fMRI for the Assessment of Functional Connectivity [chapter]

Till Nierhaus, Daniel Margulies, Xiangyu Long, Arno Villringer
2012 Neuroimaging - Methods  
Neuroimaging -Methods 30 analyses: (1) investigation of anatomy, and (2) investigation of dynamics using simultaneous EEG-fMRI measurements. History of BOLD in functional neuroimaging, and the beginnings of functional connectivity in both task-states and rest fMRI is the most widely used imaging technique in modern cognitive neuroscience. It allows for non-invasive (albeit indirect) studying of neuronal processes in the brain with excellent spatial resolution. While the first functional MRI
more » ... riment was performed with an exogenous contrast agent (Belliveau et al., 1991) , fortunately another method was developed using magnetic properties of blood itself, so that such a contrast agent was no longer required for functional imaging (Bandettini et al., 1992; Frahm et al., 1992; Kwong et al., 1992; Ogawa et al., 1992) . The fMRI signal depends on the vascular response to functional brain activation and is typically implemented by imaging of the blood oxygenation-level dependent (BOLD) contrast. The BOLD-effect The use of the BOLD effect for imaging brain activation relies on the fact that changes in neuronal activity are associated with changes in energy consumption and cerebral blood flow (Roy & Sherrington, 1890; Villringer & Dirnagl, 1995) 1 . Since changes in oxygen consumption and blood flow are associated with changes in haemoglobin oxygenation, the latter can also be used as an indirect measure of neuronal activity, assuming that activated neuronal circuits have an increased metabolic demand. The magnetic susceptibility of blood depends on the magnetic properties of haemoglobin, the molecule carrying the oxygen necessary for aerobic cellular metabolism. During brain activation, blood focally changes its oxygenation level, which depends on the proportion of oxygenated haemoglobin ([oxy-Hb]) and deoxygenated haemoglobin ([deoxy-Hb]). [oxy-Hb] is a diamagnetic molecule, whereas [deoxy-Hb] is paramagnetic. The presence of [deoxy-Hb] causes local field inhomogeneities, which are responsible for a dephasing of the local transversal magnetization 2 , leading to a reduction in the transverse relaxation time T 2 . fMRI for the Assessment of Functional Connectivity 31 As a diamagnetic molecule, [oxy-Hb] does not produce the same dephasing. Thus, changes in [deoxy-Hb] can be observed as the BOLD contrast in T 2 -weighted MR-images, serving as an indirect measure of neuronal activity. Since neuronal activation is accompanied by a focal increase in oxygenated blood, overshooting the actual metabolic demand, activated brain areas are characterised by positive BOLD responses in fMRI measurements.
doi:10.5772/23864 fatcat:ptda4zrn2rafbgewzrqvfarzty