Principles of Functional MRI
نویسنده
چکیده
Kim fMRI Methods 2 1. Introduction The idea that regional cerebral blood flow (CBF) could reflect neuronal activity began with experiments of Roy and Sherrington at 1890 (1). This concept is the basis for all hemodynamic-based brain imaging techniques being used today. The focal increase in CBF can be considered to directly relate to neuronal activity because the glucose metabolism and CBF changes are closely coupled (2). Thus, the measurement of CBF change induced by stimulation has been used for mapping brain functions. Because cerebral metabolic rate of glucose (CMRglu) and CBF changes are coupled, it is assumed that cerebral metabolic rate of oxygen (CMRO 2) and CBF changes are also coupled. However, Fox and colleagues reported, based on positron emission tomographic measurements of CBF and CMRO 2 in humans during somatosensory and visual stimulation (3, 4), that an increase in CBF increase surpassed an increase in CMRO 2. Consequently, a mismatch between CBF and CMRO 2 changes results in an increase in the capillary and venous oxygenation level, opening a new physiological parameter (in addition to CBF change) for brain mapping. In 1990, based on rat brain studies during global stimulation at 7T, Ogawa and colleagues at AT&T Bell Laboratories reported that functional brain mapping is possible by using the venous blood oxygenation level-dependent (BOLD) magnetic resonance imaging (MRI) contrast (5-7). The BOLD contrast relies on changes in deoxyhemoglobin (dHb), which acts as an endogenous paramagnetic contrast agent (5, 8). Therefore, changes in the local dHb concentration in the brain lead to alterations in the signal intensity of magnetic resonance images (MRI) (5-7, 9).
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