Monte-Carlo Simulations of BOLD Background Gradient Contributions in Diffusion-Weighted fMRI— Comparison of Spin Echo and Twice-Refocused Echo Sequences
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چکیده
Introduction. Because of its proposed potential to better localize neuronal activity and to observe activation through a mechanism different from the BOLD effect, diffusion-weighted fMRI (DFMRI) has drawn much attention [1-5]. Data from studies using lower b-values indicate an increase of the apparent water diffusion coefficient (ADC) during functional activation, which is thought to be caused predominantly by increased blood flow and/or relative volume of intravascular spins with relatively high ADC. Strong diffusion weighting (high b-value) primarily detects signals of extravascular origin. In this regime, a minute decrease of the ADC accomplished by a different temporal characteristic compared to the BOLD response was observed. The transient increase of the diffusion-weighted signal, whereby the percent signal change (ΔS/S0) rose with increasing b-value, was attributed to intracellular processes preceding the hemodynamic response [1,2]. Presently, there is still debate about the size of the effect, its observability and its true origin [3-5]. One issue that needs to be discussed due to its impact on any observation of ADC changes is the interaction (‘cross-terms’) of the diffusion-weighting (DW) gradients and susceptibility-induced background gradient fields around vessels [4]. Dedicated sequences exist that permit suppression of cross-terms by optimized schemes of sign-alternating DW gradients [6]. However, effective cross-term compensation is prevented if the background gradient contributions are non-linear and/or varying during the time course of the sequence. In case of rapidly varying background gradients ‘visited’ by the molecules, the spin phases accumulated under the influences of DW and background gradients are independent of each other. As a result, background gradients act as a mechanism that accelerates overall signal attenuation. In case of an intermediate motional regime a general predication cannot be made easily. However, it seems reasonable to assume that interactions between background gradients and DW gradients cannot be suppressed completely. As a consequence, it is necessary to survey the signal formation under the real conditions of the experiment [7]. Here, we investigate by Monte-Carlo (MC) simulations, to what extent couplings between background gradients of vascular origin and DW gradients could influences the extravascular DFMRI signal changes observed by the popular Twice Refocused Echo Sequence (TRE) [8].
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تاریخ انتشار 2008