MRI Physics & Technology
An MRI system is really two setups working together: the control room, where technologists operate the scan, and the magnet room, where the patient lies inside the machine. Because the scanner relies so heavily on radiofrequency signals, it sits inside a copper-lined enclosure called a Faraday shield, which blocks outside electromagnetic interference from corrupting the images.
Core components
- Primary magnet — generates the main magnetic field, measured in Tesla (T). Most clinical scanners run between 1.5T and 3T.
- Gradient coils — three coils (X, Y, Z) that layer a smaller, adjustable magnetic field on top of the main one, letting the scanner target a specific slice of the body. The Z coil produces axial images, the Y coil produces coronal images, and the X coil produces sagittal images. These coils are also the source of the loud knocking sound during a scan.
- Shim coils — fine-tune the field's evenness ("homogeneity"), which directly affects image quality. Passive shimming uses fixed metal inserts for broad correction; active shimming uses computer-controlled coils for finer adjustments.
- Radiofrequency (RF) coils — send the RF pulse into the body and pick up the return signal. Different coils are shaped for different regions.
Common RF coil types
| Coil type | Used for | Notes |
|---|---|---|
| Head coil | Brain and cervical spine imaging | Cage-like shape; patient can see out during the scan |
| Body / spine coil | Abdomen, chest, spine, whole-body scans | Larger surface area; secured with straps |
| Extremity coil | Feet, ankles, knees, wrists, elbows, shoulders | Sized for smaller peripheral body parts |
How the signal is generated
Roughly 70% of the human body is water, and each hydrogen atom in that water carries a single, positively charged proton. Left alone, these protons spin in random directions. Once the scanner's magnetic field is switched on, they line up either with or against it — slightly more align with the field, and that small surplus is what produces the signal used to build an image.
A brief RF pulse knocks the protons out of alignment. How far they tip depends on the pulse's strength and length: a 90° pulse rotates them into the transverse plane, while a 180° pulse flips them to the opposite direction along the main field. This only works when the pulse is tuned to the protons' natural spin frequency — a phenomenon called resonance. As the protons realign after the pulse, they generate a small electrical current, which is what the scanner actually measures to build the image.
T1 and T2 relaxation
After the RF pulse switches off, protons drift back to their resting alignment in a process called relaxation, which happens in two distinct ways:
- T1 relaxation — protons release energy to their surroundings as they return to the main field's direction. T1-weighted images are good for showing anatomical detail; water appears dark and fat appears bright.
- T2 relaxation — protons lose their phase alignment with each other in the transverse plane. T2-weighted images tend to highlight both fat and water, and are especially sensitive to differences between tissue types.
Two timing settings control how much of each effect shows up in an image: TE (time to echo) and TR (time to repeat the pulse sequence). Short TE with long TR produces T1-weighted images; short TE with short TR produces T2-weighted images; long TR with short TE minimizes both effects and produces proton-density images instead.
Common pulse sequences
| Sequence | How it works | Good for |
|---|---|---|
| Spin echo | Uses a 180° pulse at half the TE time to refocus the signal | Standard T1, T2, and proton-density images |
| Turbo spin echo | Applies several 180° pulses within one TR to speed up acquisition | Faster scans with similar image quality |
| Gradient echo | Replaces the refocusing pulse with a magnetic gradient pulse | Detecting hemorrhage and related vascular conditions |
| Inversion recovery | Starts with a 180° pulse to invert magnetization before it regrows | Suppressing signal from a specific tissue, e.g. fat |
Contrast agents
Gadolinium is the most common intravenous contrast agent used in MRI. Its magnetic properties shorten relaxation times in nearby tissue, making it show up as a bright area on T1-weighted images. Allergic reactions are uncommon — far less frequent than with iodine-based CT contrast — though patients with significant kidney impairment face a rare risk of a condition called nephrogenic systemic fibrosis, so kidney function is usually checked first.
Safety considerations
A standard 1.5T scanner produces a magnetic field roughly 21,000 times stronger than the Earth's own. That strength can pull loose metal objects toward the machine with real force, so anything metallic — hearing aids, jewelry, belts, phones, pagers — has to come off or be powered down before entering the room. Patients are also screened for internal metal, such as aneurysm clips, pacemakers, or other implants, before being cleared for a scan.
MRI during pregnancy
Because MRI doesn't use ionizing radiation, it's considered relatively safe during pregnancy. There's a theoretical concern that heat generated by the gradient coils could affect early fetal development, so scans are typically avoided during the first trimester unless necessary. Gadolinium contrast is also used sparingly in pregnant patients, only when the diagnostic benefit clearly outweighs the risk.
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