Aging Process

MK can be regarded as a general index of tissue microstructural complexity, and, already mentioned it refers to tissue structure length scales compared to or larger than the water molecule diffusion length (typically 5-30 pm) but smaller than the voxel size. The MK length covers cell membranes, axon sheaths, and myelin layers that act as barriers to free diffusion in each voxel. More specifically, MK is a sensitive indicator of diffusional heterogeneity. Falangola et al¹⁴ has demonstrated changes in kurtosis metrics in the human frontal lobes with aging. The WM MK showed a rapid shift to higher values until age 18 years, likely reflecting the intense and continuous myelination and fiber organization that occurs at this time, with a shift to lower values with aging, probably related to degenerative changes, including a decrease of myelin density and myelinated fibers. MK in GM showed a shift to higher values until age 18, which is consistent with the known increase of cortical cell-packing density. This sequence of change in MK with age is in agreement with postmortem studies sensitive for evaluation of the microstructural complexity of both GM and WM.¹⁴¹⁶ More recently, Coutu et al¹⁷ analyzed DTI and DKI metrics in 111 healthy adults between the ages of 33 and 91 years and concluded that DKI metrics provide additional unique complementary information of microstructural changes in the context of healthy aging when used in combination with DTI metrics.