Thus, fast-flowing water (such as blood) will be black on DWI. That said, ADC is the negative logarithm of the signal intensity ratio, so the tissue will have a high apparent diffusion coefficient.
In a more highly cellular environment, there is impedance of the water molecules (by intact cell membranes, for instance) and the water molecules are more likely to receive the full rephrasing gradient and retain signal (be bright on DWI). The resultant apparent diffusion coefficient is decreased. Thus, these highly cellular areas are considered to have "restricted diffusion."
These sequences in body imaging are useful for dissociating abscess from necrotic tumors, for example.
KRISTIN PORTER, MD, PHD PAMELA JOHNSON, MD