Information AboutFmri |
| CATEGORIES ABOUT FUNCTIONAL MAGNETIC RESONANCE IMAGING | |
| magnetic resonance imaging | |
| medical tests | |
| neuroimaging | |
| psychology | |
| cognitive science | |
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Functional magnetic resonance imaging (fMRI) is the use of MRI to measure the Haemodynamic Response related to Neural activity in the Brain or Spinal Cord of Human s or other animals. It is one of the most recently developed forms of Neuroimaging . BACKGROUND It has been known for over 100 years (Roy and Sherrington , 1890) that changes in Blood Flow and blood oxygenation in the Brain (collectively known as Hemodynamics ) are closely linked to neural activity. When nerve cells are active they consume Oxygen carried by Hemoglobin in Red Blood Cells from local Capillaries . The local response to this oxygen utilisation is an increase in blood flow to regions of increased neural activity, occurring after a delay of approximately 1-5 seconds. This hemodynamic response rises to a peak over 4-5 seconds, before falling back to baseline (and typically undershooting slightly). This leads to local changes in the relative concentration of oxyhemoglobin and deoxyhemoglobin and changes in local cerebral Blood Volume in addition to this change in local Cerebral Blood Flow . Hemoglobin is Diamagnetic when oxygenated but Paramagnetic when deoxygenated. The Magnetic Resonance (MR) signal of blood is therefore slightly different depending on the level of oxygenation. These differential signals can be detected using an appropriate MR pulse sequence as Blood-oxygen-level Dependent (BOLD) contrast. Higher BOLD signal intensities arise from decreases in the concentration of deoxygenated Hemoglobin since the blood Magnetic Susceptibility now more closely matches the tissue magnetic susceptibility. By collecting data in an MRI scanner with parameters sensitive to changes in magnetic susceptibility one can assess changes in BOLD contrast. These changes can be either positive or negative depending upon the relative changes in both cerebral blood flow (CBF) and oxygen consumption. Increases in CBF that outstrip changes in oxygen consumption will lead to increased BOLD signal, conversely decreases in CBF that outstrip changes in oxygen consumption will cause decreased BOLD signal intensity. NEURAL CORRELATES OF BOLD The precise relationship between neural signals and BOLD is under active research. In general, changes in BOLD signal are well correlated with changes in blood flow. Numerous studies during the past several decades have identified a coupling between blood flow and Metabolic Rate ; that is, the blood supply is tightly regulated in space and time to provide the nutrients for brain metabolism. However, Neuroscientists have been seeking a more direct relationship between the blood supply and the neural inputs/outputs that can be related to observable electrical activity and circuit models of brain function. While current data indicate that Local Field Potential s, an index of integrated electrical activity, form a better correlation with blood flow than the spiking Action Potential s that are most directly associated with neural communication, no simple measure of electrical activity to date has provided an adequate correlation with metabolism and the blood supply across a wide dynamic range. Presumably, this reflects the complex nature of metabolic processes, which form a superset with regards to electrical activity. Some recent results have suggested that the increase in cerebral blood flow (CBF) following neural activity is not causally related to the metabolic demands of the brain region, but rather is driven by the presence of Neurotransmitter s, especially Glutamate . |
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