Principles of functional MRI

Abstract

The idea that regional cerebral blood flow (CBF) could reflect neuronal-activity began with experiments of Roy and Sherrington in 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 relate directly to neuronal activity because the glucose metabolism and CBF changes are coupled closely.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 (CMRO2) and CBF changes also are coupled. However, based on positron emission tomographic measurements of CBF and CMRO2 in humans during somatosensory and visual stimulation, Fox and colleagues reported that an increase in CBF surpassed an increase in CMRO2.3,4 Consequently, a mismatch between CBF and CMRO2 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 7 Tesla (T), 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.5-7,9 © 2010 Springer Science+Business Media, LLC.