MRI Glossary
Key MRI terms explained — what each one means, why it’s used, and a worked example. Use the filters to jump to a category, or view everything at once.
T1 (longitudinal relaxation)
The time constant for spins to realign with the main magnetic field after excitation.
Why: Tissues recover at different rates, giving the contrast that distinguishes fat, fluid, and soft tissue on T1-weighted images.
Example: On a T1-weighted brain scan, fat and subacute blood appear bright while CSF is dark.
T2 (transverse relaxation)
The time constant for loss of phase coherence among spins in the transverse plane.
Why: Fluid and many pathologies have long T2 and appear bright, making T2 weighting sensitive to oedema and disease.
Example: A T2-weighted image shows CSF and joint effusions as bright white.
T2* (T-two-star)
Transverse decay including local field inhomogeneity, faster than true T2.
Why: Sensitive to iron, calcium, and blood products because these distort the local field.
Example: A T2*/GRE or SWI sequence blooms around cerebral microbleeds.
TR (repetition time)
Time between successive excitation pulses.
Why: Controls T1 weighting — short TR increases T1 contrast; long TR reduces it.
Example: A short TR (~500 ms) is chosen for T1-weighted spin echo.
TE (echo time)
Time from excitation to readout of the echo.
Why: Controls T2 weighting — long TE increases T2 contrast.
Example: A long TE (~90 ms) yields a heavily T2-weighted image.
Flip angle
The angle through which the net magnetisation is tipped by the RF pulse.
Why: Tunes contrast and signal, especially in gradient-echo; low angles allow rapid imaging.
Example: A 10–15° flip angle is used in fast 3D gradient-echo T1 sequences.
k-space
The raw spatial-frequency data matrix that is Fourier-transformed into the image.
Why: Its centre encodes contrast and its periphery encodes detail; how it is filled defines speed and artefact behaviour.
Example: Partial-Fourier acquisition samples just over half of k-space to shorten scan time.
SNR (signal-to-noise ratio)
Ratio of signal to background noise in an image.
Why: The core quality trade-off — higher field, larger voxels, and more averages raise SNR at a cost in time or resolution.
Example: Doubling voxel volume roughly doubles SNR but halves spatial detail.
Spin echo (SE) / Turbo & Fast SE (TSE/FSE)
A 90° excitation followed by a 180° refocusing pulse; TSE/FSE uses multiple refocusing pulses per TR.
Why: Refocusing corrects field inhomogeneity for clean T1/T2 contrast; TSE/FSE speeds this up dramatically.
Example: T2 TSE of the spine acquires several lines per excitation, cutting scan time from minutes to under a minute.
Gradient echo (GRE)
Echo formed by gradient reversal instead of a 180° pulse, usually with a low flip angle.
Why: Fast, and sensitive to susceptibility; the basis of most 3D and dynamic imaging.
Example: Breath-hold GRE captures the arterial phase of a liver study.
MPRAGE / BRAVO / 3D T1 TFE
Vendor names for an inversion-prepared 3D gradient-echo T1 sequence (Siemens MPRAGE, GE BRAVO, Philips 3D TFE).
Why: Produces isotropic sub-millimetre T1 volumes that reformat in any plane — the workhorse for brain volumetry and neuronavigation.
Example: A post-contrast MPRAGE is loaded into a surgical navigation system to plan a tumour resection.
FLAIR (fluid-attenuated inversion recovery)
A T2 sequence with an inversion pulse that nulls CSF signal.
Why: Suppressing bright CSF makes periventricular and cortical lesions conspicuous.
Example: FLAIR reveals MS plaques adjacent to the ventricles that T2 would obscure.
STIR (short tau inversion recovery)
Inversion recovery with a TI chosen to null fat.
Why: Robust fat suppression even where the field is uneven; highlights oedema and marrow disease.
Example: STIR of the ankle shows bone-marrow oedema as bright signal against suppressed fatty marrow.
SPACE / CUBE / VISTA (3D TSE)
Vendor names for isotropic 3D turbo-spin-echo (Siemens SPACE, GE CUBE, Philips VISTA).
Why: A single high-resolution volume reformats into any plane, reducing the number of 2D series needed.
Example: A 3D SPACE FLAIR of the brain is reformatted sagittally and coronally from one acquisition.
DWI (diffusion-weighted imaging) & ADC
Measures random water motion; the ADC map quantifies it, separating true restriction from T2 shine-through.
Why: Restricted diffusion flags acute stroke, abscess, and hypercellular tumour.
Example: Acute infarct is bright on high-b DWI and dark on ADC.
SWI (susceptibility-weighted imaging)
High-resolution GRE combining magnitude and phase to maximise susceptibility contrast.
Why: Exquisitely sensitive to blood products, calcium, and small veins.
Example: SWI detects microhaemorrhages in diffuse axonal injury after trauma.
MRA & TOF (MR angiography / time-of-flight)
Vascular imaging; time-of-flight exploits inflowing unsaturated blood to appear bright without contrast, while contrast-enhanced MRA uses gadolinium.
Why: Visualises arteries and veins non-invasively to assess stenosis, aneurysm, or occlusion — TOF needs no contrast, useful when it is contraindicated.
Example: A 3D TOF circle-of-Willis study detects an unruptured intracranial aneurysm without any injection.
Perfusion (DSC / DCE / ASL)
Measures tissue blood flow: DSC tracks a gadolinium bolus through susceptibility signal loss, DCE tracks T1 enhancement, and ASL magnetically labels arterial blood without contrast.
Why: Quantifies vascularity to grade tumours, assess stroke penumbra, and distinguish recurrence from treatment change.
Example: Elevated cerebral blood volume on DSC perfusion suggests high-grade glioma rather than radiation necrosis.
MR spectroscopy (MRS)
Measures the chemical composition of tissue, displaying metabolite peaks rather than an image.
Why: Adds biochemical information — metabolite ratios help characterise tumours and metabolic disease.
Example: A raised choline-to-NAA ratio in a brain lesion points toward an aggressive tumour.
DTI & tractography
Diffusion tensor imaging measures the direction of water diffusion to map white-matter fibre tracts, which tractography renders in 3D.
Why: Shows the relationship of eloquent tracts to a lesion, guiding safe surgical approach.
Example: Tractography displays the corticospinal tract displaced by a tumour so the surgeon can avoid it.
MRCP (MR cholangiopancreatography)
A heavily T2-weighted sequence that makes static fluid in the biliary and pancreatic ducts bright.
Why: Non-invasively maps the ducts to detect stones, strictures, and anatomical variants without ERCP.
Example: MRCP shows a filling defect in the common bile duct from an obstructing gallstone.
Phase-contrast & flow imaging
Uses velocity-encoding gradients so signal phase is proportional to flow velocity.
Why: Quantifies direction and speed of flow — blood or CSF — for shunts, valves, and hydrocephalus.
Example: Phase-contrast imaging measures CSF flow across the aqueduct to assess a suspected obstruction.
2D vs 3D acquisition
2D excites one slice at a time; 3D excites a whole slab and phase-encodes through it.
Why: 3D gives thin, contiguous, isotropic voxels that reformat in any plane — essential for navigation and fine anatomy; 2D is faster per slice and often higher-contrast.
Example: A 3D isotropic 1 mm T1 volume can be resliced axially, sagittally, and coronally without re-scanning.
Isotropic voxel
A voxel with equal dimensions in all three directions (e.g. 1×1×1 mm).
Why: Reformats in any plane with no loss of resolution — the prerequisite for high-quality multiplanar and 3D rendering.
Example: An isotropic MPRAGE feeds cleanly into surgical planning and CT-fusion software.
Parallel imaging (SENSE / GRAPPA / ASSET)
Uses multiple receiver coil elements to undersample k-space and reconstruct faster.
Why: Shortens scans or shrinks echo trains, trading some SNR for speed.
Example: An acceleration factor of 2 halves the phase-encoding steps of a breath-hold sequence.
Fat suppression
Techniques (spectral saturation, Dixon, STIR) that remove or separate fat signal.
Why: Prevents bright fat from masking enhancement or oedema and reduces chemical-shift artefact.
Example: Dixon reconstruction outputs in-phase, opposed-phase, water-only, and fat-only images from one acquisition.
Image fusion / co-registration
Spatially aligning two datasets (e.g. MRI with CT, or MRI with PET) into a common coordinate frame.
Why: Combines MRI soft-tissue detail with CT bony or metabolic data; underpins surgical navigation and radiotherapy planning.
Example: A 3D MRI tumour volume is fused with a planning CT so bone and lesion appear in one workspace.
Motion artifact
Ghosting or blurring caused by patient movement, breathing, or pulsation during acquisition.
Why: Recognising it prevents mis-reading and guides mitigation (breath-holds, triggering, faster sequences).
Example: Respiratory motion smears the liver edge along the phase-encode direction.
Chemical-shift artifact
Fat and water resonate at slightly different frequencies, mis-mapping fat along the frequency-encode axis.
Why: Explains dark/bright borders at fat–water interfaces and can be reduced with wider bandwidth or fat suppression.
Example: A black-and-white rim outlines the kidney against surrounding fat.
Susceptibility artifact
Signal loss and geometric distortion from local field disruption, often near metal or air.
Why: Warns of metal implants and is exploited by SWI; mitigated with SE-based or metal-artefact-reduction sequences.
Example: Dental amalgam blooms and distorts the adjacent facial anatomy.
Aliasing (wrap-around)
Anatomy outside the field of view folds onto the opposite side of the image.
Why: Recognising it allows correction by enlarging FOV or oversampling.
Example: A patient’s arm wraps into the abdomen on a small-FOV scan.
Gadolinium-based contrast agent (GBCA)
A paramagnetic agent that shortens T1, brightening enhancing tissue.
Why: Highlights vascular structures, breakdown of the blood–brain barrier, inflammation, and tumour.
Example: A meningioma enhances avidly on post-gadolinium T1 imaging.
Macrocyclic vs linear GBCA
Two structural classes of gadolinium chelate; macrocyclic agents hold gadolinium more stably.
Why: Stability governs safety, especially the risk of gadolinium retention, driving agent choice.
Example: A macrocyclic agent is preferred for a patient needing repeated contrast studies.
Dynamic contrast-enhanced (DCE) imaging
Rapid repeated imaging during and after contrast injection to capture enhancement over time.
Why: Enhancement kinetics help characterise lesions as benign or malignant.
Example: A breast lesion with rapid wash-in and wash-out raises suspicion of malignancy.
MR Safe / Conditional / Unsafe
Standardised labels for how a device behaves in the MRI environment.
Why: Determines whether an implant can be scanned and under what conditions — a core screening decision.
Example: An MR Conditional pacemaker may be scanned only within specified field and SAR limits.
SAR (specific absorption rate)
RF energy deposited in tissue, measured in W/kg, causing heating.
Why: Regulatory limits cap SAR to prevent burns; high-SAR sequences may be modified for at-risk patients.
Example: The scanner lengthens TR to keep a TSE sequence within SAR limits for a patient with an implant.
dB/dt (gradient slew rate)
Rate of change of the gradient magnetic field.
Why: Rapid switching can cause peripheral nerve stimulation; limits protect patient comfort and safety.
Example: A fast diffusion sequence is throttled to avoid stimulating the patient’s peripheral nerves.
Projectile (missile) effect
Ferromagnetic objects being violently pulled toward the magnet bore.
Why: A leading cause of serious MRI accidents; drives strict zoning and ferromagnetic screening.
Example: A steel oxygen cylinder brought into the scan room is drawn into the bore.
STEALTH / neuronavigation MRI protocol
A high-resolution, thin-slice 3D volumetric acquisition (typically a pre- and/or post-contrast isotropic 3D T1 such as MPRAGE/BRAVO, sometimes with 3D T2/FLAIR) acquired specifically for import into surgical image-guidance systems — named after Medtronic’s StealthStation. On many scanners the protocol is labelled “STEALTH” and is designed to merge with surgical and CT planning software.
Why: Image guidance needs contiguous, isotropic, geometrically accurate 3D data that can be co-registered with the patient in the operating theatre and fused with CT. Standard 2D clinical sequences have gaps and thick slices that are unsuitable; the STEALTH protocol provides seamless whole-head coverage, fiducial-compatible geometry, and clear lesion enhancement.
Example: Before a craniotomy, a post-contrast isotropic 3D T1 “STEALTH” volume is acquired, fused with the planning CT, and loaded into the StealthStation so the surgeon can navigate to the tumour in real time.
Image-guided surgery / stereotactic navigation
Using registered pre-operative imaging to track instruments relative to the patient’s anatomy during surgery.
Why: Improves accuracy and safety when targeting deep or eloquent-area lesions.
Example: A navigation pointer displays its tip position on the fused MRI/CT volume as the surgeon approaches a lesion.
Fiducial markers
Reference points — skin markers or anatomical landmarks — used to register imaging to the patient.
Why: Registration accuracy of the navigation system depends on well-placed fiducials.
Example: Adhesive scalp fiducials visible on the STEALTH scan are matched to the patient’s head at the start of surgery.
MRI–CT fusion for planning
Co-registering a 3D MRI with a planning CT to combine soft-tissue and bony/dose information.
Why: Surgery and radiotherapy need MRI lesion detail aligned to CT geometry used by planning and navigation systems.
Example: A radiotherapy plan overlays the MRI-defined tumour on the CT used for dose calculation.
Educational reference only — not a substitute for local imaging protocols, device labelling, or clinical judgement. Vendor sequence names are trademarks of their respective manufacturers.