Download Functional Magnetic Resonance Imaging (fMRI)

Functional Magnetic Resonance Imaging (fMRI)
Procedure: Mr
Bodypart: Head
Patient Group: Female Male Child
Technique
What it is
Other terms: Functional MRI
Functional magnetic resonance imaging (fMRI) is the use of MRI to measure the
hemodynamic response related to neural activity in the brain or spinal cord. It is
one of the most recently developed forms of neuroimaging.
MRI body scans of a man, woman, and child
© eNotes (Simon Fraser, Photo Researchers.
Reproduced by permission.)
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How it works
In fMRI, a patient is placed in a high magnetic field and delicate measurements of magnetic fields associated with processes
like blood flow are made. In this way, the functioning of organs like the brain can be monitored as they occur.
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Equipment
The signals are extrapolated from the fMRI machine onto a screen, displaying the active regions of the brain. Depending on
what regions are the most active, the technician can determine whether a subject is telling the truth or not. This technology is
in its early stages of development, and many of its proponents hope to replace older lie detection techniques.
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Procedure
Precautions
Safety is a very important issue in all experiments involving MRI. Potential subjects must ensure that they are able to enter
the MRI environment. Due to the nature of the MRI scanner, there is an extremely strong magnetic field surrounding the MRI
scanner (at least 1.5 teslas, possibly stronger). Potential subjects must be thoroughly examined for any ferromagnetic objects
(e.g. watches, glasses, hair pins, pacemakers, bone plates and screws, etc.) before entering the scanning environment.
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Duration
An fMRI experiment usually lasts between 15 minutes and 2 hours.
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Process
Depending on the purpose of study, subjects may view movies, hear sounds, smell odors, perform cognitive tasks such as
memorization or imagination, press a few buttons, or perform other tasks. Researchers are required to give detailed instructions
and descriptions of the experiment plan to each subject, who must sign a consent form before the experiment.
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Factors affecting results
Subjects participating in a fMRI experiment are asked to lie still and are usually restrained with soft pads to prevent small
motions from disturbing measurements. Some labs also employ bite bars to reduce motion, although these are unpopular as
they can cause some discomfort to subjects. It is possible to correct for some amount of head movement with post-processing
of the data, but large transient motion can render these attempts futile. Generally motion in excess of 3 millimeters will result in
unusable data. The issue of motion is present for all populations, but most notably within populations that are not physically or
emotionally equipped for even short MRI sessions (e.g., those with Alzheimer's Disease or schizophrenia, or young children). In
these populations, various and negative reinforcement strategies can be employed in an attempt to attenuate motion artifacts,
but in general the solution lies in designing a compatible paradigm with these populations.
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Consideration
Benefits
It can noninvasively record brain signals (of humans and other animals) without risks of radiation inherent in other scanning
methods, such as CT scans.
It can record on a spatial resolution in the region of 3-6 millimeters, but with relatively poor temporal resolution (in the order
of seconds) compared with techniques such as EEG. However, this is mainly because of the phenomena being measured,
not because of the technique. EEG measures electrical/neural activity while fMRI measures blood activity, which has a longer
response. The MRI equipment used for fMRI can be used for high temporal resolution, if you measure different phenomena.
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Disadvantages
Like any other technique, fMRI is as worthwhile as the design of the experiment using it. Many investigators have used fMRI
ineffectively because they were not familiar with all aspects of the technique, or because they received their academic training
in disciplines characterized by less rigor than some other branches of psychology and neuroscience. Ineffective use of the
technique is a problem for the field, but it is not a consequence of the technique itself.
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Alternatives
Aside from fMRI, there are other related ways to probe brain activity using magnetic resonance properties:
Contrast MR
An injected contrast agent such as an iron oxide that has been coated by a sugar or starch (to hide from the body's defense
system), causes a local disturbance in the magnetic field that is measurable by the MRI scanner. The signals associated
with these kinds of contrast agents are proportional to the cerebral blood volume. While this semi-invasive method presents
a considerable disadvantage in terms of studying brain function in normal subjects, it enables far greater detection sensitivity
than BOLD signal, which may increase the viability of fMRI in clinical populations. Other methods of investigating blood volume
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that do not require an injection are a subject of current research, although no alternative technique in theory can match the high
sensitivity provided by injection of contrast agent.
Arterial spin labeling
By magnetic labeling the proximal blood supply using "arterial spin labeling" ASL, the associated signal is proportional to the
cerebral blood flow, or perfusion. This method provides more quantitative physiological information than BOLD signal, and has
the same sensitivity for detecting task-induced changes in local brain function
Magnetic resonance spectroscopic imaging
Magnetic resonance spectroscopic imaging (MRS) is another, NMR-based process for assessing function within the living brain.
MRS takes advantage of the fact that protons (hydrogen atoms) residing in differing chemical environments depending upon
the molecule they inhabit (H2O vs. protein, for example) possess slightly different resonant properties. For a given volume of
brain (typically > 1 cubic cm), the distribution of these H resonances can be displayed as a spectrum.
The area under the peak for each resonance provides a quantitative measure of the relative abundance of that compound. The
largest peak is composed of H2O. However, there are also discernible peaks for choline, creatine, n-acetylaspartate (NAA) and
lactate. Fortuitously, NAA is mostly inactive within the neuron, serving as a precursor to glutamate and as storage for acetyl
groups (to be used in fatty acid synthesis)—but its relative levels are a reasonable approximation of neuronal integrity and
functional status. Brain diseases (schizophrenia, stroke, certain tumors, multiple sclerosis) can be characterized by the regional
alteration in NAA levels when compared to healthy subjects. Creatine is used a relative control value since its levels remain
fairly constant, while choline and lactate levels have been used to evaluate brain tumors.
Diffusion tensor imaging
Diffusion tensor imaging (DTI) is a related use of MR to measure anatomical connectivity between areas. Although it is not strictly
a functional imaging technique because it does not measure dynamic changes in brain function, the measures of inter-area
connectivity it provides are complementary to images of cortical function provided by BOLD fMRI. White matter bundles carry
functional information between brain regions. The diffusion of water molecules is hindered across the axes of these bundles,
such that measurements of water diffusion can reveal information about the location of large white matter pathways. Illnesses
that disrupt the normal organization or integrity of cerebral white matter (such as multiple sclerosis) have a quantitative impact
on DTI measures.
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Citations
87 - http://en.wikipedia.org/wiki/Functional_MRI
88 - http://www.answers.com/fmri
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