FIG 1. Whole-brain–diffusion-constant distribution map (diffusion histogram) from a healthy volunteer. The data (dots) are fitted with a triple gaussian function to accommodate the two-compartment nature of the data and the mixing between two compartments (solid curve). The brain tissue compartment has a narrow distribution of values around its mean. The second and third compartments have broader distributions. The mean of the brain tissue pixel distribution (also mode of the entire distribution) is recognized as a mean diffusion constant for the entire brain (BD_av). The distribution width () of the brain tissue compartment is also recorded. FIG 2. BD_av histograms from an 86-year-old subject with age-appropriate atrophy. The thicker curve is the D_av histogram from three slices through the lateral ventricles. The tissue peak (around 0.75 x 10^–5 cm²/s) and the CSF peak (around 3.2 x 10^–5 cm²/s) are well separated. The thinner curve is the whole-brain D_av histogram. The location of the tissue peaks in both histograms did not change, demonstrating that the tissue peak is truly free of CSF contamination. The histograms are scaled so that the tissue peaks are the same height