Hemodynamic Responses in the White Matter (WM): Reduced Blood Flow in Deep WM During Hypercapnia Revealed by Multi-Delay pCASL in Healthy Young Adults

The white matter (WM) cerebrovascular response remains poorly understood compared with grey matter (GM), partly due to technical challenges in perfusion quantification. Previous hypercapnia studies using BOLD MRI have reported reduced or negative WM cerebrovascular reactivity, but whether these findings reflect true reductions in cerebral blood flow (CBF) remains unclear. We used multi-delay pseudo-continuous arterial spin labeling (pCASL) to quantify hypercapnia-induced CBF changes ({Delta}CBF) while accounting for regional variability in arterial transit time. Twenty-five healthy young adults underwent MRI during normocapnia and hypercapnia (inhalation of a 4% CO2 gas mixture). Hypercapnia induced robust positive {Delta}CBF in cortical GM (26.7 {+/-} 13.5%), superficial WM (17.2 {+/-} 12.6%), periventricular regions (13.6 {+/-} 10.6%), and subcortical GM (25.7 {+/-} 14.1%) (all p < 0.0001). In contrast, deep WM exhibited a near-zero group-mean CBF response (1.0 {+/-} 8.9%, p = 0.57), with 10 of 25 participants demonstrating negative {Delta}CBF. Negative responses were consistently localized to the corona radiata, centrum semiovale, and optic radiation. Quality-control analyses showed that deep-WM {Delta}CBF estimates are robust at our long post-labeling delays, supporting the reliability of these findings. Across tissue compartments, higher baseline CBF was associated with reduced hypercapnic responsiveness, and deep-WM responses were strongly coupled with cortical GM responses across individuals. These results demonstrate that hypercapnia-induced perfusion responses are highly heterogeneous across tissue depths and provide evidence that negative CBF responses can occur in healthy deep WM. The findings challenge the assumption of uniformly positive cerebrovascular responses during hypercapnia and support a potential role for flow redistribution arising from regional differences in vascular resistance and reserve capacity.