Mean Arterial Pressure and Cerebral Hemodynamics Across The Lifespan: A Cross-Sectional Study From Human Connectome Project-Aging

Developmental order, fibre caliber, and vascularization predict tract-wise declines: Testing retrogenesis and physiological predictions in white matter aging

To understand the consistently observed spatial distribution of white-matter (WM) aging, developmentally driven theories of retrogenesis have gained traction, positing that the order WM development predicts declines. Regions that develop first are often expected to deteriorate the last, i.e. "last-in-first-out". Alternatively, regions which develop most rapidly may also decline most rapidly in aging, or the "gains-predict-loss" model. The validity of such theories remains uncertain, in part due to lack of clarity on the definition of developmental order. Our recent findings also suggest that WM degeneration may vary by physiological parameters such as perfusion. Furthermore, it is informative to link perfusion to fibre metabolic need, which varies with fibre size. Here we address the question of whether WM degeneration is determined by development trajectory or physiological state across both microstructural and perfusion measures using data drawn from the Human Connectome Project in Aging (HCP-A). Our results indicate that developmental order of tract myelination provides the strongest support for the retrogenesis hypothesis, with the last to complete myelination the first to decline. Moreover, higher mean axon diameter and lower macrovascular density are associated with lower degrees of WM degeneration across measures. Tract perfusion, in turn also tends to be higher and the arterial transit time longer for tracts that appear first. These findings suggest that WM degeneration in different tracts may be governed by their developmental trajectories and physiology, and ultimately influenced by each tract's metabolic demand.

MRI Assessment of Cerebral White Matter Microvascular Hemodynamics Across the Adult Lifespan

BACKGROUND

Changes in cerebral hemodynamics with aging are important for understanding age-related variation in neuronal health. While many prior studies have focused on gray matter, less is known regarding white matter due in part to measurement challenges related to the lower vascular density in white matter.

PURPOSE

To investigate the impact of age and sex on white matter hemodynamics in a Human Connectome Project in Aging (HCP-A) cohort using tract-based spatial statistics (TBSS).

STUDY TYPE

Retrospective cross-sectional.

POPULATION

Six hundred seventy-eight typically aging individuals (381 female), aged 36-100 years.

FIELD STRENGTH/SEQUENCE

Multi-delay pseudo-continuous arterial spin labeling (ASL) and diffusion-weighted pulsed-gradient spin-echo echo planar imaging sequences at 3.0 T.

ASSESSMENT

A skeleton of mean fractional anisotropy (FA) was produced using TBSS. This skeleton was used to project ASL-derived cerebral blood flow (CBF) and arterial transit time (ATT) measures onto white matter tracts.

STATISTICAL TESTS

General linear models were applied to white matter FA, CBF, and ATT maps, while covarying for age and sex. Threshold-free cluster enhancement multiple comparisons correction was performed for the effects of age and sex, thresholded at PFWE  < 0.05. CBF, ATT, and FA were compared between sex for each tract using analysis of covariance, with multiple comparisons correction for the number of tracts at PFDR  < 0.05.

RESULTS

Significantly lower white matter CBF and significantly prolonged white matter ATTs were associated with older age. These effects were widespread across tracts for ATT. Significant (PFDR  < 0.05) sex differences in ATT were observed across all tracts, and significant sex differences in CBF were observed in all tracts except the bilateral uncinate fasciculus. Females demonstrated significantly higher CBF compared to males across the lifespan. Few tracts demonstrated significant sex differences in FA.

DATA CONCLUSION

This study identified significant sex- and age-associated differences in white matter hemodynamics across tracts.

EVIDENCE LEVEL

3 TECHNICAL EFFICACY: Stage 3.