Unlike X-ray, CT, or nuclear medicine, MRI uses no ionizing radiation. Instead, it leverages the magnetic properties of hydrogen protons in the body to create incredibly detailed images of soft tissues. This beginner's guide covers the MRI physics that every MR technologist must understand — from basic principles through practical sequence selection.
MRI doesn't produce images by measuring how much radiation passes through or is emitted from the body. Instead, it excites hydrogen protons with radio waves, then listens to the signal they emit as they relax back to equilibrium. The spatial location is encoded by precisely varying the magnetic field across the patient. No radiation, no moving X-ray tube — just magnets, radio waves, and sophisticated mathematics.
All of MRI rests on three physical phenomena. Understanding these is the foundation for everything else:
The main static magnetic field, measured in tesla (T). Clinical strengths: 1.5 T and 3.0 T. Higher field = more signal, better resolution, but more artifacts.
Radiofrequency energy at the Larmor frequency excites protons, knocking them out of alignment with B0. The same coil that transmits RF also receives the return signal.
Three sets of gradient coils create linear variations in the magnetic field along X, Y, and Z axes. They encode spatial position into the signal.
The equation at the heart of MRI is the Larmor equation:
ω₀ = γ × B₀
At 1.5 T: Larmor frequency = 42.58 × 1.5 = ~63.9 MHz
At 3.0 T: Larmor frequency = 42.58 × 3.0 = ~127.8 MHz
The RF pulse must be at the Larmor frequency to transfer energy to the protons. This is why different field strengths need different RF frequencies — and why RF shielding must be appropriate for the operating frequency.
After an RF pulse knocks protons out of alignment, two independent relaxation processes occur simultaneously:
T1 is the time constant for the longitudinal magnetization to recover to 63% of its original value. It represents protons giving energy back to the surrounding molecular lattice.
T2 is the time constant for the transverse magnetization to decay to 37% of its initial value. It represents protons losing phase coherence due to spin-spin interactions.
| Tissue | T1-Weighted | T2-Weighted | FLAIR |
|---|---|---|---|
| CSF / Water | Dark ↓ | Bright ↑↑ | Dark ↓ (suppressed) |
| Fat | Bright ↑↑ | Intermediate | Intermediate |
| Gray matter | Gray | Gray | Gray |
| White matter | Light gray | Dark gray | Dark gray |
| Edema / Tumor | Dark ↓ | Bright ↑↑ | Bright ↑↑ |
| Bone (cortex) | Dark ↓ | Dark ↓ | Dark ↓ |
| Blood (acute) | Intermediate | Dark ↓ | Variable |
| Gadolinium contrast | Bright ↑↑ (on T1) | Variable | Variable |
"T1 — Fat is bright." Think of T1 as the "anatomy" sequence — structures are well-defined, fat is white, water is black.
"T2 — Water is bright." Think of T2 as the "pathology" sequence — anything with excess fluid (edema, tumor, inflammation) is white.
TR (Repetition Time) — The time between successive RF pulses applied to the same slice. Controls how much T1 relaxation is allowed.
TE (Echo Time) — The time between the RF pulse and the peak of the signal echo. Controls how much T2 decay is allowed.
| Sequence | TR | TE |
|---|---|---|
| T1-weighted | Short (400–800 ms) | Short (10–30 ms) |
| T2-weighted | Long (2000–5000 ms) | Long (80–120 ms) |
| PD-weighted | Long (2000–5000 ms) | Short (10–30 ms) |
| FLAIR | Long (6000–10000 ms) | Long (80–140 ms) + inversion pulse |
Without gradients, every proton would experience the same magnetic field and resonate at the same frequency — producing a signal but with no spatial information. Gradient coils create linear variations in the magnetic field along each axis:
Gradients are also responsible for the loud knocking sounds during MRI — the rapid switching of currents through gradient coils causes mechanical vibration (the Lorentz force on the coils).
RF coils function as both transmitters (sending RF pulses) and receivers (detecting the MRI signal). Different coils are optimized for different body parts:
| Coil Type | Location | Best For |
|---|---|---|
| Body coil (built-in) | Inside the bore | Whole-body imaging, large FOV |
| Head coil | Encloses the head | Brain, orbits, sinuses (highest SNR) |
| Spine coil | Embedded in table | Cervical, thoracic, lumbar spine |
| Surface / Phased-array | Placed on body surface | Knee, shoulder, breast, wrist — high resolution near the coil |
| Cardiac coil | Anterior + posterior array | Cardiac MRI, MR angiography |
Phased-array coils with multiple receiver elements allow parallel imaging (GRAPPA, SENSE) which reduces scan time by using multiple coil elements to partially replace gradient-encoded spatial information.
MRI has unique safety concerns that every technologist must know:
For more on how MRI compares to other modalities, see CT vs MRI: When to Use Which and explore our MRI modality overview.