MRI sequences explained
Read Your Scan Editorial Team·Last updated ·6 min read
Written from published radiology references and reviewed for plain language. Not written or signed by a physician, and not a diagnosis. How we write these pages →
The short answer
An MRI sequence is a recipe of magnetic pulses that makes the scanner sensitive to one tissue property. T1 shows anatomy with bright fat, T2 shows water as bright, FLAIR suppresses free fluid, STIR suppresses fat, and diffusion imaging tracks water movement. Comparing them is how tissue is identified.
- Each series in your study is the same anatomy imaged a different way, not a repeat or a different angle.
- The single most useful rule: on T1 fat is bright and water is dark; on T2 water is bright.
- Fat suppression exists so that bright fat stops hiding bright fluid next to it.
- No sequence is diagnostic alone. Identification comes from how a structure behaves across several.
What a sequence actually is
An MRI scanner does not have a single imaging mode. It works by tipping the hydrogen nuclei in your tissue out of alignment with a radio pulse and then measuring the signal as they settle back. How long the scanner waits between pulses, and how long it waits before listening, changes which tissues dominate the resulting picture. A sequence is that recipe of timings — and swapping the recipe produces a completely different-looking image of exactly the same anatomy.
This is why an MRI takes so long and why your study contains many folders. Each is one recipe, run over the same region. A radiologist chooses a location and then looks at it in every recipe, because what a structure is depends on how it behaves as the recipe changes. A bright spot is not a finding; a bright spot that stays bright on one sequence and vanishes on another is.
The sequences you will see named in a report
| Sequence | What it is sensitive to |
|---|---|
| T1-weighted | Anatomy. Fat bright, fluid dark, tissue boundaries crisp. Also the best sequence for bone marrow and for anything containing fat. |
| T2-weighted | Water. Fluid is bright, and since most disease increases local water, most abnormalities appear here. Highly sensitive, not at all specific. |
| Proton density (PD) | A middle setting, common in joints: enough anatomical detail to see cartilage and menisci while still showing fluid. |
| FLAIR | T2 with free fluid nulled. The ventricles and CSF go black, so lesions immediately beside them stop being lost in glare. A brain-imaging workhorse. |
| STIR / fat-sat | Fat suppressed instead. Bone marrow oedema, soft-tissue swelling and subtle fluid become obvious against a dark background. |
| DWI with ADC | How freely water diffuses. Restriction — bright DWI with dark ADC — is the signature of an acute infarct, and also occurs in abscess and some tumours. |
| GRE / SWI | Blood products, iron and calcium, which bloom into exaggerated dark spots. Used for microbleeds and old haemorrhage. |
| MRA / MRV | Flowing blood, imaged to produce an angiogram of arteries or veins, usually without any contrast injection. |
| Post-contrast T1 | Where injected gadolinium accumulates. Enhancement marks a leaky blood-tissue barrier: inflammation, infection, tumour. |
How to tell which image you are looking at
- Find fluid you know is there — the cerebrospinal fluid around the brain and cord, or fluid in a joint. If it is bright white, you are on T2 or a fluid-sensitive sequence. If it is dark, you are on T1.
- Then find fat — subcutaneous fat under the skin, or the marrow inside bone. Bright fat with dark fluid means T1. Dark fat with bright fluid means the fat has been suppressed: STIR or a fat-saturated T2.
- If fluid is bright everywhere except the ventricles, which are black, you are looking at FLAIR.
- If the images look grainy, low-resolution and oddly bright in places, they are probably diffusion images, which trade image quality for speed and sensitivity.
DICOM series names are set by the scanner and vary between manufacturers and hospitals, so the labels in your own study may be abbreviations you cannot decode. The appearance rules above are more reliable than the file names.
Why the combination is the diagnosis
Take a bright spot on T2. If it is also bright on T1, it probably contains fat or blood products, because very few other things are bright on both. If it is dark on T1 and stays bright with fat suppression, it is fluid. If it enhances after contrast, something is leaking there. If it restricts on diffusion, water movement inside it is impeded. Four comparisons and a shapeless bright spot has become a description with real content.
That is the whole logic of MRI, and it is also why sequences that were not acquired cannot be commented on. If a study did not include diffusion, restricted diffusion was never assessed. If no contrast was given, enhancement was never evaluated. The technique section at the top of your report is therefore not filler: it defines exactly which questions this scan was able to ask.
When the picture lies
- Motion artifact — blurring or ghosting from movement, including breathing and blood pulsation. It is the main reason you are asked to hold still, and a badly degraded study sometimes has to be repeated.
- Susceptibility artifact — metal, including dental work and surgical hardware, distorts the local magnetic field and produces a black void with a bright rim. Specific sequences reduce it, but never entirely.
- Chemical shift and partial volume — boundary effects that create thin bright or dark lines where two tissues meet, or blur a small structure into its neighbours.
- T2 shine-through — an area bright on diffusion purely because it is bright on T2, which is exactly why the ADC map is always read alongside it.
Reports comment on artefact for a reason. A sentence noting that image quality was limited is telling your doctor how much confidence to place in what follows, and occasionally that a region could not be assessed at all.
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Frequently asked questions
- What is the difference between T1 and T2 on an MRI?
- They are two different timing recipes. On T1 images fat is bright and fluid is dark, which gives crisp anatomy. On T2 images fluid is bright, and because most disease processes increase local water content, most abnormalities show up there. Radiologists use T1 to see structure and T2 to find things.
- Why does my MRI have so many folders or series?
- Each one is a different sequence covering the same body part. Identifying tissue depends on comparing how it behaves across sequences, so a routine study acquires several — typically T1, T2, a fat-suppressed sequence, and whichever specialised ones the clinical question requires.
- What is FLAIR used for?
- FLAIR is a T2 image with the signal from free fluid switched off, so cerebrospinal fluid appears black instead of white. That matters in the brain, because lesions sitting immediately next to the fluid-filled ventricles would otherwise be lost in the glare. It is the standard sequence for white matter change.
- What does STIR mean on my report?
- STIR is a fat-suppressed sequence: it turns fat dark so that anything containing extra fluid glows against it. It is heavily used in spine and musculoskeletal imaging because bone marrow oedema, subtle swelling and stress injury are obvious on STIR and can be nearly invisible on a standard T1.
- Does a brighter signal mean something is worse?
- No. Brightness is a physical property of the tissue on that particular sequence, not a severity scale. Fat is bright on T1 and normal; cerebrospinal fluid is bright on T2 and normal. What matters is whether a structure is brighter or darker than it should be for its type, and how it behaves across the other sequences.
Words on this page
Each of these has its own page in the report glossary.
Report glossary, A–Z →Sources
- Patient-facing procedure descriptions (MRI, CT, X-ray, ultrasound)RadiologyInfo.org — Radiological Society of North America & American College of Radiology
- MRI scan — overview, how it is performed and resultsNHS (United Kingdom)
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Read Your Scan is informational only — not a medical diagnosis, and not a substitute for a licensed radiologist or your doctor. This page explains what a report says and what a radiologist looks at; it cannot tell you what your own images show. If you have urgent symptoms, seek care.