Imaging of the pial arterial vasculature of the human brain in vivo using high-resolution 7T time-of-flight angiography

Abstract

The pial arterial vasculature of the human brain is the only blood supply to the neocortex, but quantitative data on the morphology and topology of these mesoscopic vessels (diameter 50-300 μm) remains scarce. Because it is commonly assumed that blood flow velocities in these vessels are prohibitively slow, non-invasive time-of-flight MRI angiography (TOF-MRA)-which is well-suited to high 3D imaging resolutions-has not been applied to imaging the pial arteries. Here, we provide a theoretical framework that outlines how TOF-MRA can visualize small pial arteries in vivo, by employing extremely small voxels at the size of individual vessels. We then provide evidence for this theory by imaging the pial arteries at 140-μm isotropic resolution using a 7T MRI scanner and prospective motion correction, and show that pial arteries one voxel-width in diameter can be detected. We conclude that imaging pial arteries is not limited by slow blood flow, but instead by achievable image resolution. This study represents the first targeted, comprehensive account of imaging pial arteries in vivo in the human brain. This ultra-high-resolution angiography will enable the characterization of pial vascular anatomy across the brain to investigate patterns of blood supply and relationships between vascular and functional architecture.

Measuring Arterial Pulsatility With Dynamic Inflow Magnitude Contrast

“…Modifying the parameters (i.e., flip angle, TR or slice thickness) to make v c closer to the maximum expected velocity in a small artery of interest maximises the dynamic range and absolute magnitude of the DIMAC signal, but at the expense of reduced static spin saturation. However, a recent study that explored this inherent trade-off in the context of imaging the pial arterial vasculature with TOF ( Bollmann et al, 2021 ), suggests a similar optimisation could be performed for the DIMAC sequence. The simulated results presented in Figure 4B suggest that with increased averaging over cardiac cycles, pulsatile flow waveforms can be recovered in low SNR scenarios, even in cases with significant extra-luminal partial volumes.…”