Regular ArticleIntervoxel Heterogeneity of Event-Related Functional Magnetic Resonance Imaging Responses as a Function of T1 Weighting
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2019, NeuroImageCitation Excerpt :The CSF is well approximated by pure water, with a spin density we define as unity. Blood is approximately 87% water by volume (Lu et al., 2002), while GM and WM have spin densities of about 0.89 and 0.73, respectively (Shen et al., 2012). We assume that the stated water proton densities represent freely diffusing, NMR-visible water compartments.
Inflow effects on functional MRI
2012, NeuroImageCitation Excerpt :One of the possible explanations for the variation in the prior studies lies in the CBV contribution for the acquisition with increased T1-weighting (e.g. short TR and large flip angle). Due to the longer T1 of blood than tissue, increased CBV during activation may result in decreased signal, a reverse trend than the inflow effect (Lu et al., 2002). Combining the inflow and the CBV effects with the partial volume averaging, Lu et al. (2002) showed that either “increased” (inflow), “decreased” (CBV) or no effects would be detected in the BOLD signals obtained from different activated voxels.
A review of the development of Vascular-Space-Occupancy (VASO) fMRI
2012, NeuroImageCitation Excerpt :Therefore, we set out to search for a method to null the blood signal yet maintaining (at least some) tissue signal. Because blood T1 was well known to be longer than tissue T1 (Duck, 1990; Hoogenraad et al., 2001; Lu et al., 2002), the use of an inversion recovery sequence with the inversion time (TI) set at the blood nulling point came to mind. We also noted that, since the blood spins in the voxel are continuously replaced by newly arrived fresh spins, a non-slice-selective inversion pulse needed to be used.
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