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May 21st, 2015 Filed under Magnetic Resonance Imaging Tagged angelo-mosso, animals, balance, bold, cambridge, energy, gradient, magnetic, nuclei, proportion, redistribution, study, the-brain Comments Off on Functional magnetic resonance imaging Wikipedia, the

FMRI redirects here. For Fault Management Resource Identifier, see OpenBSM.

Functional magnetic resonance imaging or functional MRI (fMRI) is a functional neuroimaging procedure using MRI technology that measures brain activity by detecting associated changes in blood flow.[1][2] This technique relies on the fact that cerebral blood flow and neuronal activation are coupled. When an area of the brain is in use, blood flow to that region also increases.[3]

The primary form of fMRI uses the blood-oxygen-level dependent (BOLD) contrast,[4] discovered by Seiji Ogawa. This is a type of specialized brain and body scan used to map neural activity in the brain or spinal cord of humans or other animals by imaging the change in blood flow (hemodynamic response) related to energy use by brain cells.[4] Since the early 1990s, fMRI has come to dominate brain mapping research because it does not require people to undergo shots, surgery, or to ingest substances, or be exposed to radiation, etc.[5] Other methods of obtaining contrast are arterial spin labeling [6] and diffusion MRI.

The procedure is similar to MRI but uses the change in magnetization between oxygen-rich and oxygen-poor blood as its basic measure. This measure is frequently corrupted by noise from various sources and hence statistical procedures are used to extract the underlying signal. The resulting brain activation can be presented graphically by color-coding the strength of activation across the brain or the specific region studied. The technique can localize activity to within millimeters but, using standard techniques, no better than within a window of a few seconds.[citation needed]

fMRI is used both in the research world, and to a lesser extent, in the clinical world. It can also be combined and complemented with other measures of brain physiology such as EEG and NIRS. Newer methods which improve both spatial and time resolution are being researched, and these largely use biomarkers other than the BOLD signal. Some companies have developed commercial products such as lie detectors based on fMRI techniques, but the research is not believed to be ripe enough for widespread commercialization.[7]

The fMRI concept builds on the earlier MRI scanning technology and the discovery of properties of oxygen-rich blood. MRI brain scans use a strong, permanent, static magnetic field to align nuclei in the brain region being studied. Another magnetic field, the gradient field, is then applied to kick the nuclei to higher magnetization levels, with the effect depending on where they are located. When the gradient field is removed, the nuclei go back to their original states, and the energy they emit is measured with a coil to recreate the positions of the nuclei. MRI thus provides a static structural view of brain matter. The central thrust behind fMRI was to extend MRI to capture functional changes in the brain caused by neuronal activity. Differences in magnetic properties between arterial (oxygen-rich) and venous (oxygen-poor) blood provided this link.[8]

Since the 1890s it has been known that changes in blood flow and blood oxygenation in the brain (collectively known as hemodynamics) are closely linked to neural activity.[9] When neurons become active, local blood flow to those brain regions increases, and oxygen-rich (oxygenated) blood displaces oxygen-depleted (deoxygenated) blood around 2 seconds later. This rises to a peak over 46 seconds, before falling back to the original level (and typically undershooting slightly). Oxygen is carried by the hemoglobin molecule in red blood cells. Deoxygenated hemoglobin (dHb) is more magnetic (paramagnetic) than oxygenated hemoglobin (Hb), which is virtually resistant to magnetism (diamagnetic). This difference leads to an improved MR signal since the diamagnetic blood interferes with the magnetic MR signal less. This improvement can be mapped to show which neurons are active at a time.[10]

During the late 19th century, Angelo Mosso invented the human circulation balance, which could non-invasively measure the redistribution of blood during emotional and intellectual activity.[11] However, although briefly mentioned by William James in 1890, the details and precise workings of this balance and the experiments Mosso performed with it have remained largely unknown until the recent discovery of the original instrument as well as Mossos reports by Stefano Sandrone and colleagues.[12]Angelo Mosso investigated several critical variables that are still relevant in modern neuroimaging such as the signal-to-noise ratio, the appropriate choice of the experimental paradigm and the need for the simultaneous recording of differing physiological parameters.[12] Mossos manuscripts do not provide direct evidence that the balance was really able to measure changes in cerebral blood flow due to cognition,[12] however a modern replication performed by David T Field[13] has now demonstrated using modern signal processing techniques unavailable to Mosso that a balance apparatus of this type is able detect changes in cerebral blood volume related to cognition.

In 1890, Charles Roy and Charles Sherrington first experimentally linked brain function to its blood flow, at Cambridge University.[14] The next step to resolving how to measure blood flow to the brain was Linus Paulings and Charles Coryells discovery in 1936 that oxygen-rich blood with Hb was weakly repelled by magnetic fields, while oxygen-depleted blood with dHb was attracted to a magnetic field, though less so than ferromagnetic elements such as iron. Seiji Ogawa at AT&T Bell labs recognized that this could be used to augment MRI, which could study just the static structure of the brain, since the differing magnetic properties of dHb and Hb caused by blood flow to activated brain regions would cause measurable changes in the MRI signal. BOLD is the MRI contrast of dHb, discovered in 1990 by Ogawa. In a seminal 1990 study based on earlier work by Thulborn et al., Ogawa and colleagues scanned rodents in a strong magnetic field (7.0T) MRI. To manipulate blood oxygen level, they changed the proportion of oxygen the animals breathed. As this proportion fell, a map of blood flow in the brain was seen in the MRI. They verified this by placing test tubes with oxygenated or deoxygenated blood and creating separate images. They also showed that gradient-echo images, which depend on a form of loss of magnetization called T2* decay, produced the best images. To show these blood flow changes were related to functional brain activity, they changed the composition of the air breathed by rats, and scanned them while monitoring brain activity with EEG.[15] The first attempt to detect the regional brain activity using MRI was performed by Belliveau and others at Harvard University using the contrast agent Magnevist, a ferromagnetic substance remaining in the bloodstream after intravenous injection. However, this method is not popular in human fMRI, because any medically unnecessary injection is to a degree unsafe and uncomfortable, and because the agent stays in the blood only for a short time. [16]

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