Elsevier

NeuroImage

Volume 17, Issue 2, October 2002, Pages 943-955
NeuroImage

Regular Article
Intervoxel Heterogeneity of Event-Related Functional Magnetic Resonance Imaging Responses as a Function of T1 Weighting

https://doi.org/10.1006/nimg.2002.1206Get rights and content

Abstract

Inflow effects on activation-related BOLD signal changes in event-related fMRI experiments were assessed by varying the repetition time (TR) and flip angle (FA) values for gradient-echo echo-planar imaging (GE-EPI). Surprisingly, both increases and decreases were detected in these signal changes with increased T1 weighting (reduced TR, increased FA). The well-known “positive” effect is attributed to inflow of fresh spins in the slice, leading to an apparent reduction in T1. The “negative” effect is attributed to voxels containing pure parenchyma, where large-vessel inflow effects are very small and the BOLD effect is dominated by microvascular blood volume and oxygenation changes. Because blood T1 is greater than tissue T1 at 1.5 T, the fractional BOLD effect decreases with increased T1 weighting. To aid in the interpretation of these experimental results, numerical simulations were performed based on a physiological multicompartment model, including pure tissue, large vessels (arteries, veins), microvessels (arterioles, capillaries, venules), and cerebrospinal fluid.

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