MRI_BOOK Flashcards
acetylcholine (ACh)
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.
action potential
A self-propagating wave of depolarization that travels down a neuronal axon.
adaptation
A change in the response to a stimulus following its repeated presentation.
adenosine triphosphate (ATP)
A nucleotide containing three phosphate groups that is the primary energy source for cells in the human body.
aerobic glycolysis
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.
Aliasing
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.
alternating design
A blocked design in which two conditions are presented one after another for the duration of the experimental run.
γ-aminobutyric acid (GABA)
One of the most important inhibitory neurotransmitters.
anaerobic glycolysis
The conversion of glucose to lactate in the absence of oxygen.
anastomosis
The branching and reconnection of blood vessels.
anatomical ROI
A region of interest (ROI) chosen based on anatomical criteria.
angular momentum (J)
A quantity given by multiplying the mass of a spinning body by its angular velocity.
Anisotropic
Having different properties in different directions
antiparallel state
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.
apical dendrites
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.
apparent diffusion coefficient (ADC)
The quantification of diffusivity assuming isotropic diffusion.
arachnoid
The middle membrane covering the brain
arterial spin labeling (ASL)
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.
arteries
Large, thick-walled blood vessels that carry oxygenated blood from the heart to the rest of the body.
arterioles
Small arteries.
ascending/descending slice acquisition
The collection of data in consecutive order such that slices are acquired sequentially from one end of the imaging volume to the other.
astrocyte
A type of glial cell that regulates the extracellular environment. It is the most prevalent glial cell type in the brain.
Autoradiography
Imaging by injecting a radioactive substance into tissue, then exposing the tissue to X-ray-sensitive film.
averaged epoch
The result of averaging a large number of epochs that are time-locked to similar events.
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.
B
The sum of all magnetic fields experienced by a spin.
B0
The strong static magnetic field generated by an MRI scanner.
B1eff
The effective magnetic field experienced by a spin system during excitation.
b factor
The degree of diffusion weighting applied within a pulse sequence.
balloon model
A model of the interaction between changes in blood volume and changes in blood flow associated with neuronal activity.
basal ganglia
A set of nuclei in the forebrain that includes the caudate, putamen, and globus pallidus.
base image
The image on which a statistical map is displayed, often a high-resolution anatomical image.
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.
between-subjects manipulation
A manipulation in which different conditions are assigned to different subject groups.
bias field estimation
A technique for estimating inhomogeneities in the magnetic field based on intensity variations in collected images.
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.
Biomarker
A phenotypic feature, whether physical, physiological, or behavioral, that provides a robust predictor of some experimentally or clinically important outcome.
biophysical property
A biological property (e.g., blood flow) whose physical parameters undergo measurable changes in response to internal processes (e.g., information processing).
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.
blood-oxygenation-level dependent (BOLD) contrast
The difference in signal on T2*-weighted images as a function of the amount of deoxygenated hemoglobin.
Bolus
A quantity of a substance that is introduced into a system and then progresses through that system over time.
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.
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.
brain stem
The midbrain, pons, and medulla.
Brodmann areas
Divisions of the brain based on the influential cytoarchitectonic criteria of Korbinian Brodmann.
calcium-sensitive contrast agents
A type of contrast agent with its ability for signal enhancement triggered by binding to calcium ions (Ca2+).
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.
categorical variable
A variable that can take one of several discrete values.
cathode
An attractor for positive charge or ions, and a source of free electrons.
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.
central nervous system (CNS)
The brain and spinal cord.
central sulcus
A deep fissure that separates the frontal and parietal lobes of the brain.
Cerebellum
A large cortical structure at the caudal base of the brain that plays an important role in motor function.
cerebral blood volume (CBV) contrast
A type of fMRI contrast that is sensitized to the cerebral blood volume changes subsequent to brain activation.
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.
cerebrospinal fluid (CSF)
A colorless liquid that surrounds the brain and spinal cord and fills the ventricles within the brain.
cerebrum (cerebral hemispheres)
The largest and most rostral component of the mammalian brain, composed of a left and a right hemisphere.
chemical shift imaging
A technique for measuring the concentration of particular chemicals, based on subtle differences in the resonance of the protons they contain.
circle of Willis
The interconnection between the basilar artery and the carotid arteries at the base of the cranial vault.
cluster-size thresholding
The adoption of a minimum size, in voxels, for a cluster of active voxels to be labeled as significant.
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.
collinear regressors
Model factors that are highly correlated with one another. The inclusion of collinear regressors reduces the validity of general linear model analyses.
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.
Component
In the context of data-driven analyses, a feature of a data set that represents some aspect of its intrinsic structure.
compressed sensing
A new technique that takes sparse (i.e., underdetermined) data and, by estimating the sparseness of the data, reconstructs the original signal.
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.
conditions (levels)
Different values of an independent variable.
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.
continuous ASL
A type of perfusion imaging that uses a second transmitter coil to label spins within an upstream artery while collecting images.
continuous variable
A variable that can take any value within a range.
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.
contrast agent
A substance injected into the body to increase image contrast.
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.
contrast-to-noise ratio (CNR)
The magnitude of the intensity difference between different quantities divided by the variability in their measurements.
control block
A time interval that contains trials of the control condition.
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.
converging operations
Employing two or more techniques to provide complementary evidence used to test an experimental hypothesis or scientific theory.
Coregistration
The spatial alignment of two images or image volumes.
coronal
A frontal view of the brain (along the x–z plane in MRI).
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.
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.
correlation coefficient (r-value)
A number between –1 and 1 that expresses the strength of the correlation between two variables.
cortex
The thin wrapping of cells around the outer surface of the brain.
cortical layers (cortical laminae)
The six cellular layers of the neocortex, distinguished by differences in the types, densities, sizes, and shapes of their neurons.
cost function
A quantity that determines the amount of residual error in a comparison.
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.
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.
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.
Cryogens
Cooling agents used to reduce the temperature of the electromagnetic coils in an MRI scanner.
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.
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.
current source
A source of positive ions.
cytoarchitecture
The organization of the brain into physically distinguishable regions on the basis of cellular structure.
data-driven analysis
Drawing inferences based on examination of the intrinsic structure of data.
deactivation
Decrease in BOLD activation during task blocks compared with non-task blocks.
dB/dt
The change in magnetic field strength (dB) over time (dt).
deception
The use of experimental procedures that intentionally mislead subjects.
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.
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.
de-identification
Removal of information that may link research data or medical information to a particular individual.
dendrite
A neuronal process that receives signals from other cells. A neuron typically has multiple dendrites, which perform a primarily integrative function.
deoxygenated hemoglobin (dHb)
Hemoglobin without attached oxygen
dependent variables (DVs)
Quantities that are measured by the experimenter to evaluate the effects of the independent variables.
Depolarization
A change in the cell membrane potential caused by admitting positive charge into the cell and thus reducing its negative resting potential.
descriptive statistics
Statistics that summarize the sample data but do not allow inferences about the larger population.
design matrix
In fMRI implementations of the general linear model, the specification of how the model factors change over time.
detector coil
An electromagnetic coil that measures energy emitted back to the environment after its initial absorption by the sample.
diamagnetic
A property of a substance that opposes a magnetic field (i.e., decreases the strength of the magnetic field).
diencephalon
Region of the brain lying just rostral to the midbrain and containing the hypothalamus and thalamus.
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.
diffusion
The random motion of molecules through a medium over time.
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.
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.
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.
distribution
The pattern of variation of a variable under some conditions. For example, the normal distribution has a characteristic bell shape.
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).
dot product
The scalar product of two vectors. It is created by summing the products along each dimension.
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.
dura
The outermost membrane covering the brain
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.
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.
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.
Eigenimage
Spatial maps, as generated by a principal components analysis, that reflect orthogonal components of a complex image (or time series of images).
Eigenvalue
A mathematical description of the amount of variability in a data set that is accounted for by a given component.
Eigenvector
Following principal components analysis, a set of values that describes a component of the intrinsic variability in a data set.
electric dipole
A point in space whose electrical properties can be approximated by assuming that it consists of physically separated positive and negative charges.
electroencephalography (EEG)
The measurement of the electrical potential of the brain, usually through electrodes placed on the surface of the scalp.
electrogenesis
The generation of electrical electrophysiological phenomena by a living organism.
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.
electron transport chain
The third step in aerobic glycolysis
endogenous contrast
Contrast that depends on an intrinsic property of biological tissue.
Epiphenomenal
A secondary consequence of a causal chain of processes, but playing no causal role in the process of interest.
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.
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.
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.
Estimation
Measurement of the pattern of change over time within an active voxel in response to the experimental manipulation.
event-related design
The presentation of discrete, short-duration events whose timing and order may be randomized.
event-related potentials (ERPs)
Small electrical changes in the brain that are associated with sensory or cognitive events.
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.
evoked potential
A field potential that occurs in response to a sensory stimulus.
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.
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.
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.
excitatory postsynaptic potential (EPSP)
A depolarization of the postsynaptic cell membrane.
Excitotoxicity
Damage or death of neurons caused by excess concentrations of glutamate and other substances.
exogenous contrast
Contrast that requires the injection of a foreign substance into the body.
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).
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.
experimental condition
A condition that contains the stimulus or task that is most relevant to the research hypothesis. Also called the task condition.
experimental design
The organization of an experiment to allow effective testing of the research hypothesis.
experimental regressors
Model factors that are associated with specific experimental hypotheses.
false discovery rate (FDR)
The probability of having at least one false-positive result, given the set of reported positive results.
family-wise error rate (FWER)
The probability of making at least one type I error, given the total number of statistical tests.
feature selection
An initial step in pattern classification that involves the determination of which input variables should be included in the classification algorithm.
fiber tracts
Bundles of axons that convey signals, in the form of action potentials, from one brain region to another.
field map
An image of the intensity of the magnetic field across space.
field of view (FOV)
The total extent of an image along a spatial dimension.
field potentials
Changes in electrical potential over space associated with postsynaptic neuronal activity.
field strength
The magnitude of the static magnetic field generated by a scanner, typically expressed in teslas.
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.
filling k-space
The process of collecting samples from throughout k-space to collect data sufficient for image formation.
Filter
Within the context of fMRI, an algorithm for removing temporal or spatial frequency components of data.
finite impulse response (FIR)
A signal processing approach that treats each time unit with a separate function (i.e., an impulse)
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.
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.
flip angle
The change in the precession angle of the net magnetization following excitation.
Flux
A measure of the strength of a magnetic field over an area of space.
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.
forward inference
Reasoning from the experimental manipulation (i.e., changes in the independent variable) to infer the effects on a dependent variable.
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.
Fourier transform
A mathematical technique for converting a signal (i.e., changes in intensity over time) into its power spectrum.
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.
frequency domain
The expression of a signal in terms of its power at different frequencies.
frequency-encoding gradient
A gradient that is applied during the data acquisition period so that the spin precession frequencies change over space.
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.
functional contrast
A type of contrast that provides information about a physiological correlate of brain function, such as changes in blood oxygenation.
functional hyperemia
The local increase in blood flow that occurs in response to a sensory, motor, or cognitive event.
functional magnetic resonance imaging (fMRI)
A neuroimaging technique that uses standard MRI scanners to investigate changes in brain function over time.
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.
functional resolution
The ability to map measured physiological variation to underlying mental processes or behaviors.
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.
GABA (γ)-aminobutyric acid)
One of the most important inhibitory neurotransmitters.
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.
glass-brain view
A two-dimensional projection of fMRI data, as if the brain were made transparent and only the activations were visible.
glial cells (glia)
Brain cells that support the activities of neurons but are not primarily involved with information transmission.
glutamate
The most common excitatory neurotransmitter in the brain.
Glycolysis
The process of breaking down glucose into other compounds to produce ATP.
gradient coils
Electromagnetic coils that create controlled spatial variation in the strength of the magnetic field.
gradient-echo (GRE) imaging
One of the two primary types of pulse sequences used in MRI
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.
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.
hemifield
One-half of a visual display, usually referring to the left half or the right half relative to fixation.
Hemodynamic
Having to do with changes in blood flow or other blood properties.
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.
Heteroscedastic
Having the property that the distributions of noise are different across experimental conditions.
Homogeneity
Uniformity over space and time.
homoscedastic
Having the property that the distributions of noise are similar for all experimental conditions.
homotopic
The cortex in one cerebral hemisphere that corresponds to the same region in the other hemisphere.
hyperpolarization
A change in the cell membrane potential caused by admitting negative change into the cell and thus increasing its negative resting potential.
hypothesis-driven analysis
The evaluation of data based on statistical tests of the validity of a null hypothesis.
