Premature infant’s brain undergoes important micro and macrostructural maturational changes in both WM and GM during early brain development
NDI
The longitudinal increase of FD, FC and FDC observed in premature infants’ WM tracts, from the 33rd week GA to term-equivalent age, relates to the important axonal growth, maturation and myelination processes occurring typically during the third trimester of pregnancy and leading to the establishment of long-range WM connections, facilitating brain integration (Our FBA results are in line with previous dMRI studies supporting an increased maturation of WM fibers during early brain development, characterized by an increase of NDI and fractional anisotropy (FA), and decrease of ODI and mean diffusivity (MD), relating to increased fiber organization, axonal coherence and preliminary myelination (Moreover, the increase of FD, FC and FDC in thalamic regions supports the fact that deep GM also undergoes important maturational processes, which is in line with the rapid growth and establishment of functional thalamo-cortical connections occurring during this critical period of brain development (Our results additionally show that the observed maturation of WM tracts is characterized mainly by an important increase of FC, up to 50%, reflecting thus a predominant macroscopic spatial extent of the fiber bundles, accompanied by an increase of up to 15% of FD, reflecting microstructural axonal number expansion. This greater increase of FC in comparison to FD might be related to maturational processes such as myelination, leading to a prominent increase in fiber bundle diameter and thus increased ability to transfer information across brain regions (From all WM tracts, the central projection fibers are the ones revealing the highest rates of FD, FC and FDC increase. Such is in line with the expected maturation and myelination trajectory, known to occur primarily in central regions and projection fibers (In parallel to WM maturation, our study also unveils important maturational changes occurring in cortical GM during this early period of postnatal preterm brain development. A significant increase of FC accompanied by a decrease of FD and FDC is observed in various cortical GM regions distributed among the frontal, parietal, temporal, insula, cingulate and occipital cortices, comprising all the primary sensory-motor areas (auditory, somatosensory, visual and primary motor cortex).Both FD and FDC are directional-dependent metrics, since FD is computed by integrating the FOD amplitude proportional to the volume of the fibers aligned in a certain direction (On the other hand, the observed significant increase in cortical FC, accompanying FD and FDC decrease, can also be related to myelination of intracortical axons, as well as to the expansion of basal dendrites and glial cells during development. Indeed, a decreased FC has been related to decreased myelination (