List of commonly used MRI terms and definitions. A reference resources that explains key concepts in magnetic resonance imaging. This resource is an addition to our MRI technologist resources library, where we feature MRI operator manual procedures and technical documents.
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MRI Terminology
(A-E) MRI Terms and Definitions
3D Multi Slab
An image mode used in Time-of Flight vascular imaging for acquiring multiple overlapping 3D slabs.
90° Pulse
A pulse that rotates the magnetization vector 90° from longitudinal static magnetic field direction.
This converts the longitudinal magnetization into transverse magnetization.
Anterior/Posterior (A/P)
A patient positioning selection designating the coronal plane alignment in order to ensure that the
center of the region of interest is as close as possible to isocenter. The coronal plane divides the
body into anterior (front) and posterior (back) sections.
Artifact
An error in the reconstructed image that does not correspond to the patient. There are three major
forms of artifacts that can occur in MR imaging and contribute to poor image quality: geometric
distortion, inhomogeneous signal intensity, and spurious signal.
Asymmetric Echo
An echo whose peak, at TE, is not centered in the sampling window. Also called fractional echo or
partial echo.
Asymmetric Field of View (AFOV)
- An FOV in which the vertical and horizontal dimensions are not equal. Similar to the rectangular
FOV selected. - An imaging enhancement activated by choosing one of two FOV options: square pixel or variable
FOV. Asymmetric FOV is useful for any scan which has anatomy smaller than the FOV in the phase
direction. See FOV and square pixels.
Available Imaging Time (AIT)
In cardiac gating, the time during which data can be collected by the MR system.
Average Flow
An Flow Analysis measurement. Summation of voxel values in a given flow region (ml/min),
reflecting the volume per minute passing through the defined flow region of a specified cardiac
phase or cycle.
Average Velocity
A Flow Analysis measurement. Flow Q (expressed in cm3/sec) divided by the cross-sectional area A
(expressed cm2) of a vessel: V = Q/A (cm/sec); 1/2 Vmax for laminar flow.
Averaging
A SNR-enhancing technique in which the same MR signal is added up, and then the sum is divided by
the number of signals acquired.
Bandwidth
A range within a band of frequencies that an MRI system is “tuned” to receive. The receive
bandwidth of an image determines the number of frequencies encompassed in the image. The
system’s bandwidth choice depends on the TE, matrix, and FOV you select. Bandwidth is a Scan
Timing parameter that lets you narrow the system’s receiver bandwidth to increase SNR. Narrowing
the bandwidth forces the system to detect signals from a small range of frequencies. This means
the system discards more random electronic noise, improving SNR. The system narrows the
Bandwidth only as much as the selected TE allows.
Beats per Minute (bpm)
The average heart rate as shown by the cardiac waveform display.
Bipolar Flow-Encoding Gradients
Two gradient pulses of identical shape, but opposite polarity. Used in order to encode velocities as
changes of phase, as used in Phase Contrast angiography.
Cardiac Phase Images
Images demonstrating different times or phases within a cardiac cycle.
Cine
Generated images for dynamic views of anatomy such as the heart. The Cine option employs
retrospective gating techniques and a Gradient Echo pulse sequence.
Collapsed
A Maximum Intensity Projection (MIP), also called Maximum Pixel Projection (MPP) from TOF
magnitude images, or PC weighted-phase images. The collapsed image is the MIP in the slice
direction.
Complex Difference
A flow reconstruction type for Phase Contrast Vascular Imaging providing control of the Slab
Dephasing Gradient and Phase Correction. Complex Difference reconstructions have the Dephase
Gradient off and Phase Correction on.
Contrast Resolution
An image function providing the ability to differentiate anatomical density differences with respect to
surrounding anatomical regions.
Contrast-to-Noise Ratio (CNR)
Ratio of the absolute difference in intensities between two regions to the level of fluctuations in
intensity due to noise.
Coronal
The horizontal plane along the longitudinal axis of the body dividing it into anterior (front) and
posterior (back) halves.
Decubitus Position
Describes the position of a patient lying on the left or right side.
Diastole
The period between the end of the T-wave and the beginning of the R-wave in the cardiac cycle. Also
called ventricular filling.
Dynamic-Range Compression
A method of enhancing Phase Contrast image quality by applying a projection Dephasing Gradient to
suppress signal from stationary tissues.
Echo Rephasing
Re-establishment of spin phase coherence, accomplished via a 180 degree RF pulse or gradient
switching. See Refocusing.
Echo Train Length (ETL)
The number of 180° refocusing pulses played out during one TR period.
Effective R-R interval (RR)
The inverse of bpm (Beats per Minute) measured in msec: RR = 60,000 divided by bpm.
Effective TR
The “average” repetition time, or TR, in cardiac gating. Measured as the number of RR intervals
between successive excitations of a particular slice location – e.g., RR, 2xRR, 3xRR, 4xRR.
Effective value
A typical or average value – for example, effective TR. Since you can not control your patients heart
rate, you can not control true TR in a gated study. You can control the effective TR by telling the
system not to trigger at every beat.
Even-Echo Rephasing
Rephasing of moving spins on symmetric, even echoes (e.g., 2, 4, or 6) in Multi-Echo sequences.
Extended Dynamic Range (EDR)
An imaging enhancement that uses 32-bit processing instead of the conventional 16-bits to improve
SNR.
(F-I) MRI Terms and Definitions
F/W
See Fat/Water Suppression.
Fast Cardiac Gating (FastCard)
A 2D Time-of Flight, gradient recalled, single-breath PSD for acquiring multiple phases of the cardiac cycle at a single slice location.
Fat/Water Suppression (F/W)
An imaging enhancement technique that suppresses signal within the imaging volume from either fat or water by applying a frequency-selective saturation pulse. See more GE MRI Fat Sat.
FID (Free Induction Decay)
The measurable magnetic resonance signal that occurs as the transverse magnetism, produced by the application of the 90° pulse, decays toward zero.
Field of View (Acquisition FOV)
The area of the anatomy being imaged, usually expressed in centimeters. FOV image size is a function of the acquisition matrix times the pixel size.
First-Order Phase Correction
Phase errors in a Phase Contrast image can be modeled as a linear shading across the image in the x and y direction. In first order Phase Correction, the slopes of the x and y shading are determined to reduce the shading.
Flip Angle
Flip angle is the rotational angle of the magnetization vector produced by a RF pulse relative to the
longitudinal axis of the static magnetic field. Flip angle adjusts contrast.
Flow Analysis
A flow reconstruction type for Cine-PC and 2D PC providing control of the Slab Dephasing Gradient
and Phase Correction. Flow Analysis reconstructions have the Dephase Gradient off and Phase
Correction off.
Flow Axis
The orthogonal axis (S/I, R/L, A/P) for which flow has been encoded in a flow image.
Flow Compensation (Flow Comp)
An imaging enhancement using the system’s gradients to put flowing protons into phase with
stationary protons, thereby reducing flow artifacts. Applied in the slice and frequency directions.
Flow Encoding
A technique used in MR to measure or display motion such as blood flow within vessels.
Flow Image Set
An image type produced by Phase Contrast scans. Flow images are phase images that may or may
not be magnitude weighted. If magnitude weighted, a multiplicative magnitude mask for noise
suppression is applied to the phase image. Phase correction and scaling for velocity encoding may
also be applied to the image. The default is for correction to take place, but correction can be turned
off by selecting the Flow Analysis reconstruction mode on the vascular options screen.
Flow Recon Type
A user-selectable option for selecting a Slab Dephasing Gradient and a Phase Correction technique.
See Phase Difference, Complex Difference, and Flow Analysis.
Flow-Related Enhancement
A process by which the signal intensity of moving fluids, like blood or CSF, can be increased
compared with the signal of stationary tissue. Occurs when unsaturated, fully magnetized spins
replace saturated spins between RF pulses.
FOV Center
The center of a scan image, which is ideally located at the magnet’s isocenter.
Fractional Echo
A feature instructing the system to collect just part of the data it normally would. Reduces
susceptibility and flow artifacts.
Fractional NEX
A feature instructing the system to use about half or exactly three-quarters of the phase encoding
acquired in conventional imaging. Decreases scan time significantly.
Frequency
The scanning direction associated with the frequency gradient. Usually corresponds to the image’s
long axis.
Gating
An MR technique for imaging rapidly moving anatomy such as the heart. Uses equipment such as a
standard electrocardiograph to trigger data acquisition.
GMN
See Gradient Moment Nulling.
Gradient Echo Imaging
A pulse sequence that reverses gradient polarity to rephase protons and form echoes. Permits short
TRs and flip angles of less than 90° to excite only a portion of the longitudinal magnetization.
Gradient Echo
A pulse sequence that uses pulses of 1° to 180° to excite the protons of interest and rephase them.
Gradient Echo uses gradients rather than conventional RF pulses.
Gradient Moment Nulling (GMN)
The application of gradients to correct phase errors caused by velocity, acceleration or other motion.
First-order gradient nulling is the same as Flow Compensation.
Gradient Moment
In MR angiography, the first moment describes a gradient’s effect on the phase of a spin with
constant velocity; the second moment, its effect on spins experiencing acceleration; the third
moment, its effect on spins experiencing jerk.
Gradient Coil
See https://www.medicalimagingsource.com/mri-gradient-coil/
Gradient-Recalled Acquisition in the Steady State (GRASS)
See Gradient Echo and MPGR Gradient Echo.
Gx, Gy, Gz
Symbols for MR gradients. Subscripts indicate the spatial direction of each gradient.
Intersequence Delay
The time between each image in the cardiac cycle.
Intravoxel Spin-Phase Dispersion
A loss of phase coherence and therefore, signal intensity that can result when a wide spectrum of
flow velocities exist, when higher orders of motion like acceleration are present, or when there are
minor variations in magnetic field homogeneity.
Inversion Recovery (IR)
A pulse sequence that inverts the magnetization and then measures the recovery rate as the nuclei
return to equilibrium. This rate of recovery depends on T1.
Inversion Time (TI)
The time between the center of the first (180°) inverting pulse and the beginning of the second (90°)
refocusing pulse in an IR pulse sequence.
Isocenter
The point at which the three gradient planes cross.
Isochromats
Spins sharing the same phase and frequency at a given point in time. Isometric Contraction: The
time immediately after the R-wave when the heart prepares for contraction but does not change in
volume.
Isometric contraction
The time immediately after the R-wave when the heart prepares for contraction but does not
change in volume.
(J-R) MRI Terms and Definitions
J-Coupling
Also called Spin-Spin Coupling. The interaction between multiple lines and nuclei. When this
interaction takes places the nuclei split their energy levels according to J (the spin-spin coupling
constant).
Magnetic Field Gradient
A device for varying the strength of the static magnetic field at different spatial locations. This is
used for slice selection and determining the spatial locations of protons being imaged. Also used for
Velocity Encoding, Flow Comp, and in place of RF pulses during Gradient Echo acquisitions to
rephase spins. Commonly measured in gauss per centimeter.
Magnetic Resonance (MR)
The absorption or emission of electromagnetic energy by nuclei in a static magnetic field after
excitation by a suitable RF pulse.
Magnetic Resonance Imaging (MRI)
The creation of images using the magnetic resonance phenomenon. The current application involves
imaging the distribution of hydrogen nuclei (protons) in the body. The image brightness in a given
region usually depends jointly on the spin density and the relaxation times. Image brightness is also
affected by motion such as blood flow.
Magnetic Resonance Signal
The electromagnetic signal (in the radio frequency range) produced by the precession of the
transverse magnetization of the spins. The rotation of the transverse magnetization induces a
voltage in the coil. This voltage is amplified by the receiver.
Magnetization Transfer (MT)
A technique that improves contrast by saturating the short T2 component of tissue such as
gray/white matter and skeletal muscle.
Maximum Intensity Projection (MIP)
A technique for producing multiple projection images from a volume of image data (i.e., 3D volume
or a stack of 2D slices). The volume of image data is processed along a selected angle and the pixel
with the highest signal intensity is projected onto a two-dimensional image.
MSMP
See Multi-Slice, Multi-Phase Imaging.
MSSP
See Multi-Slice, Single-Phase Imaging.
Multi-Planer Gradient Echo (MPGR)
A Gradient Echo pulse sequence that acquires multiple slice locations within the TR period.
Multi-Slice, Multi-Phase (MSMP) Imaging
Multi-slice, multi-phase cardiac gating pulse sequence that produces images at multiple heart
locations and several different cardiac phases at each location.
Multi-Slice, Single-Phase (MSSP) Imaging
Multi-slice, single-phase cardiac gating pulse sequence that produces images at multiple heart
locations, each at a different phase of the cardiac cycle.
Number of Excitations (NEX)
The number of times a pulse sequence is repeated in a given acquisition.
PD-Weighted
See Proton Density-Weighted.
Phase Difference
A flow reconstruction type for Phase Contrast Vascular imaging providing control of the Slab
Dephasing Gradient and Phase Correction. Phase difference reconstructions have the Dephase
Gradient off and Phase Correction on.
Phase Encoding
The act of localizing an MR signal by applying a gradient pulse to alter the phase of spins before
signal readout.
Phase FOV
The Phase Field of View option provides faster scans by scaling down the size of the field of view in
the phase direction. The phase FOV option is not compatible with some PSD and imaging options.
Projection Dephasing Gradient
A gradient applied to diminish signal from stationary tissues in thick slab 2D Phase Contrast
angiography.
Proton Density-Weighted (PD-weighted)
PD-weighted images have contrast that is primarily due to the number of protons in the structures.
PD-weighted images result when scan timing parameters are selected that minimize the T1 (long
TRs) and the T2 (short TEs) contrast effects.
Pulse Length or Width
The duration of a pulse, expressed in milliseconds.
Pulse Sequence Database (PSD)
A series of RF and gradient pulses and the intervals between them used in conjunction with gradient
magnetic fields to produce magnetic resonance images.
Radio Frequency (RF)
The frequency (intermediate between audio and infrared frequencies) used in magnetic resonance
systems to excite nuclei to resonance.
Radiofrequency Pulse (RF Pulse)
A burst of RF energy which, if it is at the correct Larmor frequency, will rotate the macroscopic
magnetization vector by a specific angle. This is dependent on the amplitude and duration of the RF pulse.
Ramp Pulse
An RF excitation pulse that has smaller flip angles for spins flowing into the slab. As spins penetrate
deeper into the slab, the flip angle increases. For example, a 2:1 Ramp Pulse, with a nominal flip
angle of 20°, provides the entry slices with a flip angle of about 13°, and the exit slices with a flip of
about 27°.
Readout Gradient
A gradient pulse, applied when an MR signal is collected, used for frequency encoding.
Refocusing
The re-establishment of phase coherence via gradient or RF pulse. Additionally, see Echo Rephasing, Gradient
Echo, and Gradient Moment Nulling.
Relaxation Time
The time required for 63% of the nuclei to revert to their original state in the magnetic field after the
RF pulse is turned off.
Repetition Time (TR)
The time between successive excitations of a slice. That is, the time from the beginning of one pulse
sequence to the beginning of the next. In conventional imaging, it is a fixed value equal to a userselected value. In cardiac-gated studies, however, it can vary from beat to beat depending on the
patient’s heart rate.
Rephasing Gradient
A gradient applied in the opposite direction of a recent selective excitation pulse, in order to correct
for gradient-induced phase shifts.
RF (Radio Frequency)
The frequency (intermediate between audio and infrared frequencies) used in magnetic resonance
systems to excite nuclei to resonance.
RF pulse
A burst of RF energy which, if it is at the correct Larmor frequency, will rotate the macroscopic
magnetization vector by a specific angle, dependent on the amplitude and duration of the pulse.
R-R Interval
That part of an ECG waveform representing the heart’s electrical activity showing the time between
the peak of one R-wave and the peak of the next one. Therefore, each R-R interval represents the length of one
cardiac cycle.
(S-Z) MRI Terms and Definitions
SAR
Specific Absorption Rate refers to the Radio Frequency power absorbed per unit of mass of an
object (Watts/kg). Absorption of RF energy may result in increased tissue temperature.
Saturation Pulse
A slice-selective RF pulse applied, often followed by a Dephasing Gradient, to saturate spins and
therefore minimize their signal. Used, for example, to minimize signal from flowing blood in the
slice direction.
Saturation
Repeated application of radio frequency pulses in a time that is short compared to the T1 of the
tissue, producing incomplete realignment of the net magnetization with the static magnetic field.
Scan Time
The amount of time needed to acquire data.
Sequential IR
An Inversion Recovery sequence in which the system applies the 180°/90°/180° pulses a slice at a
time. With the alternative, non-sequential, the system applies the initial 180° pulse to all slices, then
returns to each slice to complete the sequence.
Signal-to-Noise Ratio (SNR)
The ratio of signal amplitude to noise – i.e., the amplitude of signal emitted by the patient’s protons,
divided by the amount of patient noises and electronic noise inherent in any electronic instrument.
Slice Select
The scanning direction associated with the slice-select gradient. Usually corresponds to the
direction of the scanning range.
Spatial Encoding
A method by which data is collected in order to formulate a three-dimensional image in a two dimensional plane.
Spin Echo (SE) imaging
A magnetic resonance imaging technique in which the Spin Echo magnetic resonance signal rather
than the Free Induction Decay is used.
Spin-Spin Coupling
See J-Coupling.
Spoiled Gradient Echo (SPGR)
A Gradient Echo pulse sequence designed for acquiring T1-weighted images in 2D or 3D mode.
Spoiler Pulse
A gradient pulse applied to dephase spins and to minimize or eliminate residual signal.
SSFP
See Steady State Free Precession.
Steady State Free Precession (SSFP)
- A Gradient Echo pulse sequence designed for acquiring T2-weighted images in 3D mode.
- A condition achieved by repeatedly exciting an MR sample with phase-coherent RF pulses at a
repetition rate (TR) which is shorter than T2.
T1
The characteristic time constant for the magnetization’s return to the longitudinal axis after being
excited by an RF pulse. Also called Spin Lattice or Longitudinal Relaxation Time.
T1-Weighted
Scan protocols that allow the T1 effects to predominate over the other relaxation effects.
T2
The characteristic time constant for loss of transverse magnetization and MR signal due to T2 and local field inhomogeneties. Since such inhomogeneities are not compensated for by gradient reversal, contrast in gradient-echo images depends on T2.
T2-Weighted
Scan protocols that allow the T2 effects to predominate over the other contrast effects. Additionally, there are
multiple gradient echo pulse sequences that can be used to produce varying T2*-weighted images.
T2
The characteristic time constant for loss of phase coherence among spins, caused by their
interaction, and the resulting loss in the transverse-magnetization MR signal. Also referred to as
Spin-Spin or Transverse Relaxation Time.
TE Min
The shortest possible TE time for a given prescription, used to minimize flow dephasing and T2
effects.
TE1
The time from the middle of the first excitation pulse to the middle of the first readout in an
Asymmetrical Spin Echo pulse sequence.
TE2
The time between the middle of the first excitation pulse and the middle of the second readout in an
Asymmetrical Spin Echo pulse sequence.
Threshold
A technique for setting the desired pixel signal intensity values the system uses to process an image.
Through plane
A flow-encoding direction which is perpendicular to the imaging plane.
TI (Time Inversion)
The time between the center of the 180° inversion pulse and the center of the acquired k-space
segment.
Time of Echo (TE)
The time between the center of the excitation pulse and the peak of the echo, which usually occurs
at the center of the readout.
Time-of-Flight (TOF) Angiography
A 2D or 3D imaging technique that relies primarily on flow-related enhancement to distinguish
moving from stationary spins in creating MR angiograms. Blood that has flowed into the slice will not
have experienced RF pulses and will therefore appears brighter than stationary tissue.
Tmax
Time to maximum value of Residue function, which represents the tracer delay effect at a pixel
TR
Time to Repeat or Repetition Time. The time between successive excitations of a slice. That is, the
time from the beginning of one pulse sequence to the beginning of the next.
Trigger Delay
The time between the occurrence of the triggering pulse and the actual onset of imaging.
Trigger Window (TW)
In cardiac gating, a period during which no further data can be acquired. During this period, the
system waits for the next R-wave trigger, which initiates a new sequence of data acquisition.
Trigger
In cardiac/respiratory gating, signal sent by the cardiac/respiratory monitor to activate data
acquisition.
Velocity Encoding (VENC)
A value entered to prescribe the highest velocities to be encoded without aliasing in Phase Contrast
angiography.
Very Selective Saturation (VSS)
Used in spectroscopy.
Volume Imaging
An acquisition technique in which signal is collected from an entire volume rather than individual
slices. Permits reconstruction of extremely thin slices, and usually enhances SNR.
Water Suppression
The suppression of the water signal in a MR spectrum, usually by a specialized excitation sequence.
Weighted-Phase Images
Images that present flow data. Directional-flow images demonstrate flow along a single axis;
speed-flow images combine all flow information into a single presentation.
Conclusion
The most comprehensive list of MRI terms with definitions includes the most commonly used terms by MRI technologists and imaging professionals. We are dedicated to demystifying magnetic resonance imaging by providing the best information resources available. Additionally, make sure to check out the MRIPETCTSOURCE YouTube Channel for the latest MRI video resources. Related resources and links available below.
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