The Neurovascular Fingerprint of BOLD bSSFP fMRI

Analysis of the impact of vessel size, orientation and intravascular contribution on the neurovascular fingerprint of BOLD bSSFP fMRI

This work focuses on the analysis and description of the neurovascular fingerprint of pass-band bSSFP, an imaging modality that has been introduced for functional BOLD imaging in 2001 (at the stop-band), and that was further advanced by several groups. The formation of MR signal from water proton magnetization during a random walk through the neurovascular network is modeled using Monte Carlo simulations for artificial cylinders with different diameter and orientation, as well as for four different sets of neurovascular networks acquired from the mouse parietal cortex measured with two-photon laser scanning microscopy at 1 µm isotropic resolution. In addition, selected Monte Carlo simulations of GE and SE have been performed to serve as a comparison to pass band bSSFP. Signal changes as a function of vessel size, blood volume, vessel orientation to the main magnetic field B₀ as well as relations of intra- and extravascular and of micro- and macrovascular contributions have been analyzed. The results show that bSSFP is highly sensitive to extravascular and microvascular components. Furthermore, GE and bSSFP, and to a lesser extent SE, exhibit a strong dependence of their signal change on the orientation of the vessel network to B₀.

Except for larger veins that occupy a huge volume fraction of the imaging voxel, bSSFP is mostly sensitive to extravascular contributions from small vessels. The BOLD signal ratio between microvessels and intracortical veins is highest for deep layers (about 20:1) and decreases to about 4:1 for the surface layer for bSSFP. Thus, the contribution of larger cortical veins is in the range of 5% to 20%, indicating a strong sensitivity of bSSFP to the microvasculature. All sequences show a strong dependence on the orientation of the cortical layers to B₀ due to perpendicularly arranged feeding and draining.