Superconducting magnet designs and MRI accessibility: A review

Abstract

Presently, magnetic resonance imaging (MRI) magnets must deliver excellent magnetic field (B0) uniformity to achieve optimum image quality. Long magnets can satisfy the homogeneity requirements but require considerable superconducting material. These designs result in large, heavy, and costly systems that aggravate as field strength increases. Furthermore, the tight temperature tolerance of niobium titanium magnets adds instability to the system and requires operation at liquid helium temperature. These issues are crucial factors in the disparity of MR density and field strength use across the globe. Low‐income settings show reduced access to MRI, especially to high field strengths. This article summarizes the proposed modifications to MRI superconducting magnet design and their impact on accessibility, including compact, reduced liquid helium, and specialty systems. Reducing the amount of superconductor inevitably entails shrinking the magnet size, resulting in higher field inhomogeneity. This work also reviews the state‐of‐the‐art imaging and reconstruction methods to overcome this issue. Finally, we summarize the current and future challenges and opportunities in the design of accessible MRI.

Low‐field MRI: Clinical promise and challenges

“…High‐field devices are large, usually weighing over 5 tons and requiring two dedicated rooms with reinforced flooring and RF shielding. 38 Most high‐field scanners use superconducting magnets, which require additional high‐power infrastructure and a quench pipe for cryogenic cooling. These devices are sensitive to vibrations and nearby ferromagnetic objects (eg ambulances, cars, trains).…”

“…Low‐field devices already play a dominant role in MRI services in West African countries, with the majority (77.6%) of devices in Nigeria being low‐field strength (<0.3T). 38 , 50 Neuroimaging was the primary application, with one center reporting over 90% of studies requested were for brain (49.9%) or spine (45.6%) imaging. 51 However, the average MRI cost was ~500 USD and services are typically paid by patients out‐of‐pocket, making even low‐field scanners beyond the reach of a significant portion of the population.…”

Material Flow Analysis of Dysprosium in the United States

“…Before 1999, a large number of MRI machines were installed in the US, and the in-use stocks tended to saturate, leading to a decrease in the Dy demand. Moreover, compared to superconductive MRI machines with a field strength of 1.5 T and more, the field strength of the Dy-bearing MRI machines is lower (about 0.15–0.5 T), and their imaging performance (e.g., spatial resolution and stability) is limited, which restricts their use in advanced and critical imaging techniques (e.g., diffusion-weighted imaging) . After 1997, superconductive MRI machines dominated the US market, suggesting a decrease in the Dy demand in the coming years…”

Environmental Sustainability and MRI: Challenges, Opportunities, and a Call for Action

“…90 More pressingly, known helium reserves are limited to just a few countries (including Qatar, the US, Algeria, Russia, Canada, and China), and supply and costs are inconsistent and can fluctuate substantially with demand and geopolitical concerns, further highlighting the importance of sustainable management. 91 Given that helium is a finite resource, strategies for helium conservation and alternative MRI systems are important to consider. For MRI systems, helium losses can occur at any point along the manufacturing, transportation, and installation process as well as throughout the scanner lifecycle, such as during routine maintenance.…”

“…92 Cutting-edge, ultra-low volume or cryogen-free MRI systems are now available that employ fully sealed, conduction-cooled magnets that use <10 liter of helium-a hundred-fold or more reduction from earlier scanner generations. 91,93 These systems are also lower cost and therefore can potentially increase global access to MRI. Due to the exceedingly small helium volumes in these so-called "dry" magnet systems, there is no need for a vent pipe-which is required for large volume quenches for safety reasons-or helium refills.…”