What is an apparent diffusion coefficient?

The apparent diffusion coefficient is a measurement of how far water molecules spread in a tissue, notes Radiopaedia. Water moves in a random fashion as it spreads through biological tissue, and the ADC can be computed by radiologists to assist in diagnoses.

As water moves throughout tissue areas, blockages show up on diffusion-weighted images; these are helpful to locate regions of interest, such as in the brain, a uniformly watery organ that resists other methods of investigation. ADC values can be computed from magnetic-resonance imaging results, then mapped as an overlay. The ADC of a tissue is measured in units of millimeters squared per second.