Multi-Echo Investigations of Positive and Negative CBF and Concomitant BOLD Changes

Abstract

Unlike the positive blood oxygenation level-dependent (BOLD) response (PBR), commonly taken as an indication of an 'activated' brain region, the physiological origin of negative BOLD signal changes (i.e. a negative BOLD response, NBR), also referred to as 'deactivation' is still being debated. In this work, an attempt was made to gain a better understanding of the underlying mechanism by obtaining a comprehensive measure of the contributing cerebral blood flow (CBF) and its relationship to the NBR in the human visual cortex, in comparison to a simultaneously induced PBR in surrounding visual regions. To overcome the low signal-to-noise ratio (SNR) of CBF measurements, a newly developed multi-echo version of a center-out echo planar-imaging (EPI) readout was employed with pseudo-continuous arterial spin labeling (pCASL). It achieved very short echo and inter-echo times and facilitated a simultaneous detection of functional CBF and BOLD changes at 3 T with improved sensitivity. Evaluations of the absolute and relative changes of CBF and the effective transverse relaxation rate, R2*, the coupling ratios, and their dependence on CBF at rest, CBFrest, indicated differences between activated and deactivated regions. Analysis of the shape of the respective functional responses also revealed faster negative responses with more pronounced post-stimulus transients. Resulting differences in the flow-metabolism coupling ratios were further examined for potential distinctions in the underlying neuronal contributions.

Characterisation of laminar and vascular spatiotemporal dynamics of CBV and BOLD signals using VASO and ME-GRE at 7T in humans

“…Finally, the VASO signal is usually optimized to obtain functional responses in gray matter and the sequence protocol used here is exceptional in various ways; potentially leading to inflated signal changes above the cortical surface. For example, the use of very short TEs (15 ms) and incorporation of data from multiple sessions may lead to mis-corrections of BOLD contaminations in large superficial vessels due to BOLD-overcompensation and potential misregistration (Chaimow et al, 2019;Devi et al, 2022;Genois et al, 2021;Huber, 2014Huber, , 2018a. Furthermore, the strong, (flip angle dependent) contrast between CSF and gray matter in our data may lead to apparent signal changes in superficial vessels due to volume redistributions upon activation (Donahue et al, 2006;Jin and Kim, 2010;Piechnik et al, 2009).…”

Evaluating the capabilities and challenges of layer-fMRI VASO at 3T

“…At 3T, it is expected that 30% of the overall BOLD signal change is arising from intravascular signal sources (77). Ongoing 3T VASO with multi-echo depicting (78) suggests that there indeed is a residual BOLD contamination in VASO (79). It has been hypothesized that these residual BOLD contaminations might be arising from voxels of large extra-cortical veins.…”