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http://www.cis.rit.edu/htbooks/mri/gloss.htm
Aliasing: An image artefact where one part of the image overlaps with another. Generally caused by defining a FOV that is smaller than the object in the PE direction.
B0: The constant, homogeneous magnetic field used to polarize spins, creating magnetization. This can refer to both the direction and the magnitude of the field. The direction of B0 defines the longitudinal axis.
B1: An RF energy field applied perpendicular to the longitudinal axis (B0) to perturb the magnetization in some manner (e.g., excitation pulses, inversion pulses, etc).
Bandwidth: Generally, bandwidth refers to a contiguous range of frequencies. In MRI, it most often refers to the range of frequencies established across the object by imaging gradients during acquisition. In this case, it impacts on SNR and image distortion. It also refers to the range of frequencies excited by an RF pulse during a slice selection.
BOLD, or blood oxygenation level dependent: The BOLD effect is the source of contrast in FMRI. The presence of deoxygenated blood leads to signal loss due to a reduction in T2*.
CNR, or contrast-to-noise ratio: The image contrast divided by the standard deviation of the image noise. Increasing CNR increases perception of the differences between the tissues or regions of interest.
Contrast: The difference in signal intensity between two tissues or regions of interest.
Dephasing: In an ensemble of spins, each has a phase angle in the transverse plane. The ensemble can either be coherent (have the same angle) or incoherent (varying angle). Loss of coherence is referred to as "dephasing", which leads to net signal loss. BOLD and diffusion contrast are both based on dephasing.
Echo time (TE): The time between the excitation of magnetization and the acquisition of signal. For long acquisition windows, the TE is usually defined as the point at which the acquisition is closest to the center of k-space.
EPI, or echo-planar imaging: A fast imaging method that acquires multiple k-space lines following a single excitation. Most commonly acquiring data for an entire 2D image ("single shot" EPI).
Ernst angle: The flip angle at which maximum signal or contrast is achieved.
Excitation: Rotation of magnetization out of alignment with the longitudinal axis, caused by the application of an RF pulse. Generally used to refer to flip angles less than 90 degrees, but also can simply refer to the creation of any transverse (xy) magnetization.
Field of view, or FOV: The spatial extent of an acquired image.
Flip angle: The angle of rotation away from the longitudinal axis experienced by magnetization due to the application of an RF pulse.
FMRI, or Functional MRI: The detection of neuronal or metabolic activity, most typically via on the BOLD effect.
Fourier transform (FT): The mathematical operation that relates a signal or object to its frequency components, and vice versa. In MRI, the Fourier transform relates the image to k-space data. Sometimes referred to as 2DFT (for 2D images) or a 3DFT (for 3D), and is done using the Fast Fourier Transform (FFT) algorithm.
Frequency encode (FE) direction: The direction along which individual lines are acquired in k-space. Note that one may also refer to the frequency encode direction in the image (i.e., if the FE direction is along kx, one may also refer to the x direction in image space as the FE direction). See also phase encode (PE) direction.
Gradient: Spatially varying magnetic field used to manipulate the resonance frequency across an object. MRI scanners incorporate three linearly-varying gradient fields across x, y and z. The strength of the gradient is controlled by the pulse sequence and can be rapidly manipulated (or "switched").
Gradient echo, or GRE/GE: Any MRI sequence that detects an un-refocused signal (i.e., a non-spin-echo sequence). This is particularly unfortunate choice of terminology.
Inversion: Rotation of magnetization from alignment with B0 (along +z) to anti-alignment (along Mz), caused by an RF pulse with 180 degree flip angle. Note that both refocusing and inversion use flip angles of 180 degree; the difference is the intended consequence of the RF pulse. In practice, angles less than 180 degrees can be used.
Inversion time (TI): The time between inversion of magnetization and its excitation (often to acquire signal that has been "prepared" with an inversion pulse).
k-space: The conceptual space in which MRI images are acquired. Data in k-space provides a map of the amount of structure in the image that can be attributed to each spatial frequency. The image and k-space are related by the mathematical operation called the Fourier transform (FT).
Larmor frequency: See resonant frequency.
Longitudinal axis (z): The direction parallel to the main magnetic field (B0), which represents the direction along which magnetization is in equilibrium. After an RF pulse, the magnetization recovers to this equilibrium according to the rate T1. The component of magnetization along z cannot be detected.
Magnetization: The net magnetic moment induced in an object or tissue when exposed to an external magnetic field. The magnetization is at equilibrium when it is aligned parallel to the external field (along the longitudinal axis) and at its maximum magnitude. MRI experiments manipulate the magnetization away from this equilibrium, into the transverse plane to detect its signal.
Phase: In MRI, phase most commonly refers to the angle of the magnetization in the transverse plane.
Phase encode (PE) direction: The direction perpendicular to individual lines in k-space. Note that one may also refer to the phase encode direction in the image (i.e., if the PE direction is along ky, one may also refer to the y direction in image space as the PE direction). Image artefacts are often most severe along the PE direction. See also frequency encode (FE) direction.
Polarization: The tendency for spins to align in the presence of an external magnetic field, creating a net magnetic moment (or, magnetization).
Precession: Gyration of a spinning body, which traces out a cone about the axis of precession. In MRI, the magnetization that is out of alignment with the longitudinal axis precesses about this axis.
Preparation: Any manipulation of magnetization prior to its acquisition in order to change its signal level. For example, "fluid inversion" preparations invert the magnetization prior to acquisition in order to remove signal from fluid.
Proton density (PD): The concentration of mobile protons in a tissue. In PD-weighted imaging, the contrast is determined by the relative density of protons in different tissues (largely, the amount of free water).
Pulse sequence: The series of RF pulses, gradient field amplitudes and acquisition periods applied to acquire an image. Also used to refer to sets of sequences with common properties (e.g., the family of spin echo pulse sequences include structural and diffusion variants).
R1: The expression of time constant T1 as a rate: R1=1/T1.
R2: The expression of time constant T2 as a rate: R2=1/T2.
Relaxation: The process by which the magnetization slowly returns to equilibrium following the rotation of the magnetization away from the longitudinal axis with an RF pulse. Relaxation (decay) in the transverse plane has the characteristic time T2; relaxation (recovery) along the longitudinal axis has characteristic time T1.
Refocusing: The rotation of magnetization in the transverse plane by 180 degrees, usually with the intention of removing the effect of field inhomogeneities on signal. Note that both refocusing and inversion involve flip angles of 180 degrees, and that the difference is intended consequence of the RF pulse. See also spin echo.
Repetition time (TR): The time between repeated excitations of a given component of the magnetization. For multi-slice sequences that sequentially excite different imaging slices, the TR is the time between repeated excitations of the same slice. For volume-excite (3D) sequences that repeated excite the entire imaging volume, the TR is the time between repeated excitations of the volume.
Resonant frequency: The frequency at which the magnetization can be excited and detected. The frequency varies directly with magnetic field strength, and is normally in the radio frequency (RF) range. Also called resonance frequency or Larmor frequency.
RF: Energy in the radiofrequency range. RF is significant in MRI because the resonant frequency of magnetization is in this range.
RF pulse: A brief transmission of energy in the RF range. In MRI, RF pulses are used to manipulate the direction of the magnetization (e.g., to excite or invert).
SAR, or specific absorption ratio: Absorption of RF energy by tissue, which induces heating and can lead to tissue damage. One of the major safety issues in MRI.
Saturation: Removal or reduction of a component of the magnetization using excitation into the transverse plane. Most commonly used to in the context of "T1 saturation", where repeated excitations of magnetization result in incomplete T1 recovery, and therefore signal reduction. Can also refer to an excitation followed by a "spoiling" mechanism, which deliberately removes signal.
Shim, or shimming: The process of improving field homogeneity by compensating for imbalances in the main magnetic field of an MRI system. Accomplished by a combination of constant ("passive") shims and controllable ("active") shim coils.
SNR, or signal-to-noise ratio: The ratio of the signal in a given tissue of interest divided by the standard deviation of the image noise.
Spin: The property exhibited by atomic nuclei that contain an odd number of protons and/or neutrons. The property spin causes nuclei to behave as though they are spinning charges. These nuclei are often referred to colloquially as "spins".
Spin echo, or SE: Any MRI pulse sequence characterized by the use of a refocusing pulse to reverse the effect of off-resonance precession. The signal is said to form a spin echo when the off-resonance precession of all spins is reversed, such that the spins re-align to form a signal peak (or "echo").
T1: The time constant defining the rate of recovery of magnetization along the longitudinal (z) axis following an RF pulse. Also called the spin-lattice relaxation time.
T2: The time constant defining the irreversible loss of magnetization (and therefore signal) in the transverse (xy) plane following excitation. Also called the spin-spin relaxation time. In tissue, T2 is shorter than T1 (often by an order of magnitude). This signal loss cannot be recovered by a spin echo.
T2*: The time constant defining the loss of signal following excitation. Two components contribute to T2*. First, some signal loss occurs due to T2 relaxation (i.e., loss of magnetization). Second, some signal loss is caused by variation in precession angles for different spins within a voxel. This does not represent loss of magnetization, and this component can be recovered wiwth a spin echo.
TE: See echo time
TI: See inversion time
TR: See repetition time
Transverse plane: The plane orthogonal to the longitudinal axis, denoted as the xy plane. It is defined by the direction of the main (B0) field, and the plane in which the magnetization is observable (i.e., where signal can be detected).
Trajectory: The pathway in k-space an acquisition takes in order to acquire image data.
Voxel: A resolution element in a 3D imaging experiment (coming from the term "volume element"); the 3D extension of a pixel (a "picture element")