Diffusion MRI of Brain Connectivity and Microstructure:

What is a “diffusion coefficient” or “diffusion tensor” in living tissue, anyway? While the measurement of the self-diffusivity in an NMR tube represents a "gold standard", in complex media particularly in living tissue, water “diffusivity” ceases to have a clear meaning or definition. The apparent diffusion coefficient (ADC) and apparent diffusion tensor (ADT) concepts were introduced to answer the question: “What would the equivalent diffusion coefficient be if the displacement distribution were Gaussian?” The actual MR measurement is of the mean squared displacement not the diffusion coefficient, which offers improved prospects for discovering microstructural features over the ADC or ADT measurements.