Source of nonlinearity in echo‐time‐dependent BOLD fMRI

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Abstract
Stimulation‐induced changes in transverse relaxation rates can provide important insight into underlying physiological changes in blood oxygenation level‐dependent (BOLD) contrast. It is often assumed that BOLD fractional signal change (ΔS/S) is linearly dependent on echo time (TE). This relationship was evaluated at 9.4 T during visual stimulation in cats with gradient‐echo (GE) and spin‐echo (SE) echo‐planar imaging (EPI). The TE dependence of GE ΔS/S is close to linear in both the parenchyma and large vessel area at the cortical surface for TEs of 6–20 ms. However, this dependence is nonlinear for SE studies in the TE range of 16–70 ms unless a diffusion‐weighting of b = 200 s/mm2 is applied. This behavior is not caused by inflow effects, T2* decay during data acquisition in SE‐EPI, or extravascular spin density changes. Our results are explained by a two‐compartment model in which the extravascular contribution to ΔS/S vs. TE is linear, while the intravascular contribution can be nonlinear depending on the magnetic field strength and TE. At 9.4 T, the large‐vessel IV signal can be minimized by using long TE and/or moderate diffusion weighting. Thus, stimulation‐induced relaxation rate changes should be carefully determined, and their physiological meanings should be interpreted with caution. Magn Reson Med, 2006.

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