van der Kouwe, A. J. | Search bioRxiv

bioRxiv2025

Spatial Image Gradient Estimation from the Diffusion MRI Profile

PurposeIn the course of diffusion, water molecules experience varying values for the relaxation-time properties of the underlying tissue. This factor, which has rarely been accounted for in diffusion MRI (dMRI), is modeled in this work, allowing for the estimation of the gradient of the relaxation-time properties from the dMRI signal. MethodsWith the aim of mining the dMRI data for information about the spatial variations in the tissue relaxation-time properties, a new mathematical relationship between the diffusion signal and the spatial gradient of the image is derived, enabling the estimation of the latter from the former. The hypothesis was validated on human brain dMRI images from three datasets: the public Human Connectome Project Young Adults database, 10 healthy volunteers and 1 ex vivo sample scanned in-house with stimulated-echo diffusion encoding and a long diffusion time of 1 second (which will be made publicly available), and 3 subjects from the public Multi-TE database. The effects of the confounding factor of "fiber continuity" were furthermore measured. ResultsThe image spatial gradient estimated from the diffusion signal was compared to the gold-standard spatial gradient approximated through the finite difference. The former gradient was significantly related to the latter in all datasets (i.e., with a difference significantly smaller than chance), with an effect distinct from fiber continuity. ConclusionThe results support the hypothesized relationship between within-voxel dMRI signal and image gradient, with an effect that was not explainable by the confounding factor of fiber continuity.

bioengineering↗

bioRxiv2019

Brain wiring and supragranular-enriched genes linked to protracted human frontal cortex development.

The human frontal cortex is unusually large compared with many other species. The expansion of the human frontal cortex is accompanied by both connectivity and transcriptional changes. Yet, the developmental origins generating variation in frontal cortex circuitry across species remain unresolved. Nineteen genes, which encode filaments, synapse, and voltage-gated channels (e.g., NEFH, SYT2, VAMP1) are especially enriched in the supragranular layers of the cerebral cortex in humans relative to mice. The increased expression of these genes suggests enhanced cortico-cortical projections emerging from layer III in humans. We confirm that the expression of these supragranular-enriched genes is preferentially expressed in frontal cortex layer III in humans relative to mice. We demonstrate a concomitant expansion in cortico-cortical pathways projecting within the frontal cortex white matter in humans with diffusion MR tractography. To identify developmental sources of such variation, we compare frontal cortical white matter growth and developmental trajectories of transcriptional profiles of supragranular-enriched genes in humans and mice. We also use temporal changes in gene expression during postnatal development to control for variation in developmental schedules across species. The growth of the frontal cortex white matter and transcriptional profiles of supragranular genes are both protracted in humans relative to the timing of other transformations. These findings demonstrate that an expansion of projections emerging from the human frontal cortex is achieved by extending the duration of cortical circuitry development. Integrating RNA sequencing with neuroimaging level phenotypes is an effective strategy to assess deviations in developmental programs leading to variation in connections across species.

neuroscience↗