Investig Magn Reson Imaging.  2019 Sep;23(3):179-201. 10.13104/imri.2019.23.3.179.

Portable Low-Cost MRI System Based on Permanent Magnets/Magnet Arrays

• Huang S
• Ren ZH
• Obruchkov
• Gong J
• Dykstra R
• Yu W

• KMID: 2459873
• DOI: http://doi.org/10.13104/imri.2019.23.3.179

Abstract

Portable low-cost magnetic resonance imaging (MRI) systems have the potential to enable "point-of-care" and timely MRI diagnosis, and to make this imaging modality available to routine scans and to people in underdeveloped countries and areas. With simplicity, no maintenance, no power consumption, and low cost, permanent magnets/magnet arrays/magnet assemblies are attractive to be used as a source of static magnetic field to realize the portability and to lower the cost for an MRI scanner. However, when taking the canonical Fourier imaging approach and using linear gradient fields, homogeneous fields are required in a scanner, resulting in the facts that either a bulky magnet/magnet array is needed, or the imaging volume is too small to image an organ if the magnet/magnet array is scaled down to a portable size. Recently, with the progress on image reconstruction based on non-linear gradient field, static field patterns without spatial linearity can be used as spatial encoding magnetic fields (SEMs) to encode MRI signals for imaging. As a result, the requirements for the homogeneity of the static field can be relaxed, which allows permanent magnets/magnet arrays with reduced sizes, reduced weight to image a bigger volume covering organs such as a head. It offers opportunities of constructing a truly portable low-cost MRI scanner. For this exciting potential application, permanent magnets/magnet arrays have attracted increased attention recently. A magnet/magnet array is strongly associated with the imaging volume of an MRI scanner, image reconstruction methods, and RF excitation and RF coils, etc. through field patterns and field homogeneity. This paper offers a review of permanent magnets and magnet arrays of different kinds, especially those that can be used for spatial encoding towards the development of a portable and low-cost MRI system. It is aimed to familiarize the readers with relevant knowledge, literature, and the latest updates of the development on permanent magnets and magnet arrays for MRI. Perspectives on and challenges of using a permanent magnet/magnet array to supply a patterned static magnetic field, which does not have spatial linearity nor high field homogeneity, for image reconstruction in a portable setup are discussed.

Keyword

Potable MRI; Low cost; Permanent magnets; Permanent magnet arrays; Spatial encoding magnetic field (SEM); SEM

MeSH Terms

Diagnosis
Head
Image Processing, Computer-Assisted
Magnetic Fields
Magnetic Resonance Imaging*

Figure

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  Fig. 1. Examples of open MRI (a) Siemens 1.5T MAGNETON Aera, short cylindrical superconducting magnet, 70 cm bore diameter, 137 cm long, system weight of 4.8 tons, minimum room size of 30 m2, FoV of 50 × 50 × 50 cm, gradients 33 mT/m @ 125 T/m/s (4), (b) Siemens 0.35T MAGNETON C, C-shaped dipolar permanent magnet with a vertical magnetic field, bore gap size of 41 cm, 270o accessibility, pole diameter of 137 cm, system weight of 17.6 tons, system dimension of 233 × 206 × 160 cm, minimum room size of 30 m2, FoV of 0.5–40 cm, gradients 24 mT/m @ 55 T/m/s (5), (c) 0.6T UprightTM MRI from Fonar, dipolar electromagnet with a horizontal magnetic field, bore gap size (pole-to-pole) of 46 cm, power requirement of 380–480V, FoV of 6 cm, gradients 12 mT/m, closed-loop water cooling, active and passive shimming, unreported system weight, system dimension, or minimum room size (6), (d) 0.5T PARAmed open MRI, dipolar superconducting magnet using MgB2 with a horizontal magnetic field, cryogen free, low power consumption, bore gap size (pole-to-pole) of 46 cm (7).

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  Fig. 2. O-scan from Esaote (9).

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  Fig. 3. The organ specific superconducting magnets (10) (a) A bagel-shaped superconducting magnet for breast imaging (b) a helmet-shaped superconducting magnet for head imaging.

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  Fig. 4. Dipolar magnets (a) C-shaped, (b) H-shaped.

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  Fig. 5. A conventional C-shaped magnet.

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  Fig. 6. A C-shaped table-top permanent magnet array (0.21T) for MRI imaging (12), (a) a photograph, (b) a side view of the system with dimensions.

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  Fig. 7. The magnet built by the Institute of Electrical Engineering of the Chinese Academy of Sciences in Beijing (75).

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  Fig. 8. Halbach permanent magnet array (a) 1D (35) (b) 2D (c) 3D (76).

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  Fig. 9. Side views of a Halbach cylinder (a) relationship of ⇀ dipolar (n = 1) (d) inner-field, quadrupolar (n = 2). M and angles, (b) inner-field, dipolar (n = 1), (c) outer-field,

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  Fig. 10. The inside-out well-logging NMR sensor designed by Jackson (45).

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  Fig. 11. Magnets for unilateral NMR (a) a simple bar magnet (b) a U-shaped open magnet.

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  Fig. 12. An inward-outward (IO) ring pair (55–57).

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  Fig. 13. A segmented Albert ring pair using magnet cubes (60).

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  Fig. 14. IO Ring-pair aggregates for head imaging (65) (a) 3D view (b) side view.

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  Fig. 15. Optimization using GA (65) (a) the GA flow (b) the results of optimization.

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  Fig. 16. The magnetic field generated by the proposed IO ring-pair aggregate in FoV in (a) the rø-plane, (b) the rz-plane (65).

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  Fig. 17. The magnetic field generated by the original IO ring pair in FoV in (a) the rø-plane, (b) the rz-plane (65).

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  Fig. 18. The segmented optimized IO magnet array made up of fan-shaped magnets (a) The 3D view, the calculated magnetic field (b) on the rø-plane in FoV on the rz-plane in the FoV. COMSOL Multiphysics were used for the calculation (65).

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