Regional and depth-dependence of cortical blood-flow assessed with high-resolution Arterial Spin Labeling (ASL)

Abstract

Methods for imaging of cerebral blood flow do not typically resolve the cortex and thus underestimate flow. However, recent work with high-resolution MRI has emphasized the regional and depth-dependent structural, functional and relaxation times variations within the cortex. Using high-resolution Arterial Spin Labeling (ASL) and T₁ mapping acquisitions, we sought to probe the effects of spatial resolution and tissue heterogeneity on cortical cerebral blood flow (CBF) measurements with ASL. We acquired high-resolution (1.6mm)³ whole brain ASL data in a cohort of 10 volunteers at 3T, along with T₁ and transit-time (ATT) mapping, followed by group cortical surface-based analysis using FreeSurfer of the different measured parameters. Fully resolved regional analysis showed higher than average mid-thickness CBF in primary motor areas (+15%,p<0.002), frontal regions (+17%,p<0.01) and auditory cortex, while occipital regions had lower average CBF (-20%,p<10^-5). ASL signal was higher towards the pial surface but correction for the shorter T₁ near the white matter surface reverses this gradient, at least when using the low-resolution ATT map. Similar to structural measures, fully-resolved ASL CBF measures show significant differences across cortical regions. Depth-dependent variation of T₁ in the cortex complicates interpretation of depth-dependent ASL signal and may have implications for the accurate CBF quantification at lower resolutions.

Keywords: ASL; cerebral blood-flow; cortical perfusion; perfusion; surface-based analysis.

Figure 1.

Example of (a) T₁-weighted, (b) T₁ map, (c) T₂-FLAIR and (d) high-resolution 1.6mm isotropic ASL perfusion-weighted data.

Figure 2.

Integrated and mid-distance sampled CBF, T₁ and ATT projected on an inflated pial surface of the left hemisphere

Figure 3.

Group Z-score maps showing CBF regional variation.

Figure 4.

Uncorrected, as well as T₁/ATT, T₁ only and ATT-only corrected CBF sampled at mid-distance in the cortex (left-hemisphere). A relative color scale is shown because of different CBF values depending on which correction is used.

Figure 5.

depth-dependence of cortical blood-flow before and after T₁/ATT correction as well as T₁, averaged across cortical ROIs (a) and in the precentral gyrus (b). The S/M/D marks represent grouping into superficial, middle and deep cortical layers.