背景:磁体的磁场匀场是磁共振成像(MRI)系统中的常规做法。对于临床使用的1.5T或3TMRI超导磁体,它通常是直接实现所需的磁场均匀性与被动垫片技术。相比之下,通常将具有较高匀场效率的超导垫片与无源匀场结合使用,以满足超高场磁体(≥7特斯拉)的较高磁场均匀性要求。然而,超导垫片通常涉及复杂的绕组结构和低温环境,在实践中带来了相当大的工程挑战和额外的成本。
目的:在本研究中,我们旨在改进被动匀场方法,该方法可以结合超高场MRI磁体的独特电磁特性,因此对于7T及以上的场校正更有效。
方法:在这项工作中,我们为7T全身MRI超导磁体提出了一种专用的无源匀场策略。在这种方法中,由于铁场的相互作用,铁的使用和磁力受到严格的管理,以确保垫片托盘插入件可通过人力操作(无需专门设计的工具)。
结果:为了验证建议的匀场策略,在7T/800mm超导磁体上进行了匀场实验。在我们的两轮操作中,与奇数和偶数垫片托盘交替,磁场不均匀性从85.36ppm成功校正到7.91ppm,达到一个数量级以上的磁场质量提升。
结论:实验结果表明,所提出的电磁技术有望有效地开发超高场MRI仪器。
BACKGROUND: Magnetic field shimming of the magnet is a routine practice in a magnetic resonance imaging (MRI) system. For clinically-used 1.5 T or 3 T MRI superconducting magnets, it is generally straightforward to achieve desired magnetic field uniformity with the passive shim technique. In comparison, superconducting shims with higher shimming efficiency are usually introduced in combination with passive shimming to satisfy the higher magnetic field uniformity requirement for ultrahigh field magnets (≥7 Tesla). However, superconducting shim usually involves a complex winding structure and low-temperature environment, bringing considerable engineering challenges and extra costs in practice.
OBJECTIVE: In this study, we aimed to improve the passive shimming method that can incorporate the unique electromagnetic properties of ultrahigh-field MRI magnets and is thus more effective for field corrections at 7T and above.
METHODS: In this work, we propose a dedicated passive shimming strategy for a 7 T whole-body MRI superconducting magnet. In this method, the iron usage and magnetic force due to the iron-field interaction are strictly managed to ensure a shim tray insert is operable by manpower (without specially designed tools).
RESULTS: To validate the proposed shimming strategy, a shimming experiment was implemented on a 7 T/800 mm superconducting magnet. Alternating with the odd and even shim trays in our two-round operation, the magnetic field inhomogeneity was successfully corrected from 85.36 to 7.91 ppm, achieving the magnetic field quality elevation of more than one order of magnitude.
CONCLUSIONS: The experimental results indicated that the proposed electromagnetic technology is expected to be effective for developing ultrahigh-field MRI instruments.