Figure 4. Hypothesized Neurobiological Mechanisms Associated With Increased Restricted Isotropic Diffusion.

Hypothesized neurobiological underpinning of particulate matter 2.5 μm or less in diameter (PM_2.5)–associated increase in restricted isotropic intracellular diffusion (rN0). Given that rN0 represents diffusion within cells, it is hypothesized that increases of rN0 in white matter at 9 to 10 years may reflect an increase in the size or number of support and glial cells in response to exposure to PM_2.5. An overall increase or change in cell numbers within a given white matter region could also contribute to overall decreases in mean diffusivity as measured by diffusion tensor imaging. hD indicates total hindered diffusion; rND, restricted directional intracellular diffusion; RSI, restriction spectrum imaging.