MRI
Meaning of MRI: No x-rays are involved, which is one reason this scan is used so heavily in neurology, orthopedics and pediatric imaging.
Definition of MRI
MRI, short for magnetic resonance imaging, is a scan that uses a strong magnetic field and radio waves rather than x-rays to build detailed cross-sectional pictures of the inside of the body. It distinguishes one soft tissue from another unusually well, which makes it a standard choice for the brain, the spinal cord, joints and bone marrow.
How does the scanner make a picture?
The body is mostly water and fat, and both are rich in hydrogen. Inside the scanner's magnetic field, the nuclei of those hydrogen atoms line up. A pulse of radio energy knocks them out of alignment, and as they settle back they emit a faint signal that receiver coils pick up. How quickly they settle depends on the tissue around them, so fat, muscle, fluid, gray matter and white matter each return a slightly different signal, and a computer converts those differences into an image.
Varying the timing of the pulses produces different sequences, each weighted to show something particular: fluid and swelling on one, fat and anatomy on another, blood flow, diffusion of water or blood supply on others. A radiologist reads several together rather than a single picture. An injected contrast agent based on gadolinium can be added, which changes the signal where blood vessels are leaky, highlighting inflammation, infection and many tumors.
How does it compare with a CT scan?
| Aspect | MRI | CT |
|---|---|---|
| Energy used | Magnetic field and radio waves | X-rays |
| Strongest at | Brain and spinal cord, ligaments and cartilage, muscle, bone marrow | Bone detail, fresh bleeding, lung tissue, urgent chest and abdominal questions |
| Time in the scanner | Several minutes per sequence, so a study often runs far longer | Seconds |
| Ionizing radiation | None | A radiation dose with each scan |
Neither replaces the other. The claim that MRI simply produces better images than a CT scan holds only for soft tissue contrast, and CT remains faster, more widely available and better for several specific questions.
What are its practical limits?
The magnet is always on, even between scans, so ferromagnetic metal anywhere near it is a hazard and implanted devices, surgical clips and retained metal fragments are screened for beforehand. The bore is narrow and the sequences are loud, which some people find difficult. Images also blur with movement, so a study depends on lying still, and that is the main reason young children may need sedation for one.
Also known as
Used in a sentence
An MRI was requested to obtain detailed images of the knee.
Good to know
The scanner's magnetic field remains on continuously, so implanted devices, surgical hardware and retained metal fragments are screened for before a scan and can make MRI unsuitable.
Word origin
Where “MRI” comes from
Named for magnetic resonance, the behavior of atomic nuclei placed in a magnetic field that produces the signal the scanner reads.