What is the role of the apparent diffusion coefficient (ADC) in MRI, and how does it differ from diffusion-weighted imaging (DWI)?

The apparent diffusion coefficient (ADC) quantifies the degree of water diffusion within tissues, providing a map that is free from T2 shine-through effects seen in diffusion-weighted imaging (DWI). ADC values are crucial for differentiating between cytotoxic edema, which shows restricted diffusion and low ADC values, and vasogenic edema, which typically has higher ADC values. Clinically, ADC is vital in stroke imaging to identify acute ischemic changes and in oncology to assess tumor cellularity. A common pitfall is misinterpreting T2 shine-through on DWI as restricted diffusion, which can be avoided by correlating with ADC maps.