MRI_BOOK Flashcards
(500 cards)
1
Q
acetylcholine (ACh)
A
An important neurotransmitter used throughout the central and peripheral nervous systems and at the neuromuscular junction. Within the brain, ACh projections from certain cell groups in the basal forebrain may stimulate widespread changes in blood flow.
2
Q
action potential
A
A self-propagating wave of depolarization that travels down a neuronal axon.
3
Q
adaptation
A
A change in the response to a stimulus following its repeated presentation.
4
Q
adenosine triphosphate (ATP)
A
A nucleotide containing three phosphate groups that is the primary energy source for cells in the human body.
5
Q
aerobic glycolysis
A
The process, consisting of glycolysis, the TCA cycle, and the electron transport chain, that breaks down glucose in the presence of oxygen, resulting in a gain of 36 ATP molecules.
6
Q
Aliasing
A
The sampling of a signal at a rate insufficient to resolve the highest frequencies that are present. The energy at those frequencies becomes artifactually expressed at lower frequencies, distorting the measured signal.
7
Q
alternating design
A
A blocked design in which two conditions are presented one after another for the duration of the experimental run.
8
Q
γ-aminobutyric acid (GABA)
A
One of the most important inhibitory neurotransmitters.
9
Q
anaerobic glycolysis
A
The conversion of glucose to lactate in the absence of oxygen.
10
Q
anastomosis
A
The branching and reconnection of blood vessels.
11
Q
anatomical ROI
A
A region of interest (ROI) chosen based on anatomical criteria.
12
Q
angular momentum (J)
A
A quantity given by multiplying the mass of a spinning body by its angular velocity.
13
Q
Anisotropic
A
Having different properties in different directions
14
Q
antiparallel state
A
The high-energy state in which an atomic spin precesses around an axis that is antiparallel (i.e., opposite) to that of the main magnetic field.
15
Q
apical dendrites
A
The dendrites that are distant from the neuronal cell body. For typical pyramidal cells in the cortex, the apical dendrites extend to the superficial layers of cortex, whereas the cell bodies are located in deeper layers.
16
Q
apparent diffusion coefficient (ADC)
A
The quantification of diffusivity assuming isotropic diffusion.
17
Q
arachnoid
A
The middle membrane covering the brain
18
Q
arterial spin labeling (ASL)
A
A family of perfusion imaging techniques that measures blood flow by labeling spins with excitation pulses and then waiting for the labeled spins to enter the imaging plane before data acquisition.
19
Q
arteries
A
Large, thick-walled blood vessels that carry oxygenated blood from the heart to the rest of the body.
20
Q
arterioles
A
Small arteries.
21
Q
ascending/descending slice acquisition
A
The collection of data in consecutive order such that slices are acquired sequentially from one end of the imaging volume to the other.
22
Q
astrocyte
A
A type of glial cell that regulates the extracellular environment. It is the most prevalent glial cell type in the brain.
23
Q
Autoradiography
A
Imaging by injecting a radioactive substance into tissue, then exposing the tissue to X-ray-sensitive film.
24
Q
averaged epoch
A
The result of averaging a large number of epochs that are time-locked to similar events.
25
axon hillock
A region of the neuronal cell body located at the emergence of the axon. Changes in its electrical potential lead to the generation of action potentials.
26
B
The sum of all magnetic fields experienced by a spin.
27
B0
The strong static magnetic field generated by an MRI scanner.
28
B1eff
The effective magnetic field experienced by a spin system during excitation.
29
b factor
#The degree of diffusion weighting applied within a pulse sequence.
30
balloon model
A model of the interaction between changes in blood volume and changes in blood flow associated with neuronal activity.
31
basal ganglia
A set of nuclei in the forebrain that includes the caudate, putamen, and globus pallidus.
32
base image
The image on which a statistical map is displayed, often a high-resolution anatomical image.
33
basis functions
A set of functions whose linear combination can take on a wide range of functional forms. In fMRI analyses, researchers often replace a single hemodynamic response function with basis functions in order to improve the flexibility of their design matrices.
34
between-subjects manipulation
A manipulation in which different conditions are assigned to different subject groups.
35
bias field estimation
A technique for estimating inhomogeneities in the magnetic field based on intensity variations in collected images.
36
Biofeedback
Providing an explicit indicator of some physiological process, such as the beating of the heart or activation within a particular brain region, so that an individual can attempt to regulate that activation or guide behavior.
37
Biomarker
A phenotypic feature, whether physical, physiological, or behavioral, that provides a robust predictor of some experimentally or clinically important outcome.
38
biophysical property
A biological property (e.g., blood flow) whose physical parameters undergo measurable changes in response to internal processes (e.g., information processing).
39
Bloch equation
An equation that describes how the net magnetization of a spin system changes over time in the presence of a time-varying magnetic field.
40
blood-oxygenation-level dependent (BOLD) contrast
The difference in signal on T2*-weighted images as a function of the amount of deoxygenated hemoglobin.
41
Bolus
A quantity of a substance that is introduced into a system and then progresses through that system over time.
42
Bonferroni correction
A stringent form of family-wise error rate correction for multiple comparisons in which the alpha value is decreased proportionally to the number of independent statistical tests.
43
brain extraction
A step during preprocessing that removes unwanted parts of the imaging volume, like bones and scalp, leaving only the desired brain tissue for subsequent analyses.
44
brain stem
The midbrain, pons, and medulla.
45
Brodmann areas
Divisions of the brain based on the influential cytoarchitectonic criteria of Korbinian Brodmann.
46
calcium-sensitive contrast agents
A type of contrast agent with its ability for signal enhancement triggered by binding to calcium ions (Ca2+).
47
capillaries
Small. thin-walled blood vessels. The extraction of oxygen and glucose from the blood and the removal of waste carbon dioxide occur in the capillaries.
48
categorical variable
A variable that can take one of several discrete values.
49
cathode
An attractor for positive charge or ions, and a source of free electrons.
50
central autoregulation
Autonomic regulatory mechanisms that maintain a constant perfusion of the brain despite large variations in blood pressure that can occur over the course of a day.
51
central nervous system (CNS)
The brain and spinal cord.
52
central sulcus
A deep fissure that separates the frontal and parietal lobes of the brain.
53
Cerebellum
A large cortical structure at the caudal base of the brain that plays an important role in motor function.
54
cerebral blood volume (CBV) contrast
A type of fMRI contrast that is sensitized to the cerebral blood volume changes subsequent to brain activation.
55
cerebral cortex (neocortex)
The thin wrapping of cells around the outer surface of the cerebral hemispheres. It has a layered structure, referred to as cortical columns or cortical layers.
56
cerebrospinal fluid (CSF)
A colorless liquid that surrounds the brain and spinal cord and fills the ventricles within the brain.
57
cerebrum (cerebral hemispheres)
The largest and most rostral component of the mammalian brain, composed of a left and a right hemisphere.
58
chemical shift imaging
A technique for measuring the concentration of particular chemicals, based on subtle differences in the resonance of the protons they contain.
59
circle of Willis
The interconnection between the basilar artery and the carotid arteries at the base of the cranial vault.
60
cluster-size thresholding
The adoption of a minimum size, in voxels, for a cluster of active voxels to be labeled as significant.
61
Coactivation
The simultaneous activation of two or more brain regions within a single experimental task. Coactivation of brain regions does not imply that the regions are functionally connected.
62
collinear regressors
Model factors that are highly correlated with one another. The inclusion of collinear regressors reduces the validity of general linear model analyses.
63
complex cell
A neuron in the visual cortex with a larger receptive field than a simple cell that responds to a stimulus with preferred orientation anywhere within its receptive field.
64
Component
In the context of data-driven analyses, a feature of a data set that represents some aspect of its intrinsic structure.
65
compressed sensing
A new technique that takes sparse (i.e., underdetermined) data and, by estimating the sparseness of the data, reconstructs the original signal.
66
concentration gradient
A difference in the density of a substance across space. Substances diffuse along concentration gradients from areas of high concentration to areas of low concentration.
67
conditions (levels)
Different values of an independent variable.
68
confounding factor
Any property that co-varies with the independent variable within the experiment but could be distinguished from the independent variable using a different experimental design.
69
continuous ASL
A type of perfusion imaging that uses a second transmitter coil to label spins within an upstream artery while collecting images.
70
continuous variable
A variable that can take any value within a range.
71
Contrast
(1) The intensity difference between different quantities being measured by an imaging system. (2) The physical quantity being measured (e.g., T1 contrast). (3) A statistical comparison of the activation evoked by two (or more) experimental conditions, in order to test a research hypothesis.
72
contrast agent
A substance injected into the body to increase image contrast.
73
contrast weights
A vector that expresses the predictions of a research hypothesis for the different regressors in a design matrix. When multiplied by the parameter weights from a fMRI regression analysis, the result can be evaluated for statistical significance.
74
contrast-to-noise ratio (CNR)
The magnitude of the intensity difference between different quantities divided by the variability in their measurements.
75
control block
A time interval that contains trials of the control condition.
76
control condition
A condition that provides a standard to which the experimental condition(s) can be compared. Also called the baseline condition or the non-task condition.
77
converging operations
Employing two or more techniques to provide complementary evidence used to test an experimental hypothesis or scientific theory.
78
Coregistration
The spatial alignment of two images or image volumes.
79
coronal
A frontal view of the brain (along the x–z plane in MRI).
80
corpus callosum
The large white-matter bundle that is the primary connection between the cerebral hemispheres. The anterior portion is known as the genu, the posterior portion as the splenium.
81
correlation analysis
A type of statistical test that evaluates the strength of the relation between two variables. For fMRI studies, correlation analyses typically evaluate the correspondence between a predicted hemodynamic response and the observed data.
82
correlation coefficient (r-value)
A number between –1 and 1 that expresses the strength of the correlation between two variables.
83
cortex
The thin wrapping of cells around the outer surface of the brain.
84
cortical layers (cortical laminae)
The six cellular layers of the neocortex, distinguished by differences in the types, densities, sizes, and shapes of their neurons.
85
cost function
A quantity that determines the amount of residual error in a comparison.
86
counterbalancing
A process for removing confounding factors by ensuring that they have equal influence on the different conditions of the independent variable, usually by matching values across conditions.
87
cross product
The vector product of two vectors. Its direction is perpendicular to the plane defined by those vectors, and its magnitude is given by multiplying their product times the sine of the angle between them.
88
cross-validation
In pattern classification analysis, an approach to evaluating the effectiveness of classification using a given feature set. It involves the iterated generation and testing of classifiers based on different parts of the same training set.
89
Cryogens
Cooling agents used to reduce the temperature of the electromagnetic coils in an MRI scanner.
90
current dipole
An idealized pairing of a current source and a current sink separated by an infinitesimal distance. A current dipole is used as a simple and convenient model for the electromagnetic fields produced by an activated neuron.
91
current sink
An attractor of positive ions. A depolarized patch of neuronal membrane is a current sink because positively charged ions will flow toward it.
92
current source
A source of positive ions.
93
cytoarchitecture
The organization of the brain into physically distinguishable regions on the basis of cellular structure.
94
data-driven analysis
Drawing inferences based on examination of the intrinsic structure of data.
95
deactivation
Decrease in BOLD activation during task blocks compared with non-task blocks.
96
dB/dt
The change in magnetic field strength (dB) over time (dt).
97
deception
The use of experimental procedures that intentionally mislead subjects.
98
default network
A set of brain regions whose activation tends to decrease during the performance of active, engaging tasks, but to increase during conditions of resting and reflection.
99
degrees of freedom (df)
The number of independent observations within a data set. For many statistical tests, there are n – 1 degrees of freedom associated with n data points.
100
de-identification
Removal of information that may link research data or medical information to a particular individual.
101
dendrite
A neuronal process that receives signals from other cells. A neuron typically has multiple dendrites, which perform a primarily integrative function.
102
deoxygenated hemoglobin (dHb)
Hemoglobin without attached oxygen
103
dependent variables (DVs)
Quantities that are measured by the experimenter to evaluate the effects of the independent variables.
104
Depolarization
A change in the cell membrane potential caused by admitting positive charge into the cell and thus reducing its negative resting potential.
105
descriptive statistics
Statistics that summarize the sample data but do not allow inferences about the larger population.
106
design matrix
In fMRI implementations of the general linear model, the specification of how the model factors change over time.
107
detector coil
An electromagnetic coil that measures energy emitted back to the environment after its initial absorption by the sample.
108
diamagnetic
A property of a substance that opposes a magnetic field (i.e., decreases the strength of the magnetic field).
109
diencephalon
Region of the brain lying just rostral to the midbrain and containing the hypothalamus and thalamus.
110
diffusion tensor imaging (DTI)
The collection of images that provide information about the magnitude and direction of molecular diffusion. It is often used to create maps of fractional anisotropy.
111
diffusion
The random motion of molecules through a medium over time.
112
diffusion weighting
The application of magnetic gradients to cause changes in the MR signal that are dependent on the amplitude and/or direction of diffusion.
113
direct cortical stimulation
Applying small currents directly to brain tissue to excite or disrupt neural activity. Direct cortical stimulation is usually conducted in humans to localize critical brain regions in the context of neurosurgery.
114
dispersion derivative
A regressor that when added to a model improves the robustness of that model to small variations in the width of the hemo-dynamic response.
115
distribution
The pattern of variation of a variable under some conditions. For example, the normal distribution has a characteristic bell shape.
116
dopamine
An important neurotransmitter that is produced within cells in the substantia nigra and ventral tegmentum that project broadly to the striatum and cortex (especially the frontal lobe).
117
dot product
The scalar product of two vectors. It is created by summing the products along each dimension.
118
double dissociation
The demonstration that two experimental manipulations have different effects on two dependent variables. One manipulation affects the first variable but not the second, and the other manipulation affects the second but not the first.
119
dura
The outermost membrane covering the brain
120
dynamic causal modeling (DCM)
A statistical approach for testing models of the connectivity between brain regions based on hypotheses about how experimental manipulations alter activation and connectivity between regions.
121
echo-planar imaging (EPI)
A technique that allows collection of an entire two-dimensional image by changing spatial gradients rapidly following a single excitation pulse from a transmitter coil.
122
echo time (TE)
The time interval between an excitation pulse and data acquisition (defined as the collection of data from the center of k-space), usually expressed in milliseconds.
123
Eigenimage
Spatial maps, as generated by a principal components analysis, that reflect orthogonal components of a complex image (or time series of images).
124
Eigenvalue
A mathematical description of the amount of variability in a data set that is accounted for by a given component.
125
Eigenvector
Following principal components analysis, a set of values that describes a component of the intrinsic variability in a data set.
126
electric dipole
A point in space whose electrical properties can be approximated by assuming that it consists of physically separated positive and negative charges.
127
electroencephalography (EEG)
The measurement of the electrical potential of the brain, usually through electrodes placed on the surface of the scalp.
128
electrogenesis
The generation of electrical electrophysiological phenomena by a living organism.
129
electromotive force (emf)
A difference in electrical potential that can be used to drive a current through a circuit. The MR signal is the electromotive force caused by the changing magnetic field across the detector coil.
130
electron transport chain
The third step in aerobic glycolysis
131
endogenous contrast
Contrast that depends on an intrinsic property of biological tissue.
132
Epiphenomenal
A secondary consequence of a causal chain of processes, but playing no causal role in the process of interest.
133
equipotentiality
The concept that a function is so widely distributed within the brain that it depends on the activity of the brain as a whole. Equipotentiality is the antithesis of localization of function.
134
equivalent dipole
A simplifying model that represents the electromagnetic field produced by a population of neurons as though it were produced by a single dipole.
135
ERP component
A stereotypic feature of an ERP waveform, such as a peak or trough at a particular latency, that has a presumed functional significance.
136
Estimation
Measurement of the pattern of change over time within an active voxel in response to the experimental manipulation.
137
event-related design
The presentation of discrete, short-duration events whose timing and order may be randomized.
138
event-related potentials (ERPs)
Small electrical changes in the brain that are associated with sensory or cognitive events.
139
evoked magnetic fields (EMFs)
A change in the MEG signal that occurs in response to a particular stimulus. An EMF is the magnetic equivalent of an evoked potential or event-related potential in EEG.
140
evoked potential
A field potential that occurs in response to a sensory stimulus.
141
Excitation
The process of sending electromagnetic energy to a sample at its resonant frequency (also called transmission). The application of an excitation pulse to a spin system causes some of the spins to change from a low-energy state to a high-energy state.
142
90º excitation pulse
A quantity of electromagnetic energy that, when applied to a spin system during MR excitation, results in equal numbers of nuclei in the low- and high-energy states.
143
180º excitation pulse
A quantity of electromagnetic energy that, when applied to a spin system during MR excitation, results in a flipping of the usual net magnetization, such that there are now more nuclei in the high-energy state than in the low-energy state.
144
excitatory postsynaptic potential (EPSP)
A depolarization of the postsynaptic cell membrane.
145
Excitotoxicity
Damage or death of neurons caused by excess concentrations of glutamate and other substances.
146
exogenous contrast
Contrast that requires the injection of a foreign substance into the body.
147
expected Euler characteristic
The number of clusters of significant activation expected due to chance, as estimated from the number of independent statistical tests (i.e., resels).
148
experiment
The controlled test of a hypothesis. Experiments manipulate one or more independent variables, measure one or more dependent variables, and evaluate those measurements using tests of statistical significance.
149
experimental condition
A condition that contains the stimulus or task that is most relevant to the research hypothesis. Also called the task condition.
150
experimental design
The organization of an experiment to allow effective testing of the research hypothesis.
151
experimental regressors
Model factors that are associated with specific experimental hypotheses.
152
false discovery rate (FDR)
The probability of having at least one false-positive result, given the set of reported positive results.
153
family-wise error rate (FWER)
The probability of making at least one type I error, given the total number of statistical tests.
154
feature selection
An initial step in pattern classification that involves the determination of which input variables should be included in the classification algorithm.
155
fiber tracts
Bundles of axons that convey signals, in the form of action potentials, from one brain region to another.
156
field map
An image of the intensity of the magnetic field across space.
157
field of view (FOV)
The total extent of an image along a spatial dimension.
158
field potentials
Changes in electrical potential over space associated with postsynaptic neuronal activity.
159
field strength
The magnitude of the static magnetic field generated by a scanner, typically expressed in teslas.
160
field uniformity
In the context of MRI, a uniform magnetic field is one that has a constant strength throughout a wide region near the center of the scanner bore.
161
filling k-space
The process of collecting samples from throughout k-space to collect data sufficient for image formation.
162
Filter
Within the context of fMRI, an algorithm for removing temporal or spatial frequency components of data.
163
finite impulse response (FIR)
A signal processing approach that treats each time unit with a separate function (i.e., an impulse)
164
fixed-effects analysis
An analysis that assumes that the effect of the experimental manipulation has a constant effect, with differences between successive observations caused by random noise.
165
flat map
An unfolded and flattened representation of the cortical sheet to allow viewing of topographic changes over cortical space. Flat maps are most commonly used in fMRI to illustrate the organization of the visual cortex.
166
flip angle
The change in the precession angle of the net magnetization following excitation.
167
Flux
A measure of the strength of a magnetic field over an area of space.
168
fMRI-adaptation
Reduction in the BOLD response to the repeated presentation of a set of stimuli that differ in some attribute, indicating that the brain region being studied is insensitive (as measured by fMRI) to the stimulus attribute being varied.
169
forward inference
Reasoning from the experimental manipulation (i.e., changes in the independent variable) to infer the effects on a dependent variable.
170
forward solution
The direct calculation of the electric and magnetic fields that would occur at an array of sensors based on a given distribution of dipoles with known orientations and magnitudes.
171
Fourier transform
A mathematical technique for converting a signal (i.e., changes in intensity over time) into its power spectrum.
172
fractional anisotropy (FA)
The preference for molecules to diffuse in an anisotropic manner. An FA value of 1 indicates that diffusion occurs along a single preferred axis, while a value of 0 indicates that diffusion is similar in all directions.
173
frequency domain
The expression of a signal in terms of its power at different frequencies.
174
frequency-encoding gradient
A gradient that is applied during the data acquisition period so that the spin precession frequencies change over space.
175
functional connectivity
A pattern of functional relationships among regions, inferred from common changes in activation over time, that may reflect direct or indirect links between those regions.
176
functional contrast
A type of contrast that provides information about a physiological correlate of brain function, such as changes in blood oxygenation.
177
functional hyperemia
The local increase in blood flow that occurs in response to a sensory, motor, or cognitive event.
178
functional magnetic resonance imaging (fMRI)
A neuroimaging technique that uses standard MRI scanners to investigate changes in brain function over time.
179
functional neuroimaging
A class of research techniques that create images of the brain’s functional properties, notably different aspects of cognition and related information processing. Common functional neuroimaging techniques include fMRI, PET, and optical imaging.
180
functional resolution
The ability to map measured physiological variation to underlying mental processes or behaviors.
181
functional signal-to-noise ratio (functional SNR)
The ratio between the intensity of a signal associated with changes in brain function and the variability in the data due to all sources of noise. Functional SNR is sometimes called dynamic CNR or functional CNR.
182
GABA (γ)-aminobutyric acid)
One of the most important inhibitory neurotransmitters.
183
general linear model (GLM)
A class of statistical tests that assume that the experimental data are composed of the linear combination of different model factors, along with uncorrelated noise.
184
glass-brain view
A two-dimensional projection of fMRI data, as if the brain were made transparent and only the activations were visible.
185
glial cells (glia)
Brain cells that support the activities of neurons but are not primarily involved with information transmission.
186
glutamate
The most common excitatory neurotransmitter in the brain.
187
Glycolysis
The process of breaking down glucose into other compounds to produce ATP.
188
gradient coils
Electromagnetic coils that create controlled spatial variation in the strength of the magnetic field.
189
gradient-echo (GRE) imaging
One of the two primary types of pulse sequences used in MRI
190
Granger causality
A form of time series analysis that quantifies the information gained by using the past history of one variable to improve predictions of future values of another variable.
191
Health Insurance Portability and Accountability Act (HIPAA)
An act passed by the U.S. Congress that requires specific procedures for ensuring the privacy of health information.
192
hemifield
One-half of a visual display, usually referring to the left half or the right half relative to fixation.
193
Hemodynamic
Having to do with changes in blood flow or other blood properties.
194
hemodynamic response (HDR)
The change in MR signal on T2* images following local neuronal activity. The hemodynamic response results from a decrease in the amount of deoxygenated hemoglobin present within a voxel.
195
Heteroscedastic
Having the property that the distributions of noise are different across experimental conditions.
196
Homogeneity
Uniformity over space and time.
197
homoscedastic
Having the property that the distributions of noise are similar for all experimental conditions.
198
homotopic
The cortex in one cerebral hemisphere that corresponds to the same region in the other hemisphere.
199
hyperpolarization
A change in the cell membrane potential caused by admitting negative change into the cell and thus increasing its negative resting potential.
200
hypothesis-driven analysis
The evaluation of data based on statistical tests of the validity of a null hypothesis.
201
image
In MR imaging, a visual description of how one or more properties of the atomic nuclei within a sample vary over space.
202
image reconstruction
The process by which the raw MR signal, as acquired in k-space form, is converted into spatially informative images.
203
imaging genomics
A field that investigates the effect of genetic variation on brain structure and function.
204
imaging plane
The plane in which changes in the MR signal are recorded during perfusion imaging.
205
impulse
A single input to a system. Impulses are assumed to be of infinitely short duration.
206
incidental finding
The discovery of an abnormality, usually on a structural MR image, in an asymptomatic individual who was scanned for nonmedical reasons.
207
independent components analysis (ICA)
An important class of data-driven analyses that identify stationary sets of voxels whose BOLD time courses vary together over time and are maximally distinguishable from other sets.
208
independent variables (IVs)
Aspects of the experimental design that are intentionally manipulated by the experimenter and are hypothesized to cause changes in the dependent variables.
209
indicators
Regressors that have integral values to indicate a qualitative level.
210
individual differences
Measures of variation among individuals within a population.
211
inferential statistics
Statistics that make inferences about the characteristics of a population based on data obtained from a smaller sample.
212
inflated brain
A transformation of the cortical sheet into a balloon-like structure, removing gyral and sulcal folds so that activation can be more easily viewed.
213
inhibitory postsynaptic potential (IPSP)
A hyperpolarization of the postsynaptic cell membrane.
214
initial dip
The short-term decrease in MR signal immediately following the onset of neuronal activity, before the main positive component of the hemodynamic response. The initial dip may result from initial oxygen extraction before the later overcompensatory response.
215
Institutional Review Board (IRB)
An independent group of individuals who evaluate the ethical practice of research conducted at an institution. Researchers who wish to conduct research with human subjects must have the approval of their IRB.
216
integrated RF and shimming array
A new class of array coil that can simultaneously acquire MRI signal and perform local magnetic field shimming.
217
integrative activity
The collection of inputs from other neurons through dendritic or somatic connections.
218
intellectual property
The protected legal status given to an idea, product, or invention so that its owner has exclusive rights for its use, commercial or otherwise.
219
interleaved slice acquisition
The excitation of slices in an alternating order. Data are first acquired from the odd-numbered slices and then from the even-numbered slices, so as to minimize the influence of excitation pulses on adjacent slices.
220
interleaved stimulus presentation
The presentation of events of interest at different points within a TR over trials (e.g., one-forth, one-half, and three-fourths of TR in addition to TR onset), increasing the effective sampling rate of an experiment at the expense of fewer trials per condition.
221
Interneuron
A neuron that is connected locally to other neurons. Interneurons participate in local brain circuits, but do not project to distant cortical regions.
222
interocular trauma test
An intuitive evaluation of data based on highly visible effects of the experimental manipulation. It states that data are significant if, when plotted, they hit you between the eyes.
223
interstimulus interval (ISI)
The separation in time between successive stimuli. Usually refers to the time between the end of one stimulus and the onset of the next, with the term “stimulus-onset asynchrony†(SOA) used to define the time between successive onsets.
224
intersubject correlations
Functional MRI time courses that are shared by different individuals while performing the same experimental tasks or experiencing the same stimuli.
225
intersubject variability
Variability in fMRI data across a set of subjects
226
intrasubject variability
Variability in the fMRI data from a single subject associated with thermal, system, and physiological noise, as well as with variability in the pattern of brain activity during task performance.
227
inverse problem
The mathematical impossibility of determining the distribution of electrical sources within an object based on the measurement of electric or magnetic fields at the surface of the object.
228
inversion recovery
A technique for increasing T1 contrast by adding a 180º inversion pulse before a standard pulse sequence.
229
ion
An atom or molecule that carries an electrical charge.
230
ion channel
A pore in the membrane of a cell that allows passage of particular ions under certain conditions.
231
ionizing radiation
Electromagnetic radiation with sufficient energy to break chemical bonds.
232
isomorphic
Having an identical form. A physiological measurement that is isomorphic with a psychological construct would vary over time consistently with the postulated changes in the construct.
233
Isotropic
Having similar properties in all directions.
234
item-related processes
Changes in the brain that are assumed to be caused by the properties of individual stimuli, or items. Item-related processes are more easily measured with event-related designs than with mixed designs.
235
jittering
Randomizing the intervals between successive stimulus events over some range.
236
k-space
A notation scheme used to describe MRI data acquisition. The use of k-space provides mathematical and conceptual advantages for describing the acquired MR signal in image form.
237
k-space trajectory
A path through k-space. Different pulse sequences adopt different k-space trajectories.
238
labeling plane
The plane in which initial excitation pulse(s) are applied during perfusion imaging.
239
laboratory frame
The normal reference frame that is aligned with the magnetic field of the scanner.
240
large-vessel effects
Signal changes in veins that drain a functionally active region but are distant from the neuronal activity of interest.
241
Larmor frequency
The resonant frequency of a spin within a magnetic field of a given strength. It defines the frequency of electromagnetic radiation needed during excitation to make spins change to a high-energy state, as well as the frequency emitted by spins when they return to the low-energy state.
242
latent variable
A variable whose value is not directly measured, but is inferred based on the values of other variables.
243
least-squares error
A commonly used cost function, the sum of the squared residuals.
244
ligand-gated ion channel
An ion channel that opens or closes in response to binding of chemical signals (ligands). The ligand is often a neurotransmitter molecule.
245
linear system
A system that obeys the principles of scaling and superposition.
246
localization of function
The idea that the brain may have distinct regions that support particular mental processes.
247
localizer task
A simple experimental paradigm designed to identify a set of voxels based on a known functional property in preparation for subsequent analyses of that region in different paradigms.
248
logistic regression
A subset of regression analysis that uses a set of independent variables to predict a binary outcome variable.
249
longitudinal relaxation (spin–lattice relaxation)
The recovery of the net magnetization along the longitudinal direction as spins return to the parallel state.
250
magnetic field mapping
The collection of explicit information about the strength of the magnetic field at different spatial locations.
251
magnetic moment (μ)
The torque (rotational force) exerted on a magnet, moving electrical charge, or current-carrying coil when it is placed in a magnetic field.
252
magnetic resonance
The absorption of energy from a magnetic field that oscillates at a particular frequency.
253
magnetic resonance angiography (MRA)
The creation of images of the vascular system using MRI.
254
magnetic susceptibility
The intensity of magnetization of a substance when placed within a magnetic field.
255
magnetoencephalography (MEG)
A noninvasive functional neuroimaging technique that measures very small changes in magnetic fields caused by the electrical activity of neurons, with potentially high spatial and temporal resolution.
256
matched filters
The principle that a filter of the same frequency as the signal of interest provides the maximal signal-to-noise ratio.
257
membrane potential
The difference in electrical charge between the inner and outer surfaces of a cell membrane, the result of a difference in the distribution of ions.
258
mixed design
A design that contains features of both blocked and event-related approaches.
259
mixed-effects analysis
In the context of fMRI, the common practice of modeling the effect of the experimental manipulation as stable within a participant (fixed effects) but variable across participants (random effects).
260
mixing matrix
A statistical description of how a set of hypothesized sources (e.g., components) combine to form the observed data.
261
MNI space
A commonly used space for normalization of fMRI data
262
motion contrasts
Contrast mechanisms that are sensitive to the movement of spins through space (e.g., diffusion, perfusion).
263
MR signal
The current measured in a detector coil following excitation and reception.
264
MR signal equation
A single equation that describes the MR signal as a function of the properties of the object being imaged under a spatially varying magnetic field.
265
multi-band (MB) imaging (simultaneous multi-slice imaging)
A class of accelerated imaging techniques that can simultaneously acquire multiple slices and separate the individual slices via postprocessing, using the sensitivity profiles of individual coils in an RF array.
266
multi-channel imaging (parallel imaging)
The use of multiple receiver channels to acquire data following a single excitation pulse.
267
multiple comparison problem
The increase in the number of false-positive results (i.e., type I errors) with an increasing number of statistical tests. It is of particular consequence for voxel-wise fMRI analyses, which may have many thousands of statistical tests.
268
multiple regression
A family of statistical approaches that evaluate the relative contributions of several independent variables to a dependent variable.
269
multi-voxel pattern analysis (MVPA)
An approach for pattern classification in fMRI research that uses as its input data the relative changes in activation across a set of voxels.
270
mutual information
In the context of MRI, the amount of information about one image that is provided by knowledge of another image.
271
myelin
A fatty substance that forms sheaths surrounding axons that serve to speed the transmission of action potentials.
272
net magnetization (M)
The sum of the magnetic moments of all spins within a spin system.
273
neuroelectromagnetic contrast
A new generation of contrast mechanism aimed at detecting the minute electric and/or magnetic field changes as the result of neuronal firing.
274
neuroessentialism
The intuition that neuroscience data are isomorphic with the subjective self.
275
neurological convention
The practice of displaying images of the brain so that the left and right sides of the image correspond to the same sides of the brain, as if one were behind the subject.
276
neurorealism
The tendency to consider a behavioral or mental phenomenon to be more real based on the presence of neuroscience data.
277
neurotransmitters
Chemicals released by presynaptic neurons that travel across the synaptic cleft to influence receptors on postsynaptic neurons.
278
neurovascular unit
A functional unit consisting of astrocytes and neurons that impinge on a local microvessel to control blood flow.
279
nitric oxide (NO)
A rapidly diffusing soluble gas that is produced in pyramidal cells, interneurons, astrocytes, and endothelial cells
280
NMR property
A label for atomic nuclei that have both a magnetic moment and angular momentum, which together allow them to exhibit nuclear magnetic resonance effects.
281
Noise
Nonmeaningful changes in some quantity. There are many sources of noise in fMRI studies, and some changes may be classified as either noise or signal, depending on the goals of the study.
282
nonlinearity
The property whereby the combined response to two or more events is not equivalent to the summation of the responses to the individual events in isolation.
283
noradrenaline (NA)
Neurotransmitter used extensively in the central and peripheral nervous systems. Within the brain, NA projections from the locus coeruleus nuclei of the brain stem plays a role in a number of psychological processes, including attention and alertness. Also known as norepinephrine (NE).
284
normalization
The transformation of MRI data from an individual subject to match the spatial properties of a standardized image, such as an averaged brain derived from a sample of many individuals.
285
nuclear induction
The initial term for nuclear magnetic resonance effects, as labeled by Bloch and colleagues.
286
nuclear magnetic resonance (NMR)
The measurable changes in magnetic properties of atomic nuclei induced by the application of an oscillating magnetic field at the resonant frequency of the nuclei.
287
nuclei
Anatomically discrete and identifiable clusters of neurons within the brain that typically serve a particular function.
288
nuisance regressors
Model factors that are associated with known sources of variability that are not related to the experimental hypotheses.
289
null-task block
A control block in which there are no task requirements for the subject. Also called a baseline block or non-task block.
290
Nuremberg Code
A set of principles for the ethical conduct of research involving human subjects, developed following the trials of Nazi war criminals in the 1940s.
291
nutation
The spiraling change in the precession angle of the net magnetization during an excitation pulse.
292
Nyquist frequency
The highest frequency that can be identified in a digitally sampled signal
293
occipital lobe
The most posterior lobe of the brain
294
ocular dominance columns
Cellular columns in the primary visual cortex that receive separate inputs from each eye. â€Ocular dominance†refers to the degree to which a given neuron responds more to stimuli presented to one eye than to stimuli presented to the other eye.
295
ocular dominance
The degree to which a given neuron in the visual cortex responds more to stimuli presented to one eye than to stimuli presented to the other eye.
296
off-resonance excitation
The presentation of an excitation pulse at a frequency other than the resonant frequency of the sample, resulting in reduced efficiency.
297
oligodendrocyte
A type of glial cell that constructs the myelin sheaths around axons.
298
on-resonance excitation
The presentation of an excitation pulse at the resonant frequency of the sample, resulting in maximum efficiency.
299
orientation column
Vertically organized collections of neurons in visual cortex that respond preferentially to stimuli of a particular orientation, such as lines.
300
orthogonalize
To remove correlation between two variables. In the context of fMRI data analysis, orthogonalizing one regressor (with regard to one or more other regressors) changes that regressor so that it is no longer correlated with the other regressor(s).
301
oscillating magnetic field
A magnetic field whose intensity changes periodically over time. Most such fields used in MRI oscillate at the frequency range of radio waves (megahertz, or MHz), and as such they are often called radiofrequency fields.
302
oxygen extraction fraction (OEF)
The proportion of available oxygen that is removed from the blood.
303
oxygenated hemoglobin (Hb)
Hemoglobin with attached oxygen
304
parallel imaging (multi-channel imaging)
The use of multiple receiver channels to acquire data following a single excitation pulse.
305
parallel state
The low-energy state in which an atomic spin precesses around an axis that is parallel to that of the main magnetic field.
306
paramagnetic
A property of some substances that concentrates magnetic field lines, thus increasing the strength of the local magnetic field.
307
parameter matrix
A matrix that describes the relative contributions of each model to the observed data for each voxel.
308
parameter weights
For most fMRI analyses, quantities that reflect the relative contributions of the different model factors to the observed data within a given voxel.
309
parametric effect
A manipulation of some independent variable so that it takes a number of levels to evoke regular changes (e.g., a linear increase) in the dependent variable.
310
parietal lobe
The lobe on the posterior and dorsal surfaces of the cerebrum
311
partial least squares (PLS)
An approach to analyzing functional neuroimaging data that is used to identify components whose amplitude is influenced by an experimental manipulation.
312
partial volume effects
The combination, within a single voxel, of signal contributions from two or more distinct tissue types or functional regions.
313
pattern classification
An attempt to separate individual examplars into different categories by constructing a set of decision rules based on some combination of their features.
314
perfusion MRI
A type of MRI that provides information on blood flow through tissue.
315
pericytes
Contractile cells that wrap around the endothelial cells of capillaries and that can constrict the vessel, thus influencing local blood flow patterns.
316
periodic event-related design
An experimental design in which events of interest occur at regular intervals.
317
Permutation
In the context of significance testing, approaches that involve resampling the original data to directly determine the size of an effect that might be observed with a given alpha level.
318
pH-dependent imaging
An imaging technique that is sensitive to the pH changes, currently based on amide-proton transfer contrast.
319
phase
Accumulated change in angle as the result of rotation over time.
320
phase-encoding gradient
A gradient that is applied before the data acquisition period so that spins can accumulate differential phase offsets over space.
321
phased array
A method for arranging multiple surface detector coils to improve spatial coverage while maintaining high sensitivity.
322
physiological noise
Fluctuations in MR signal intensity over space and time due to physiological activity of the human body. Sources of physiological noise include motion, respiration, cardiac activity, and metabolic reactions.
323
pia
The innermost membrane covering the brain
324
Pixel
A two-dimensional picture element.
325
polygraph
A device that measures multiple physiological indicators to judge whether an individual is lying. Despite their pervasiveness, polygraphs have proven to have minimal validity for detecting lies in real-world settings.
326
polymorphism
A common variation in a gene or segment of DNA.
327
pons
Part of the brain stem
328
positron emission tomography (PET)
A functional neuroimaging technique that creates images based on the movement of injected radioactive material.
329
postsynaptic potential (PSP)
Any postsynaptic potential, excitatory or inhibitory, that results from synaptic activity.
330
power spectrum
A representation of the strength of different frequency components within a signal. The Fourier transform converts a signal (i.e., changes in intensity over time) into its power spectrum.
331
precession
The gyroscopic motion of a spinning object, in which the axis of the spin itself rotates around a central axis, like a spinning top.
332
Precoloring
The introduction of autocorrelations to a time series of data so that the time series will have known statistical properties
333
preprocessing
Computational procedures that are applied to fMRI data following image reconstruction but before statistical analysis. Preprocessing steps are intended to reduce the variability in the data that is not associated with the experimental task, and to prepare the data for statistical testing.
334
presurgical planning
The use of fMRI or another neuroscience technique to map particular functions in a single individual to guide clinical decisions about the potential course and consequences of neurosurgery in that individual.
335
Prewhitening
The application of filters to remove autocorrelations in a time series of data
336
primary current
The current flow within a neuron caused by the inflow of ions through ionic channels opened by synaptic activity.
337
principal components analysis (PCA)
A common technique for data reduction that simplifies high-dimensional data into a smaller set of components that retains most of the variability in the original data.
338
principle of reciprocity
The rule stating that the quality of an electromagnetic coil for transmission is equivalent to its quality for reception (i.e., if it can generate a homogeneous magnetic field at excitation, it can also receive signals uniformly).
339
projectile effect
The movement of an untethered ferro-magnetic object through the air toward the bore of the MRI scanner.
340
proton-density imaging
The creation of MR images that are sensitive to the number of protons present within each voxel.
341
psychophysiological interaction (PPI)
A statistical approach for identifying the effect of an experimental manipulation on the functional connectivity between two brain regions.
342
pulse sequence
A series of changing magnetic field gradients and oscillating electromagnetic pulses that allows the MRI scanner to create images sensitive to a particular physical property.
343
pulsed ASL
A type of perfusion imaging that uses a single coil both to label spins in one plane and to record the MR signal in another plane, separated by a brief delay period.
344
pump
A transport system that moves ions across a cell membrane against their concentration gradient.
345
pyramidal cell
A common neuronal type of the cerebral cortex. These cells have a pyramid-shaped soma, extensive spined dendrites, and are characterized by a long, branching axon that can extend for many centimeters.
346
quality assurance (QA)
A set of procedures designed to identify problems with fMRI data so that they do not compromise experimental analyses.
347
quantitative susceptibility mapping (QSM)
A relatively new MRI technique that generates imaging contrast based on the magnetic susceptibility of the tissue, which can provide exquisite details in the cortical and subcortical areas.
348
radiofrequency (RF) coils
Electromagnetic coils used to generate and receive energy at the sample’s resonant frequency, which for field strengths typical to MRI is in the radiofrequency range.
349
radiological convention
The practice of displaying images of the brain so that the left side of the image is the right side of the brain and vice versa, as if one were facing the subject.
350
random field theory
A branch of mathematics that deals with the properties of smooth, spatially extended data. Using random field theory, researchers can better calculate the number of inÂdependent tests within fMRI analyses.
351
random-effects analysis
An analysis that assumes that the effects of the experimental manipulation are randomly sampled from some larger population, as when participants are drawn from the community, so that there is a distribution of effects across possible observations.
352
randomization
A process for removing confounding factors by ensuring that they vary randomly with respect to the independent variable.
353
raster
A depiction of the firing rate of action potentials by a neuron in which time is represented along the horizontal axis and a dot indicates the occurrence of an action potential.
354
raw signal-to-noise ratio (raw SNR)
The ratio between the MR signal intensity associated with a sample (e.g., the brain) and the thermal noise that is measured outside the sample.
355
reaction time
The time required for someone to make a simple motor response to the presentation of a stimulus. Note that this is distinct from response time, which applies to situations in which someone must choose between two or more possible responses.
356
real-time analyses
A set of computational steps designed for the rapid analysis of fMRI data so that statistical tests can be conducted immediately following the acquisition of images.
357
reception
The process of receiving electromagnetic energy emitted by a sample at its resonant frequency (also called detection). As spins return to a low-energy state following the cessation of the excitation pulse, they emit energy that can be measured by a receiver coil.
358
receptive field
The part of the visual field that, when stimulated, will result in an increase in firing of a particular neuron.
359
recovery of function
The improvement in a previously impaired ability over time due to functional or structural changes within the brain.
360
redundant safeguards
The use of multiple independent procedures that could each identify potential problems or unsafe situations.
361
reference volume
A target image volume to which other image volumes are to be aligned.
362
refocusing pulse
A 180º electromagnetic pulse that compensates for the gradual loss of phase coherence following initial excitation.
363
refractory effects
Changes in the amplitude and timing of a response based on the characteristics of preceding responses.
364
refractory period
A time period following the presentation of a stimulus during which subsequent stimuli evoke a reduced response. For BOLD fMRI, the refractory periods for many types of stimuli last approximately 6 s.
365
region-of-interest (ROI) analyses
Evaluations of hypotheses about the functional properties of brain regions (i.e., aggregated over a predetermined set of voxels), often chosen to reflect a priori anatomical distinctions within the brain.
366
registry
A patient database. A registry might include information about the locations of brain lesions in a large population of individuals who might then be asked to participate in experimental studies.
367
regressor
A hypothesized time course of BOLD activation caused by the manipulations of an independent variable or by another known source of variability.
368
relaxation
A change in net magnetization over time.
369
rendered image
A display of MRI data in three-dimensional perspective.
370
repetition enhancement
The increase in magnitude in the hemodynamic response when a stimulus is repeated.
371
repetition suppression
The decrease in magnitude in the hemodynamic response when a stimulus is repeated.
372
repetition time (TR)
The time interval between successive excitation pulses, usually expressed in seconds.
373
repetitive TMS (rTMS)
The delivery of an extended series of closely spaced TMS stimulation pulses so as to effect long-lasting changes in brain function.
374
residual
The variability in the data that remains unexplained after accounting for the model factors.
375
resistance vessels
Arterioles that control the flow of blood through the capillary bed.
376
resolution elements (resels)
The independent statistical tests within an fMRI volume.
377
Resonance
The repeated application of small amounts of energy at a particular frequency to induce large changes in a system.
378
resonant frequency
The frequency of oscillation that provides maximum energy transfer to the system.
379
respect for persons
The principle that individuals should be treated as autonomous decision makers who should be given the opportunity to make informed decisions about their participation in research studies. Individuals with an impaired capacity for autonomous decision making must be granted additional protection.
380
response time
The time required for someone to execute a choice between two or more possible responses. Note that this is distinct from reaction time, which applies to situations in which only one possible response is present.
381
resting-state connectivity
The functional connectivity of a given brain region when measured while the participant is not performing any coordinated, purposeful task.
382
reverse inference
Reasoning from the outcome of a dependent variable to infer the state of an independent variable (or an intervening unobservable variable).
383
right-hand rule
A method used to determine the direction of a magnetic moment generated by a moving charge or electrical current. If the fingers of the right hand are curled around the direction of spin, the magnetic moment will be in the direction indicated by the thumb.
384
rigid-body transformation
A spatial transformation that does not change the size or shape of an object
385
rotating frame
A reference frame that rotates at the Larmor frequency of the spin of interest. The rotating frame is adopted to simplify mathematical descriptions of the effects of excitation.
386
Rotation
The turning of an object around an axis in space (in the absence of translation).
387
sagittal
A side view of the brain (anywhere along the y–z plane in MRI).
388
sampling rate
The frequency in time with which a measurement is made.
389
scaling
A principle of linear systems that states that the magnitude of the system output must be proportional to the system input.
390
scanner drift
Slow changes in voxel intensity over time.
391
Searchlight
An approach to feature selection in the pattern classification of fMRI data. As its name implies, a searchlight reflects a geometrically defined ROI (e.g., a sphere of 5-voxel radius) that can be moved throughout the brain.
392
seed voxel
A voxel chosen as a starting point for a connectivity analysis.
393
Segmentation
The process of partitioning an image into constituent parts, typically types of tissue (e.g., gray matter, white matter) orÆ’ topographical divisions (e.g., different structural regions like Brodmann areas).
394
semirandom design
A type of event-related design in which the probability that an event will occur within a given time interval changes systematically over the course of the experiment.
395
Shimming
A procedure to improve main magnetic field homogeneity by adjusting the electric currents within a set of electromagnetic coils in the MRI scanner.
396
shimming coils
Electromagnetic coils that compensate for inhomogeneities in the static magnetic field.
397
signal
Meaningful changes in some quantity. For fMRI, an important class of signals includes changes in intensity associated with the BOLD response across a series of T2* images.
398
signal averaging
The combination of data from multiple instances of the same manipulation in order to improve functional SNR.
399
signal-to-noise ratio (SNR)
The relative strength of a signal compared with other sources of variability in the data.
400
significance testing
The process of evaluating whether the null hypothesis is true. Also known as hypothesis testing.
401
simple cell
A neuron in the visual cortex that responds with increased firing to a stimulus with a preferred orientation in its receptive field and responds with decreased firing to a stimulus in the region surrounding its receptive field.
402
single dissociation
The demonstration that an experimental manipulation has an effect on one variable but not on a second variable.
403
single-pulse TMS
The delivery of a single TMS stimulation pulse so as to disrupt some ongoing brain process.
404
single-unit recording (single-cell recording)
Collection of data about the electrophysiological activity (e.g., action potentials) of a single neuron.
405
Sinuses
(1) Air-filled cavities in the skull. (2) Long venous channels that form the primary draining system for the brain.
406
slice selection
The combined use of a spatial magnetic field gradient and a radiofrequency pulse to excite spins within a slice.
407
small-volume correction
The restriction of analyses to specific regions of interest, defined a priori, to reduce the total number of statistical tests and thus allow for a more liberal significance threshold.
408
smoothness
The degree to which the time courses of nearby voxels are temporally correlated.
409
sodium–potassium pump
A transport system that removes three sodium ions from within a cell while bringing two potassium ions into the cell.
410
spatial extent
The number of active voxels within a cluster of activation (i.e., the size of the active region).
411
spatial frequency
The frequency with which some pattern occurs over space.
412
spatial gradient (G)
A magnetic field whose strength varies systematically over space. Note that since a given spatial location experiences only one magnetic field, which represents the sum of all fields present, spatial gradients in MRI act to change the effective strength of the main magnetic field over space.
413
spatial ICA
A form of independent component analysis that generates components that have minimal spatial redundancy.
414
spatial interpolation
The estimation of the intensity of an image at a spatial location that was not originally sampled, using data from nearby locations.
415
spatial resolution
The ability to distinguish changes in an image (or map) across different spatial locations.
416
spatial smoothing
The blurring of fMRI data across adjacent voxels to improve the validity of statistical testing and maximize the functional signal-to-noise ratio at a cost of spatial resolution.
417
specific absorption rate (SAR)
A quantity that describes how much electromagnetic energy is absorbed by the body over time.
418
speed–accuracy trade-off
The improvement in the speed of a response at the expense of accuracy, or vice versa, within an experimental task.
419
spin system
A collection of atomic nuclei that possess the NMR property within a spatial location.
420
spin-echo (SE) imaging
One of the two primary types of pulse sequences used in MRI
421
Spins
Atomic nuclei that possess the NMR property
422
spiral imaging
A technique for fast image acquisition that uses sinusoidally changing gradients to trace a corkscrew trajectory through k-space.
423
standard deviation
A commonly used measure of the variability within a sample of data points.
424
standard error
A commonly used estimate of the likely discrepancy between a measured value and a true value, often calculated from a measure of variability in the data (i.e., the standard deviation) and the number of data points in the sample.
425
state-related processes
Changes in the brain that are assumed to reflect distinct modes, or states, of function. State-related processes are more easily measured with blocked designs.
426
static contrasts
Contrast mechanisms that are sensitive to the type, number, relaxation properties, and local environment of spins (e.g., T1, T2, proton density).
427
static magnetic field (B0)
The strong magnetic field at the center of the MRI scanner whose strength does not change over time. The strength of the static magnetic field is expressed in teslas (T).
428
statistical map (statistical parameter map)
In fMRI, the labeling of all voxels within the image according to the outcome of a statistical test.
429
statistical power
The probability of detecting an effect of the experimental manipulation.
430
stereotaxic space
A precise mapping system (e.g., of the brain) using three-dimensional coordinates.
431
structural connectivity
A pattern of structural connections between regions based on known axonal projections.
432
structural equation modeling (SEM)
A statistical approach for testing a model of the relationships among a set of independent and dependent variables.
433
structural neuroimaging
A class of research and clinical techniques that create images of the brain’s physical structure, often to provide insight into the locations and distribution of different types of tissue.
434
subsequent memory
An approach to fMRI analyses that sorts experimental stimuli based on whether they were remembered or forgotten in a later testing session
435
superconducting electromagnets
A set of wires made of metal alloys that have no resistance to electricity at very low temperatures. By cooling the electromagnet to near absolute zero, a strong magnetic field can be generated with minimal electrical power requirements.
436
superparamagnetic iron oxide (SPIO) nanoparticles
A new class of exogenous contrast agents that are super paramagnetic and can greatly enhance the imaging contrast when injected into humans.
437
superposition
A principle of linear systems that states that the total response to a set of inputs is equivalent to the summation of the independent responses to the inputs.
438
support vector machines (SVMs)
A class of algorithm used in pattern classification that attempts to identify the combination of features in the original data set that can most effectively differentiate between two categories.
439
surface coil
A radiofrequency coil that is placed on the surface of the head, very close to the location of interest. Surface coils have excellent sensitivity to the signal from nearby regions but poor sensitivity to signal from distant regions.
440
susceptibility artifacts
Signal losses on T2-dependent images due to magnetic field inhomogeneities in regions where air and tissue are adjacent.
441
Sylvian fissure
The deep sulcus separating the temporal lobe from the frontal and parietal lobes.
442
synapse
A junction between neurons where the presynaptic process of an axon is apposed to the postsynaptic process of a dendrite or cell body.
443
synaptic cleft
A gap between presynaptic and postsynaptic membranes.
444
synaptic plasticity
A change in the strength of a synapse as a consequence of functional activation.
445
system noise
Fluctuations in MR signal intensity over space or time that are caused by imperfect functioning of the scanner hardware.
446
T1 (recovery)
The time constant that describes the recovery of the longitudinal component of net magnetization over time.
447
T1-weighted (T1-dependent)
Images that provide information about the relative T1 values of tissue
448
T2 (decay)
The time constant that describes the decay of the transverse component (i.e., transverse relaxation) of net magnetization due to accumulated phase differences caused by spin–spin interactions.
449
T2-weighted (T2-dependent)
Images that provide information about the relative T2 values of tissue
450
T2* (decay)
The time constant that describes the decay of the transverse component of net magnetization due to both accumulated phase differences and local magnetic field inhomogeneities. T2* is always shorter than T2. BOLD-contrast fMRI relies on T2* contrast.
451
T2* blurring
Distortions in T2* images that result from having a data acquisition window that is sufficiently long that significant T2* decay occurs over that interval.
452
T2*-weighted (T2 *-dependent)
Images that provide information about the relative T2* values of tissue. T2*-weighted images are commonly used for BOLD-contrast fMRI.
453
Talairach space
A commonly used space for normalization of fMRI data
454
task frequency
The rate of presentation of a periodic experimental task.
455
TCA cycle
The tricarboxylic acid cycle, the second step in aerobic glycolysis. The TCA cyle, also known as the citric acid cycle or the Krebs cycle, involves the oxidation of pyruvate.
456
TE
See echo time
457
telencephalon (forebrain)
The largest and most evolutionarily advanced region of the human brain, containing the cerebral hemispheres, including the cerebral cortex and important subcortical structures such as the hippocampus, amygdala, and basal ganglia.
458
temporal derivative
A regressor that when added to a model improves the robustness of that model to small variations in the timing of the hemodynamic response.
459
temporal ICA
A form of independent component analysis that generates components that have minimal temporal redundancy.
460
temporal interpolation
The estimation of the value of a signal at a time point that was not originally collected, using data from nearby time points.
461
temporal lobe
The lobe on the ventral surface of the cerebrum
462
temporal resolution
The ability to distinguish changes in a signal (or map) across time.
463
Tensor
A collection of vector fields governed by three principal axes.
464
testing set
In pattern classification analysis, a novel part of the data set used to evaluate the robustness of the classification algorithm.
465
thalamus
A structure composed of a number of nuclei that are important for relaying many aspects of perception and cognition. The nuclei of the thalamus are highly interconnected with many regions of the cerebral cortex.
466
thermal noise
Fluctuations in MR signal intensity over space or time that are caused by the thermal motion of electrons within the sample or scanner hardware.
467
time course
The change in MR signal over a series of fMRI images.
468
time domain
The expression of a signal in terms of its intensity at different points in time.
469
time-locking
Synchronization of analyses to events of interest, usually for the extraction of data from specific epochs in time around those events.
470
time-of-flight (TOF) MRA
A type of magnetic resonance angiograpy that generates contrast by suppressing the signal from spins within an imaging plane so that voxels with inflowing spins (i.e., those with blood vessels) have a high signal.
471
time series
A large number of fMRI images collected at different points in time.
472
torque
A force that induces rotational motion.
473
Torsion
Rotation (twisting) of an object. Even if the motion of an object is restricted so that it cannot translate, a strong magnetic field will still exert a torque that may cause the object to rotate so that it becomes aligned with the magnetic field.
474
TR
See repetition time
475
Tractography
The identification and measurement, often using diffusion tensor imaging, of white-matter tracts that connect distant brain regions.
476
training set
In pattern classification analysis, that part of the data set used to develop the classification algorithm.
477
transcranial direct current stimulation (tDCS)
An inexpensive and safe neuroscience technique in which a weak electrical current is generated between a pair of electrodes on the scalp
478
transcranial magnetic stimulation (TMS)
A technique for temporarily stimulating a brain region to disrupt its function. TMS uses an electromagnetic coil placed close to the scalp
479
Translation
The movement of an object along an axis in space (in the absence of rotation).
480
transmissive activity
The relaying of the outcome of an integrative process from one neuron to another, typically through signals sent via axons.
481
transmitter coil
An electromagnetic coil that generates an oscillating magnetic field at the resonant frequency of atomic nuclei within a sample.
482
transverse
Perpendicular to the main magnetic field of the scanner, in the x–y plane.
483
transverse relaxation (spin–spin relaxation)
The loss of net magnetization within the transverse plane due to the loss of phase coherence of the spins.
484
tripartite synapse
A synapse formed by a presynaptic axon and a postsynaptic dendrite, with the addition of the astrocytic process that ensheathes and modulates the synapse.
485
type I error
Rejecting the null hypothesis when it is in fact true. Also known as a false positive.
486
type II error
Accepting the null hypothesis when it is in fact false. Also known as an incorrect rejection or false negative.
487
undershoot
The decrease in MR signal amplitude below baseline due to the combination of reduced blood flow and increased blood volume.
488
unknown risks
Possible negative outcomes that, although not anticipated based on the best available science, nevertheless influence research or clinical procedures. Most fMRI centers do not scan pregnant women because of potential unknown risks to the developing fetus.
489
vascular conducted responses
The rapid spread of vasodilation or constriction along small blood vessels that is not dependent on neural activity or a passive response to changes in blood flow.
490
vascular steal
The idea that increased blood flow in one region comes at the cost of decreased blood flow in a closely adjacent region
491
vascular tone
The degree to which a blood vessel resists blood flow.
492
vasoactive substances
Substances that change the diameter of blood vessels.
493
vector
A quantity with both magnitude and direction. Vectors are in boldface in this text.
494
veins
Blood vessels that carry blood from the body to the heart. Blood in the veins (except for the pulmonary vein) is partially deoxygenated.
495
velocity-encoded phase contrast (VENC-PC) MRA
A type of magnetic resonance angiography that uses gradient fields to induce phase differences associated with vascular flow so that the flow velocity of vessels can be measured.
496
ventral
Toward the front (belly) of the body, or toward the bottom of the brain.
497
ventricles
Fluid-filled cavities within the brain.
498
venules
Small veins.
499
voltage-dependent ion channel
An ion channel that opens or closes in response to changes in membrane potential.
500
volume coil
A radiofrequency coil that surrounds the entire sample, with roughly similar sensitivity throughout.
