Find the answers you need with this quick reference MRI glossary that contains MRI terminology, definitions, synonyms, acronyms, and links to helpful related resources. Get started below.
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A 3D Multi Slab technique is an advanced acquisition method commonly used in time-of-flight (TOF) vascular imaging to cover a larger anatomical region while maintaining high spatial resolution. Instead of acquiring one thick 3D volume, the scanner collects multiple thinner, overlapping 3D slabs that are later combined during reconstruction. This overlap minimizes signal loss at slab boundaries and provides a more continuous and accurate visualization of vascular structures, making it particularly valuable in neurovascular and peripheral angiographic studies.
A 90° pulse is a fundamental radiofrequency (RF) excitation that rotates the net magnetization vector by 90 degrees, moving it from alignment with the static magnetic field (longitudinal plane) into the transverse plane. This transition is crucial because it creates transverse magnetization (the signal that is actually measured in MRI). 90° pulses are used in many standard pulse sequences, including spin echo and gradient echo, and play a key role in determining image contrast.
Used in parallel imaging techniques (e.g., GRAPPA, SENSE) to describe the degree of undersampling.
Number of phase and frequency encoding steps. Determines in-plane resolution.
Time interval during which the MR signal is collected after excitation.
Real-time adjustment of magnetic field homogeneity using shim coils.
Artifact caused when anatomy outside the FOV is mapped incorrectly into the image.
Method of encoding MR signals by varying gradient amplitude.
Non-invasive vascular imaging technique, including TOF, PC, and CE-MRA.
Direction-dependent properties of tissues (important in DTI).
Anterior/Posterior (A/P) refers to a patient positioning selection aligned with the coronal plane, ensuring that the region of interest (ROI) is centered as close to the magnet’s isocenter as possible. This orientation divides the body into front (anterior) and back (posterior) sections. Proper A/P alignment is especially important in body and spine imaging to optimize image uniformity, reduce artifacts, and improve diagnostic accuracy.
An artifact is any signal abnormality or error in an MRI image that does not correspond to actual patient anatomy or pathology. Common categories include geometric distortion, inhomogeneous signal intensity, and spurious signal. Artifacts can arise from patient motion, hardware imperfections, or sequence design. Understanding their origin is crucial for technologists and radiologists, as some artifacts may mimic pathology or obscure important findings.
Multi-channel surface coil used to improve SNR and enable parallel imaging.
An asymmetric (or fractional) echo occurs when the echo peak at echo time (TE) is not centered within the data sampling window. This technique allows partial acquisition of k-space, which can help shorten minimum TE and overall scan time. Clinically, asymmetric echo is often used in fast imaging protocols such as gradient echo or echo planar imaging, where time efficiency is critical.
Asymmetric Field of View refers to using non-square imaging dimensions, where the frequency and phase encoding directions have different sizes. This approach can reduce scan time when the anatomy of interest doesn’t fill a square FOV—such as in imaging of the spine or extremities. By tailoring the FOV to the region, AFOV helps optimize resolution and reduce unnecessary data acquisition while preserving image quality.
Automated adjustment of RF gain and center frequency before scanning (Siemens term, similar functions across vendors).
Available Imaging Time (AIT) is the period during a cardiac cycle in which the MRI system can collect data. In cardiac-gated studies, AIT falls between physiological events like systole and diastole and is often influenced by the patient’s heart rate and rhythm. Accurate AIT estimation ensures data are acquired during motion-minimized phases, improving cardiac image sharpness.
Average flow represents the total volumetric blood flow through a defined vessel or region over time, typically expressed in mL/min. It is calculated by summing the voxel-based flow values within a region of interest. Clinically, this measurement is essential in assessing hemodynamics, such as quantifying flow in major arteries or evaluating shunt volumes in congenital heart disease.
Average velocity is calculated by dividing volumetric flow (Q) by the cross-sectional area (A) of a vessel: V = Q / A. This provides the mean blood flow velocity in cm/s. In laminar flow, it’s typically half of the peak velocity. This parameter is routinely used in phase contrast angiography to assess vascular conditions, including stenosis severity and flow patterns.
Averaging, also known as signal averaging or number of excitations (NEX), is a method for improving signal-to-noise ratio (SNR) by acquiring the same MR signal multiple times and averaging the results. This reduces random noise fluctuations, making subtle structures more visible. Although increasing averages improves image quality, it also lengthens scan time, so it’s typically balanced against clinical priorities.
See Center Frequency.
In MRI, bandwidth refers to the range of frequencies received by the system during signal acquisition. Adjusting bandwidth can significantly impact image quality and scan parameters. A narrower bandwidth improves SNR by excluding more electronic noise but can increase susceptibility to chemical shift artifacts. A wider bandwidth allows shorter TE and can reduce distortion, particularly in sequences like echo planar imaging. Optimizing bandwidth is a key part of protocol customization.
Bandwidth normalized to pixel size; affects chemical shift artifact.
Main static magnetic field of the MRI system.
RF transmit magnetic field used to excite spins.
Processing step to remove background signal from flow or spectroscopy data.
Fast gradient echo sequence with high SNR and excellent blood/myocardium contrast.
Beats per Minute (bpm) reflects the patient’s heart rate, measured directly from the cardiac waveform during gated MRI acquisitions. Accurate bpm readings are crucial for synchronizing data collection with cardiac cycles, especially in cardiac cine and perfusion imaging, where motion artifacts can otherwise degrade image quality.
Bipolar flow-encoding gradients consist of two gradient pulses with identical shape but opposite polarity. They encode velocity information as phase shifts in moving spins and are the foundation of phase contrast angiography. By applying these gradients, stationary tissues experience no net phase shift, while moving blood develops measurable phase differences, enabling quantitative flow measurements.
MRI contrast mechanism used in fMRI to detect changes in deoxyhemoglobin.
Common approach to reduce motion artifacts in thoracic and abdominal MRI.
Synchronizing data acquisition with cardiac cycle to reduce motion artifacts.
Cardiac phase images represent the heart at different time points within the cardiac cycle, typically acquired using ECG gating. This allows visualization of both systolic contraction and diastolic filling. These images are essential for assessing wall motion, valvular function, and cardiac output in both congenital and acquired heart disease.
Frequency corresponding to the resonance of the spins in the center of the imaging volume.
Frequency difference between fat and water that can cause misregistration artifacts.
Spectroscopic imaging technique to map metabolite distribution.
Primarily refers to RF receiver/transmitter device (e.g., body coil, surface coil, phased array).
Process of adjusting the RF coil for optimal energy transfer.
Cine imaging refers to a technique that produces dynamic, movie-like reconstructions of anatomical motion, most commonly used in cardiac MRI. By using retrospective ECG gating and gradient echo sequences, cine acquisitions provide high temporal resolution images across the cardiac cycle. This enables detailed assessment of ventricular function, wall motion, and ejection fraction.
Advanced acceleration technique combining undersampling and iterative reconstruction.
A “collapsed” image, often called a Maximum Intensity Projection (MIP) or Maximum Pixel Projection (MPP), is generated by projecting the highest signal intensity along a given viewing direction—typically the slice direction. In vascular imaging, this technique highlights bright blood flow in TOF or phase contrast studies, providing a clear angiographic view without the need for contrast agents.
Complex Difference is a reconstruction method used in phase contrast MRI that combines magnitude and phase information to enhance flow signal while suppressing background tissue. By deactivating the slab dephasing gradient and enabling phase correction, it improves visualization of vessels, particularly when background suppression is needed for accurate flow quantification.
Gadolinium-based or iron-based substances used to enhance tissue signal.
Contrast resolution describes an imaging system’s ability to distinguish subtle differences in tissue signal intensity. High contrast resolution is essential for differentiating structures with similar densities, such as gray and white matter in neuroimaging or tumor margins in soft tissue. It is influenced by sequence selection, parameters like TR and TE, and post-processing techniques.
Contrast-to-Noise Ratio (CNR) measures how well two tissues with different signal intensities can be distinguished relative to background noise. A high CNR improves lesion conspicuity and diagnostic confidence. Optimizing CNR often involves selecting appropriate pulse sequences, contrast agents, and scan parameters.
The coronal plane is an anatomical orientation that divides the body into anterior (front) and posterior (back) halves. In MRI, coronal acquisitions are useful for visualizing structures along the body’s longitudinal axis, such as the spine, sinuses, or abdominal organs, and are often combined with axial and sagittal planes for comprehensive evaluation.
Overlap of RF excitation between adjacent slices leading to signal loss.
Used in flow and vessel measurements.
Cine phase contrast sequences to visualize cerebrospinal fluid motion.
Reducing mutual inductance between RF coils to minimize interference.
The decubitus position refers to a patient lying on their left or right side, often used when positioning for certain abdominal, thoracic, or spine studies. This orientation can help displace or redistribute internal structures, improve visualization, or accommodate patient comfort and respiratory status.
Diastole is the phase of the cardiac cycle between the end of the T-wave and the beginning of the R-wave, representing ventricular filling. In cardiac MRI, data are often acquired during diastole to minimize motion artifacts and capture the heart at rest, providing clearer anatomical detail.
Standard format for storing and transmitting medical imaging data.
Imaging technique sensitive to molecular water motion, critical in stroke imaging.
Extension of DWI used to characterize white matter tracts.
Slow variation in the magnetic field over time, often corrected by system calibration.
Non-reconstructed acquisitions at the start of a sequence to allow signal steady state.
Time-resolved imaging (e.g., perfusion studies, contrast-enhanced MRA).
Dynamic-range compression enhances phase contrast image quality by applying a projection dephasing gradient to suppress signal from stationary tissues. This improves visualization of flowing blood while reducing background interference, especially in vessels adjacent to static structures.
Transient currents induced in conductors by changing magnetic fields; can cause artifacts or distortions.
Ultra-fast imaging technique that collects all or most of k-space after a single excitation. Widely used in fMRI and DWI.
Echo rephasing refers to the realignment of dephased spins to restore signal coherence, typically achieved with a 180° RF refocusing pulse (as in spin echo sequences) or gradient reversal (as in gradient echo sequences). Rephasing is what produces the detectable MR signal or “echo” at TE.
Time interval between successive echoes in an EPI or FSE train.
Time between RF excitation and signal acquisition; critical for contrast weighting.
Echo Train Length (ETL) is the number of 180° refocusing pulses applied within a single TR during fast spin echo sequences. A longer ETL shortens scan time by collecting multiple echoes per excitation but may increase blurring and alter image contrast. ETL selection is key in balancing efficiency and image sharpness.
The TE at which the center of k-space is sampled in multi-echo sequences.
Effective TR is the average repetition time used in cardiac-gated studies. Because heart rate varies, the actual TR can’t be strictly controlled, but the effective TR reflects how often a slice is excited over multiple cardiac cycles. Adjusting effective TR can help optimize contrast and reduce motion artifacts in gated acquisitions.
An effective value is a representative or average measurement used in scenarios where the true value fluctuates, such as heart rate during a gated study. For example, effective TR allows consistent imaging even when the patient’s heart rate is irregular.
Specific cardiac cycle phases captured in cardiac cine imaging.
Option to increase SNR through coil combination strategies.
Geometric distortion inherent to EPI due to long readout times and susceptibility effects.
Number of echoes acquired per excitation in an EPI sequence; affects resolution and distortion.
Flip angle that maximizes signal for a given TR and T1.
Adjusting echo train length to balance scan time, image quality, and SAR limits.
Angles used to describe fiber orientation in 3D space.
Even-echo rephasing occurs in multi-echo sequences, where moving spins naturally realign on even-numbered echoes (e.g., 2, 4, 6). This effect can help reduce flow-related signal loss, improving vessel and CSF visualization without additional flow compensation gradients.
Extended Dynamic Range (EDR) uses 32-bit signal processing—rather than the standard 16-bit—to improve the signal-to-noise ratio and expand the range of detectable signal intensities. This enhancement is particularly useful in sequences with a wide range of tissue signal intensities, improving subtle contrast visualization.
See Fat/Water Suppression.
A 2D time-of-flight gradient-echo pulse sequence that captures multiple phases of the cardiac cycle within a single breath-hold. Fast cardiac gating allows dynamic imaging of cardiac motion, wall motion, and flow at a single slice location, reducing motion artifacts and improving temporal resolution in cardiac studies. Equivalent methods exist across vendors (e.g., Siemens “cine GRE,” Philips “Fast Cardiac”).
An imaging enhancement technique used to selectively suppress either fat or water signal based on their distinct resonance frequencies. Frequency-selective saturation pulses or inversion recovery methods (such as STIR for fat suppression) can be applied. Fat suppression improves visualization of edema, contrast-enhanced lesions, and musculoskeletal pathology, while water suppression can be useful in spectroscopy or fat quantification studies. See also: Fat Saturation, Dixon, STIR.
The measurable MR signal that occurs immediately after excitation with a 90° RF pulse, as the transverse magnetization vector begins to decay due to T2* relaxation. The FID is the raw signal from which all MR images are derived. In spectroscopy and gradient echo imaging, understanding FID characteristics is essential for proper signal processing and reconstruction.
Variations in the main magnetic field (B0) across the imaging volume that can cause image distortion or non-uniform signal intensity. These inhomogeneities arise from imperfect magnet design, patient anatomy, or nearby ferromagnetic materials. Field shimming and auto-tuning are used to minimize these effects, improving image uniformity and spectral accuracy.
The spatial area of anatomy being imaged, usually expressed in centimeters. The FOV determines how much of the body is included in the image and influences resolution and scan time. Larger FOVs cover more anatomy but reduce spatial resolution, while smaller FOVs increase detail but risk aliasing if anatomy extends beyond the selected region. Vendors may label this as “FOV read” and “FOV phase.”
A post-processing step used in phase contrast imaging to correct for linear phase errors across the image, typically caused by eddy currents or gradient delays. By estimating and removing these linear shading effects in both x and y directions, first-order correction improves the accuracy of velocity and flow measurements.
The angle by which the magnetization vector is rotated away from the longitudinal axis by an RF pulse. Flip angle is a key contrast-determining parameter—smaller flip angles are used in fast gradient echo sequences to maintain steady-state conditions, while larger angles enhance T1 weighting. Optimal flip angles vary depending on tissue type, TR, and desired contrast (Ernst angle optimization).
A reconstruction method used in phase contrast (PC) and cine PC MRI that provides quantitative flow measurements through a region of interest. In this mode, dephasing and phase correction gradients can be toggled to optimize for either noise suppression or velocity accuracy. Clinically applied in quantifying cardiac output, regurgitant flow, and shunt fraction.
The directional axis (typically superior/inferior (S/I), right/left (R/L), or anterior/posterior (A/P)) along which velocity encoding is applied during phase contrast imaging. Selecting the appropriate flow axis ensures that motion (such as blood or CSF flow) is accurately captured in the direction of physiologic movement.
A gradient-based technique that corrects for motion-induced phase errors caused by constant velocity or acceleration. Also known as Gradient Moment Nulling (GMN), this method reduces flow-related signal loss and ghosting, especially near vessels or CSF spaces. Flow comp is often applied in spin echo or gradient echo sequences for neuro and spine imaging.
A phase contrast MRI technique that uses bipolar gradients to encode velocity into phase shifts. The encoding velocity (VENC) is user-specified and determines the sensitivity of the sequence to flow speed—higher VENC values prevent aliasing in fast flow, while lower VENC values enhance sensitivity to slow flow. Essential for quantifying blood or CSF dynamics.
A group of images produced from phase contrast MRI acquisitions, typically including magnitude and phase images. Magnitude-weighted flow images improve vessel definition, while phase images represent velocity information. These can be processed to calculate peak velocity, volumetric flow, or flow direction within vessels.
A mechanism responsible for bright-blood appearance in time-of-flight (TOF) angiography. FRE occurs when fully magnetized inflowing spins enter a region of partially saturated stationary tissue, resulting in a higher signal from moving blood compared to background tissue.
A technique that acquires only part of k-space before echo peak symmetry, reducing minimum TE and total scan time. Also called partial echo acquisition, it’s useful for minimizing motion artifacts and susceptibility effects, particularly in fast imaging or EPI-based sequences.
A method that collects a fraction (e.g., 0.5 or 0.75) of the usual number of signal averages (NEX) to shorten scan time. While it reduces acquisition time, some loss of signal-to-noise ratio can occur, so it’s typically used when patient motion or breath-hold constraints are significant.
The gradient applied during readout that determines spatial encoding along the frequency axis. Variations in this gradient’s polarity or strength influence image directionality and distortion patterns.
A pre-saturation technique that targets fat protons at their specific resonance frequency, suppressing their signal while preserving water-based tissue contrast. Widely used in musculoskeletal, breast, and contrast-enhanced imaging to enhance lesion conspicuity.
A synchronization technique that coordinates image acquisition with physiological signals such as cardiac or respiratory motion. Cardiac gating (ECG or vectorcardiogram-based) is used to minimize motion blur in cardiac and vascular imaging, while respiratory gating helps in thoracic and abdominal scans. Modern systems employ prospective, retrospective, or navigator-based gating strategies.
See Flow Compensation. This technique rephases spins that have undergone motion, reducing artifacts from flowing blood or CSF. Especially beneficial in neuroimaging to maintain vessel signal clarity.
One of the three orthogonal coils (Gx, Gy, Gz) responsible for spatial encoding in MRI. Each coil generates linear magnetic field gradients that define slice selection, frequency encoding, and phase encoding directions. Gradient performance, characterized by amplitude and slew rate, directly affects spatial resolution, scan speed, and echo times.
A family of pulse sequences that form echoes by reversing gradient polarity rather than using a 180° RF refocusing pulse. Gradient echoes allow very short TRs and flip angles, enabling rapid imaging. They’re commonly used in angiography, cardiac cine, fMRI (BOLD), and dynamic contrast studies. Vendor-specific variants include FLASH (Siemens), FFE (Philips), and SPGR (GE).
A broad category encompassing sequences that rely on gradient rephasing for signal formation. By adjusting flip angle, TR, and TE, these sequences can emphasize T1, T2*, or proton density contrast. Balanced versions (e.g., TrueFISP, FIESTA, bTFE) yield high SNR and bright-blood contrast for cardiac and abdominal imaging.
A measure describing how a gradient field influences moving spins. The first moment relates to velocity, the second to acceleration, and the third to changes in acceleration (jerk). Gradient moment control is key in flow encoding and motion compensation.
A gradient echo sequence that maintains a steady-state magnetization by using rapid, low flip-angle excitations. It provides T2/T1-weighted contrast and is suited for cardiac, vascular, and musculoskeletal imaging. Equivalent names include FISP (Siemens) and FGR (GE).
Standard notation for MRI’s three orthogonal gradient axes—x (right-left), y (anterior-posterior), and z (superior-inferior). These gradients are applied individually or in combination to achieve spatial encoding and slice selection.
A data acquisition method that collects slightly more than half of k-space and mathematically reconstructs the remainder using symmetry assumptions. This reduces scan time and TE, beneficial for single-shot and EPI-based sequences. Equivalent vendor terms include Half-Fourier (Siemens) and Partial NEX (GE).
The degree to which the magnetic field (B0) is consistent across the imaging volume. High homogeneity is critical for uniform signal intensity and accurate fat-water separation. It is optimized through shimming—either passive (using shim plates) or active (using shim coils).
The time interval between successive image acquisitions in a cardiac cycle. Adjusting this delay can help optimize temporal coverage or improve visualization of specific cardiac phases in gated studies.
Loss of phase coherence within a voxel due to varying velocities or local magnetic field inhomogeneities. This results in signal loss, especially in regions of turbulent flow or diffusion. Compensation techniques like flow compensation or shorter echo times can mitigate its effects.
A pulse sequence that begins with a 180° inversion pulse, followed by a delay (TI), and then a 90° excitation pulse. The signal’s recovery from inversion depends on T1 relaxation, allowing selective tissue contrast. Variants include STIR (fat suppression), FLAIR (CSF suppression), and DIR (double inversion recovery).
The time between the center of the inversion (180°) pulse and the center of the excitation (90°) pulse in an IR sequence. TI controls which tissues are nulled or emphasized. For example, TI ≈ 2000 ms nulls CSF (FLAIR), while TI ≈ 150–180 ms nulls fat (STIR).
The precise point at the intersection of the three orthogonal gradient planes, representing the magnetic field’s most homogeneous region. Positioning the region of interest at isocenter ensures optimal image quality and accurate gradient performance.
Groups of spins that precess at the same frequency and phase at a given moment. Isochromatic behavior leads to constructive signal formation; dephasing among isochromats results in signal decay. Managing isochromat coherence is central to pulse sequence design.
The brief phase of the cardiac cycle immediately after the R-wave when the ventricles contract without volume change. In cardiac MRI, identifying this phase helps distinguish between systolic and diastolic motion in cine or tagged imaging sequences.
A voxel with equal dimensions in all three spatial directions (e.g., 1 × 1 × 1 mm). Isotropic acquisitions enable high-quality 3D reconstructions and multiplanar reformats without loss of resolution (common in neuro, musculoskeletal, and angiographic imaging).
A method where slices or k-space lines are acquired in alternating order to minimize crosstalk and improve temporal efficiency. Frequently used in multislice spin echo or EPI sequences for functional and diffusion imaging.
Also known as spin–spin coupling, J-coupling is the interaction between nuclear spins that are bonded together within a molecule. This interaction splits resonance signals into multiple components, providing important chemical environment information. Although most clinical MRI sequences don’t rely directly on J-coupling, it is highly relevant in MR spectroscopy and advanced tissue characterization.
The mathematical domain in which raw MRI data is acquired and stored before image reconstruction. Each point in k-space represents spatial frequency information rather than anatomical structure. The center of k-space contributes to image contrast and signal intensity, while the periphery determines spatial resolution. Understanding k-space behavior is key for techniques like parallel imaging, partial Fourier, and motion correction.
An accelerated imaging technique where only the central portion of k-space is repeatedly updated over time, while the peripheral data is acquired once or infrequently. Commonly applied in dynamic contrast-enhanced studies (e.g., MR angiography, perfusion) to improve temporal resolution without compromising spatial detail.
A smooth, orderly fluid flow pattern where adjacent layers move parallel without significant mixing. In MRI, laminar flow is easier to characterize and produces more predictable phase shifts compared to turbulent flow. This concept is fundamental in velocity-encoded (VENC) phase-contrast angiography.
The frequency at which protons precess in a magnetic field, calculated by multiplying the gyromagnetic ratio by the strength of the static magnetic field. Accurate determination of this frequency ensures precise RF excitation, slice selection, and signal detection.
The component of net magnetization aligned with the main static magnetic field (B₀). After an RF pulse, the recovery of longitudinal magnetization over time reflects T1 relaxation. This property is manipulated in T1-weighted imaging and inversion recovery sequences.
A repetition time that is long relative to the T1 relaxation of the tissue, minimizing T1 weighting and emphasizing proton density or T2 contrast. Used in sequences such as spin echo and fast spin echo when maximizing SNR or reducing T1 contrast is desired.
A controlled variation in magnetic field strength along a specific spatial axis, enabling slice selection, frequency encoding, and phase encoding. Gradients are also essential for flow encoding, echo formation in gradient echo sequences, and motion compensation.
A physical phenomenon in which atomic nuclei absorb and re-emit electromagnetic energy when exposed to a strong magnetic field and a matching RF pulse. MR principles are the foundation of MRI, MRS, and functional MRI.
An imaging technique that generates detailed anatomical and functional images using magnetic resonance principles. Signal intensity reflects proton density and relaxation properties (T1, T2, T2*), as well as flow and diffusion characteristics, allowing for exceptional soft-tissue contrast.
The RF signal generated by precessing transverse magnetization, which induces a voltage in the receiver coil. The quality and stability of this signal directly affect image SNR and artifact levels.
A technique that uses off-resonance RF pulses to saturate macromolecular protons, leading to reduced signal from background tissue and improved contrast between structures. Frequently used in neuroimaging and angiographic applications.
A post-processing technique that projects the highest intensity voxel along each ray through a 3D dataset onto a 2D image. Commonly used in MR angiography to highlight vascular structures.
A gradient echo pulse sequence that acquires multiple slices within a single TR, improving efficiency. Known by various names across vendors (e.g., GRE, FLASH, SPGR), it’s often used in angiography, cardiac imaging, and 3D anatomical scans.
A cardiac gating technique that captures multiple phases of the cardiac cycle across several slice locations. Provides dynamic functional assessment of the heart.
A cardiac imaging technique that acquires multiple slice locations at a single phase of the cardiac cycle, often used for anatomical coverage.
Also called NSA (Number of Signal Averages). The number of times the same data is acquired and averaged to improve SNR. Increasing NEX reduces random noise but lengthens scan time.
An RF pulse that excites all spins in the imaging volume, not limited to a specific slice. Used in 3D acquisitions and some MRA techniques.
A signal processing principle dictating that data must be sampled at least twice the highest frequency present to accurately reconstruct the signal. In MRI, inadequate sampling leads to aliasing artifacts.
A condition where spins precess at a frequency different from the applied RF pulse frequency, often caused by magnetic field inhomogeneities, chemical shift, or susceptibility differences. Can lead to artifacts or be intentionally exploited (e.g., MT contrast).
A technique that acquires images when fat and water signals are 180° out of phase. Useful for detecting microscopic fat (e.g., in adrenal or liver lesions).
Acquiring additional data beyond the minimum required to reduce aliasing artifacts and improve image quality. Often applied in the phase-encoding direction or in parallel imaging protocols.
A data acquisition strategy where only part of k-space is collected, and the missing data is estimated using symmetry properties. This shortens scan time but can reduce SNR and increase sensitivity to artifacts.
A technique that uses multiple receiver coils and coil sensitivity information to reduce the number of phase encoding steps required, enabling faster scans. Examples include SENSE, GRAPPA, and ASSET.
An image contrast determined primarily by the number of hydrogen nuclei in tissues, with minimal T1 or T2 weighting. Achieved by long TR and short TE.
A reconstruction method for phase-contrast vascular imaging that highlights flow-induced phase shifts while suppressing stationary tissue signal.
A spatial encoding technique achieved by applying a gradient that alters the phase of spins prior to signal readout. Phase encoding determines spatial location along one axis in the image.
A parameter that adjusts the field of view along the phase encoding direction, enabling reduced scan times while risking aliasing if anatomy extends beyond the reduced FOV.
The range of frequencies per pixel in the readout direction. Higher pixel bandwidth reduces susceptibility and chemical shift artifacts, while lower bandwidth improves SNR.
A gradient pulse applied to reduce stationary tissue signal, often used in phase-contrast angiography to highlight flow.
The concentration of hydrogen nuclei (primarily in water and fat) in tissue. A fundamental determinant of signal intensity in MRI.
A type of RF coil that uses two orthogonal channels to transmit or receive signal, improving SNR compared to linear coils. Quadrature design is standard in many head and body coils.
An approach that measures absolute tissue properties (e.g., T1, T2, proton density, diffusion) rather than relative contrast, supporting advanced tissue characterization, disease monitoring, and standardized imaging biomarkers.
An RF excitation pulse with a gradual flip angle profile along the slab direction. This reduces entry slice artifacts by smoothly exciting inflowing spins, commonly used in TOF MRA.
The gradient applied during signal acquisition, determining frequency encoding direction and contributing to spatial localization.
The total frequency range the system listens to during signal readout. Narrow bandwidth increases SNR but susceptibility to artifacts, while wider bandwidth reduces artifacts at the cost of lower SNR.
The restoration of spin phase coherence after dephasing, accomplished via gradient reversal or RF pulses (e.g., 180° spin echo pulses). Essential for signal formation in SE and GRE sequences.
The time constants (T1, T2, T2*) that describe how magnetization returns to equilibrium after RF excitation. Key parameters in determining image contrast.
The time interval between successive excitations of a slice or volume. TR strongly influences image contrast, SNR, and scan time.
A gradient pulse applied opposite to a previous slice-select gradient to counteract phase dispersion and ensure echo formation.
The time between successive R-waves on an ECG tracing, corresponding to the duration of one cardiac cycle. Used for cardiac gating and timing in cardiac MRI.
Specific Absorption Rate is a measure of the RF energy absorbed by tissue per unit mass, expressed in watts per kilogram (W/kg). RF absorption can lead to tissue heating, so SAR is strictly regulated to ensure patient safety. SAR is influenced by factors such as flip angle, TR, duty cycle, body habitus, and field strength. Modern scanners use real-time SAR monitoring and auto-adjustment of sequence parameters to remain below regulatory limits.
A slice-selective RF pulse, typically followed by a dephasing gradient, used to null or minimize signal from a specific region. Commonly applied to suppress inflowing blood signal in the slice direction (e.g., in TOF angiography), or to suppress background tissue prior to contrast arrival.
Occurs when RF pulses are applied repeatedly with TR shorter than the T1 of the tissue, preventing full longitudinal recovery of net magnetization. This leads to reduced signal intensity and is used intentionally for background suppression.
The total duration required to acquire image data. Scan time depends on TR, number of phase-encoding steps, number of signal averages (NSA/NEX), parallel imaging factors, acceleration techniques (e.g., SENSE, GRAPPA, ARC), and the number of slices.
A type of IR sequence in which the 180°–90°–180° pulses are applied slice-by-slice. In non-sequential IR, the inversion pulse is applied to all slices simultaneously, followed by sequential readouts. Typically used in T1-weighted and FLAIR protocols.
The ratio of the true MR signal to background noise. Higher SNR improves image quality and diagnostic confidence. SNR is affected by factors such as voxel size, coil type, field strength, TR/TE, bandwidth, and acceleration methods. SNR is also improved using phased array coils, parallel receive chains, and 3D volume acquisitions.
The gradient direction applied during RF excitation that determines the location and thickness of the slice. It is typically orthogonal to the readout and phase-encoding directions.
The combination of slice selection, phase encoding, and frequency encoding that allows spatial localization of the MR signal and reconstruction of the image. Also known as “K-space encoding”
An MR imaging technique in which the signal used for image formation is generated by a spin echo, produced by a 90° RF pulse followed by a 180° refocusing pulse. SE is less sensitive to magnetic field inhomogeneities and is a workhorse for T1 and T2-weighted imaging.
See J-Coupling. Refers to interactions between nuclear spins, relevant in MR spectroscopy.
A gradient echo sequence in which residual transverse magnetization is dephased or “spoiled” to achieve T1-weighted contrast. Used extensively for 3D anatomical imaging. Also known as FLASH (Siemens), SPGR (GE), and T1‑FFE (Philips).
A gradient pulse applied to dephase residual transverse magnetization, eliminating unwanted signals or coherence pathways.
See Steady State Free Precession.
A gradient echo technique in which a steady state of magnetization is maintained by using short TR (shorter than T2), leading to high SNR and characteristic contrast. Commonly used in cardiac and MRCP imaging. Also known as TrueFISP (Siemens), FIESTA (GE), and bTFE (Philips).
The longitudinal relaxation time, representing the time constant for recovery of longitudinal magnetization after RF excitation. Also known as spin-lattice relaxation time. T1 varies with tissue type, field strength, and temperature.
Pulse sequence designed to emphasize T1 differences between tissues. Typical features: short TR, short TE, strong T1 contrast. Applications include anatomical imaging, post-contrast studies, and dynamic imaging.
The transverse relaxation time, representing the loss of transverse magnetization due to spin–spin interactions and local magnetic field variations. Also called spin–spin relaxation time.
Pulse sequence designed to emphasize T2 contrast between tissues. Typical features: long TR, long TE. Applications include pathology detection (e.g., edema, demyelination).
The shortest achievable TE for a given protocol. Used to minimize flow dephasing and T2 effects.
In asymmetric spin echo sequences, TE1 is the time from the middle of the excitation pulse to the first readout; TE2 is to the second readout. This allows acquisition of multi-echo data for T2 mapping or Dixon techniques.
A method for setting pixel intensity limits used during image processing, e.g., segmentation or angiographic masking.
Flow direction perpendicular to the imaging plane, often encoded in phase-contrast imaging.
The time between the center of the 180° inversion pulse and the center of data acquisition (or k-space segment). Adjusting TI allows nulling of specific tissues, e.g., CSF in FLAIR or fat in STIR.
Time between the center of the RF excitation pulse and the peak of the echo. TE affects T2-weighting and contrast.
A 2D or 3D angiographic technique that uses flow-related enhancement to distinguish moving spins (bright blood) from stationary tissue. No contrast agent is required. Commonly used in neurovascular imaging.
Time to maximum value of the residue function in perfusion imaging, representing tracer delay at a voxel. Used in stroke perfusion analysis.
Time between successive RF excitations of a slice. TR strongly influences T1 weighting, SNR, and scan time.
The time between the trigger event (e.g., R-wave in ECG) and the onset of data acquisition.
In cardiac gating, the period during which the scanner waits for the next trigger. No data is acquired during this interval.
Signal from a cardiac or respiratory monitor used to synchronize data acquisition.
The maximum velocity value to be encoded without aliasing in phase contrast MRI. Setting VENC appropriately is essential to avoid under- or over-sampling of flow velocities.
A localized saturation technique used in MR spectroscopy to suppress unwanted signals outside the volume of interest.
Signal is collected from an entire volume rather than individual slices, allowing high SNR, isotropic voxels, and multi-planar reconstructions.
Techniques designed to suppress the dominant water signal in MR spectroscopy or imaging, enabling better visualization of metabolites or fat.
Phase contrast images that display flow velocity information. Directional flow images show velocity along a single axis. Speed flow images combine flow in multiple directions into one image.
MRI terminology is the foundation of clear communication in medical imaging. Many terms are shared across major scanner manufacturers, though there may be slight variations in naming conventions. Learning the standardized concepts behind these labels helps technologists, radiologists, and students adapt seamlessly to different systems and workflows. Technical vocabulary like SAR, TR, TE, FOV, and SNR isn’t just “jargon”, it directly influences image quality, patient safety, and diagnostic accuracy. A strong grasp of these fundamentals builds confidence in protocol optimization, improves communication across teams, and supports better clinical outcomes.
Check out some of the related resources below to learn more.
Read more on Larry’s author page.
MRI laser alignment landmark assembly with class 2 laser
CT scan ring artifact explained. CT scan machine pictured left, CT ring artifact example pictured right.
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Visual representation of MRI magnetic field lines. License this image
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