Superconducting vs permanent
Conventional clinical MRI at 1.5 T, 3.0 T, and above uses superconducting solenoids — not permanent magnets. Permanent magnets serve a different, growing part of the field:
- Open & low-field MRI — permanent-magnet systems up to roughly 0.4 T give open patient access with no cryogens, no quench risk, and no standby power.
- Portable / point-of-care imaging — recent low-field scanners built around NdFeB Halbach arrays (tens of millitesla) bring imaging to the bedside, the clinic, and underserved settings.
- Benchtop & industrial NMR — compact permanent-magnet arrays polarize samples for spectroscopy, relaxometry, and process analysis without a cryomagnet.
Passive operation — no cooling, no power, always on, open geometry, far lower cost — bought at the price of lower field strength and sensitivity to temperature. The engineering of a permanent-magnet MR system is the engineering of homogeneity and thermal stability at whatever field the array can reach.
Two permanent architectures
| Yoked dipole (C / H) | Halbach cylinder | |
|---|---|---|
| Structure | Two pole pieces bridged by an iron yoke | Ring of magnets with rotating magnetization |
| Field uniformity | Excellent — large iron shapes the field | Good; set by segmentation & length |
| Weight | Heavy (massive iron return path) | Lighter for the same bore |
| Fringe field | Escapes around the yoke | Largely self-shielded (ideal Halbach: none) |
| Access | Open, patient-friendly | Bore / cylinder |
| Typical use | Open low-field MRI, dedicated extremity | Portable MRI, benchtop NMR |
The Halbach cylinder is the permanent-magnet analog of a solenoid: rotating the magnetization angle around the ring produces a uniform transverse field in the bore, with the flux nearly cancelled outside. Its field follows B₀ = Br·ln(ro/ri) for the ideal case — real arrays fall short of that because of the gaps between discrete segments and finite length. Both architectures lean on precise magnetization direction in every element.
Homogeneity is the spec
In magnetic resonance, resonance frequency is proportional to field. A field that varies across the imaging volume smears the signal — so homogeneity, quoted in parts per million (ppm) over the field of view, is the governing specification, ahead of raw strength.
- Segment placement tolerance — angular and positional errors in each magnet translate directly into field ripple. Precision matters more here than in almost any other magnet application.
- Shimming — small trays of trim magnets (passive shims) correct residual inhomogeneity after assembly, tuning the field to spec.
- Verification — the field map, not a single surface-gauss reading, is the acceptance artifact; it's mapped point-by-point across the bore (field mapping & verification).
Field strength & material
Field comes from the array geometry and the magnet's remanence, so NdFeB is the near-universal choice for its high Br. Portable head-scale Halbach systems reported in the literature run in the 50–100 mT range at ~120 kg (with ~80 kg of rare-earth material), built from arrays of N42–N52 NdFeB cubes; yoked open systems reach a few tenths of a tesla. Higher field needs more magnet mass, quickly.
- Grade: high-remanence NdFeB (N50/N52 class) maximizes B₀ per unit mass (grades chart).
- Geometry over grade: as with sensors, a grade step buys only a few percent of field — array radius ratio and length do the heavy lifting (block/cube magnets are the usual building blocks).
Temperature stability
NdFeB remanence drifts about −0.12%/°C. In an MR magnet that means the field — and therefore the resonance frequency — moves with temperature, which is why permanent-magnet systems are so sensitive to it.
- Thermal control — enclosures, heaters, and stabilization hold the array at a set temperature so the field stays put.
- Material compensation — a clever route pairs NdFeB with SmCo elements of opposite temperature coefficient so the assembly's net drift approaches zero — the same temperature-stability argument that makes SmCo the instrument-magnet choice (material comparison, temperature guide).
A permanent-magnet MR system is calibrated at a working temperature. Let it drift and the images degrade. Decide early whether you'll stabilize thermally, compensate with SmCo, or both — it changes the magnet spec.
Fringe field & safety
- The 5-gauss line — the safety boundary for pacemakers and implants. A Halbach cylinder's self-shielding keeps that boundary tight; a yoked dipole leaks more and needs more siting clearance.
- Projectile hazard — any MR-strength magnet turns loose ferromagnetic objects into projectiles. Screening and access control are part of the system, not an afterthought.
- Assembly forces — the arrays themselves are dangerous to build (next section).
Assembly & handling
Building an MR array means bringing many strong magnets into precise, permanent registration against enormous forces:
- Non-magnetic fixtures — aluminum, brass, fiberglass tubes and trays locate each element; the structure must survive the array's own internal forces.
- Element-by-element build — cube-array (mandhala) construction places magnetized blocks one at a time into a fixture that holds the geometry.
- Retention & bonding — every element is captured and bonded so it can't shift under load or temperature; a micron of migration is a ppm of inhomogeneity (bonding & mounting).
- Coating — elements are sealed against corrosion even inside the housing.
Specifying an MR magnet
For an MRI or NMR magnet or array, beyond the standard RFQ checklist:
- Target field B₀ and bore / field-of-view.
- Homogeneity in ppm over the FOV — the make-or-break number.
- Temperature-stability requirement and the chosen approach (thermal, SmCo compensation, or both).
- Element grade, geometry, and placement tolerance — angular and positional, per element.
- Fixture / retention and non-magnetic housing constraints.
- Verification — the field-map method and homogeneity acceptance limits.