Glossary
The vocabulary, without the jargon.
Short, plain definitions of the terms that come up when talking about low-field, portable and open MRI.
A–Z
Terms.
- Low-field MRI
- MRI performed at a lower magnetic flux density than the 1.5T and 3T systems that dominate hospital imaging. In practice, low-field usually means anything below roughly 1T, and ultra-low-field below 0.1T.
- Field strength
- The magnetic flux density of the main field, measured in tesla (T). Signal-to-noise ratio scales roughly with field strength, which is why higher field was pursued for decades.
- Permanent magnet
- A magnet that produces its field without an electrical current or cryogenic cooling — typically NdFeB or SmCo. It cannot be switched off, but it removes liquid helium, the quench risk and the compressor.
- Halbach array
- An arrangement of permanent magnets in a ring whose field is strong inside and weak outside. It allows a compact, lightweight magnet with a usable bore.
- Open magnet
- A geometry that leaves the patient space accessible rather than fully enclosed — C-type and H-type designs are common. Useful for claustrophobic patients, interventional access and standing or seated scanning.
- SNR (signal-to-noise ratio)
- How much real signal there is compared with random noise. The central constraint in low-field imaging, and the reason reconstruction algorithms matter so much.
- EMI (electromagnetic interference)
- External noise from mains wiring, motors, lifts and broadcast sources that corrupts the signal. A shielded room is the traditional answer.
- EMI cancellation
- Measuring ambient noise with dedicated sensing coils and subtracting it in real time using deep learning — the reason a system can image in an ordinary room.
- Shielded room
- A copper or steel enclosure that blocks external electromagnetic noise. Costly to build on site and impossible to move — which is why here it ships as a modular enclosure instead of being built.
- Larmor frequency
- The precession frequency of a nucleus in a magnetic field, proportional to field strength. It sets the operating frequency of the system and the design of its coils.
- T1 / T2 relaxation
- The time constants that describe how nuclear magnetisation recovers and decays. Image contrast comes from the difference between these values across tissues.
- Compressed sensing
- A reconstruction approach that recovers an image from fewer measurements than classical theory requires, by exploiting the fact that images are sparse in some representation.
- Parallel imaging
- Using multiple receiver coils to acquire less data while keeping the image — the coil geometry supplies the missing spatial information.
- Metal artifact
- Distortion around implants and prostheses. It scales with field strength, so implants are noticeably less troublesome at low field.
- Standing / weight-bearing MRI
- Scanning a patient upright and load-bearing, so that the anatomy is imaged in the position that produces the symptoms.
- Point-of-care imaging
- Imaging brought to where the patient already is — the bedside, the ward, the operating theatre — rather than moving the patient to the scanner.
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