Quick reference guide
General Principles
MRI is an imaging technique that allows us to "listen" to the signals emitted by hydrogen protons in our body when we subject them to a very powerful magnetic field. That's why it's so good for seeing soft tissue, we're full of H₂O!
The term "nuclear" is a little scary because it is associated with radiation, but in this case it has nothing to do with it. It is called this simply because the phenomenon originates in the nucleus of hydrogen atoms. To avoid this confusion in patients, in clinical practice it is abbreviated to "Magnetic Resonance" (MRI) or MRI for its acronym in English. Does not use ionizing radiation, unlike X-rays or CT.
How does it work?
Imagine that the hydrogen protons in the patient's tissues are like little microscopic compasses.
- In normal state: Outside of a magnetic field, these "compasses" are disordered, pointing in all directions, as if they have no clear north. The net effect is zero.
- Enter the magnet (the resonator): When we put the patient into the resonator, we apply a very powerful magnetic field. This field is like an incredibly strong "Magnetic North" that forces almost all of those little compasses (protons) to align with it. Some point "up" and others "down", but all on the same axis.
- The radiofrequency pulse (the "question"): Once the protons are aligned and "calm," we send them a pulse of radio waves, as if giving them a little lateral push. This pulse is just the right frequency for the protons to "resonate" (hence the name) and move out of alignment, spinning to one side. It's like asking them a specific question that only they can "hear."
- Relaxation and the signal (the "response"): Here comes the magic. When we turn off the radiofrequency pulse, the protons tend to "relax" and return to their original position, aligned with the large magnetic field. By doing so, they release the energy they absorbed from the radio pulse. That energy released is a very weak radio signal.
- Creating the image: A very sensitive antenna, like a microphone, picks up these signals. A powerful computer processes millions of these signals and, based on the time it takes for the protons to relax and the intensity of the signal they emit, builds a very detailed three-dimensional image.
Why do we see different fabrics?
The key is in the relaxation. Protons from different tissues (fat, muscle, cerebrospinal fluid, gray matter, white matter) relax at different rates.
- The protons in the free water (such as in a cyst or CSF) take longer to relax.
- The protons in the fat They relax very quickly.
By playing with the timing and sequences of the radiofrequency pulses (the famous T1, T2, FLAIR sequences, etc.), we can "weight" the images to highlight these differences. That is why in one sequence the fat looks bright (hyperintense) and in another it looks dark (hypointense). It is like applying different "filters" to highlight one tissue over another and thus characterize the lesions.
In short, MRI is an incredibly sophisticated way to map the distribution of hydrogen protons in the body and differentiate tissues based on how these protons respond to magnetic manipulation. We do not see the tissues directly, but rather the radio "echo" that their hydrogen atoms emit.
The Key: Relaxation Times T1 and T2
The contrast of the image does not come from the signal itself, but from the speed to which protons relax in different tissues. There are two simultaneous processes:
Relaxation T1 (Longitudinal)
It is the recovery of alignment with the main magnetic field (B₀). It measures how quickly protons "give up" their energy to the molecular environment.
Analogy: Compasses that point back to magnetic North after being deflected. Some do it quickly (short T1), others slowly (long T1).
- Short T1 (fast): Fat, Gadolinium. appear bright at T1.
- T1 Long (slow): Water (CSF, edema). appear dark at T1.
Relaxation T2 (Transverse)
It is the loss of synchronization (phase coherence) between the protons. It is due to the magnetic interactions between them.
Analogy: A group of compasses rotating in perfect synchrony. Little by little, they begin to get out of phase and point in different directions.
- T2 Long (slow): Water (pure liquids). They take time to get out of phase. appear bright in T2.
- Short T2 (fast): Solid, paramagnetic tissues (hemosiderinAn iron-containing pigment, derived from hemoglobin. It is found in old bleeding sites.). They go out of date quickly. appear dark in T2.
To Know More: TR and TE
By controlling two key parameters, we "force" the image to display T1 or T2 contrast:
TR (Repetition Time)
It is the time between one RF pulse and the next. Control the boost T1.
- Short TR: It does not give tissues with long T1 (water) time to recover. Your signal will be weak (dark). Highlight the T1 differences.
- TR Long: Give all tissues time to recover. Minimizes T1 contrast.
TE (Echo Time)
It is the time we wait to "hear" the signal after the RF pulse. Control the boost T2.
- Short TE: We hear the signal early, before the protons get out of phase. Minimizes T2 contrast.
- Long TE: We listen late, giving time for tissues with short T2 (solid) to lose their signal. Highlight tissues with long T2 (water).
Test your knowledge
On a T1-weighted image, which tissue would you expect to see brightest?
The Arsenal of Sequences
T1: The Anatomical Reference
The fat is shiny. The water is dark. Excellent for morphology and is the base sequence for post-contrast.
T2: The Pathology Detector
Water (edema, CSF, cysts) and fat are bright. Almost every lesion is hyperintense here. It is sensitive but not specific.
FLAIR: Optimized Brain T2
It is a T2 with suppression of the CSF signal. The "bad" fluid (edema, gliosis) is bright, the "good" fluid (CSF) is dark. Essential in neuroimaging.
Diffusion (DWI): The Molecular Motion Detector
This sequence does not look at the anatomy, but at the physiology: measures the Brownian (random) motion of water molecules.
In healthy tissues, water moves freely. In certain pathologies, movement is limited. This is called restriction on diffusion.
What causes restriction?
- Cytotoxic edema (Acute stroke): The cells swell, trapping water.
- Hypercellularity (Tumors, Abscesses): Too many cells packed together leave little room for water to move.
How is it interpreted?
You should always look at the image DWI next to the map ADC (Apparent Diffusion Coefficient):
- True Constraint: Bright on DWI + Dark on ADC.
- T2 "Shine-Through" effect: Bright on DWI + Bright on ADC (it's just a very bright T2 lesion, not a real restriction).
GRE / SWI: The "Metal Detector"
Gradient Echo sequences very sensitive to the inhomogeneity of the magnetic field. Perfect for detecting blood (hemosiderin), calcium and air, which appear very dark (hypointense).
Fat Suppression (FatSat, STIR)
Essential to nullify the bright signal of fat (which can mask pathology). STIR is more robust and homogeneous than FatSat. Key to seeing bone edema or inflammation in soft tissues.
Quick Signal Table
| Tissue | Visual | T1 | T2 | FLAIR | DWI (Restriction) |
|---|---|---|---|---|---|
| Water / CSF / Edema | ⚫ Dark | ✨ Brilliant | ⚫ (CSF) / ✨ (Edema) | - | |
| Fat | ✨ Brilliant | ✨ Brilliant | ✨ Brilliant | - | |
| Gray Matter | dark gray | light gray | light gray | - | |
| Acute Stroke | ⚫ Dark | ✨ Brilliant | ✨ Brilliant | ✨ Brilliant | |
| Blood (Chronicle) | Variable | ⚫⚫ Very Dark | ⚫⚫ Very Dark | ⚫ Dark | |
| T1 + Contrast | ✨ Brilliant | - | - | - |
Test your knowledge
What sequence is essential in neuroimaging to differentiate edema from CSF?
Common Artifacts
Motion
Causes blurry or "ghosting" images. It is the most common artifact. It is solved with the patient's collaboration or quick sequences.
Magnetic Susceptibility
It occurs near metal objects (prostheses, fillings) or air. Causes distortions and signal gaps. The GRE/SWI sequences are the most sensitive.
Aliasing (Wrap-around)
If the field of view (FOV) is very small, the structures that are left out "fold" and appear on the opposite side of the image.
Chemical Displacement
Protons from fat and water preprocess at slightly different frequencies. Causes a black outline at fat-water interfaces.
Advanced Studies
🧠 Functional MRI (fMRI)
Detects changes in blood oxygenation (BOLD effectBlood-Oxygen-Level-Dependent. Deoxyhemoglobin is paramagnetic and alters the signal, oxyhemoglobin does not.). Maps brain activity for neurosurgical planning.
🩸 MR Angio (MRA)
Visualize blood vessels. It can be without contrast (Time-of-Flight, TOF) or with contrast for greater anatomical and functional detail.
🗺️ Tractography (DTI)
An advanced DWI that maps the directionality of water movement along white matter tracts. It is the "wiring map" of the brain.
📈 Perfusion (PWI)
Evaluates cerebral blood flow. Crucial in stroke to identify the ischemic penumbra (tissue at risk but salvageable).
🧪 Spectroscopy (MRS)
A "virtual biopsy." Analyzes the chemical composition of a voxel, measuring metabolites such as NAA, Hill y Lactate.
Contrast with Gadolinium
Gadolinium is a paramagnetic agent that dramatically shortens T1 of nearby water protons. Therefore, the areas where it accumulates are seen hyperintense (bright) on T1.
Clinical Considerations and Safety
Key Indications: Tumor characterization, detection of active inflammation/infection, perfusion studies and angiography.
Security: The main risk is Nephrogenic Systemic Fibrosis (NSF) in patients with severe renal failure (GFR < 30 mL/min/1.73m²). It is always mandatory to evaluate kidney function.
Types of Agents: The agents macro cyclic They are more stable and have a lower risk of long-term tissue gadolinium deposition.
Putting it into Practice: Clinical Cases
Case 1: Acute Stroke
Essential Protocol: DWI/ADC (confirms ischemia), GRE/SWI (rules out bleeding), FLAIR (chronology), MR Angio (vessel occlusion).
Case 2: Knee Injury
Essential Protocol: PD with fat suppression (edema, injuries), T1 (anatomy, fractures), T2 (fluid).
Case 3: Brain Tumor
Essential Protocol: T2/FLAIR (localization), T1 without and with contrast (enhancement), DWI/ADC (cellularity), GRE/SWI (bleeding).
Case 4: Appendicitis in a Child
Essential Protocol (without contrast): T2 HASTE (freeze movement), T2 with fat suppression (edema), DWI/ADC (pus restriction).
Case 5: Lumbar Disc Herniation
Essential Protocol (sagittal and axial planes): T2 (better to see the disc, thecal sac and nerve roots), T1 (anatomy of the vertebrae and foramina).