Innovation, Advanced Therapies, Siemens Healthineers GmbH, Forchheim, Germany.
ACRF Image X Institute, Sydney School of Health Sciences, University of Sydney, Australia.
Phys Med Biol. 2021 Feb 2;66(4):045004. doi: 10.1088/1361-6560/abd010.
Rotating MRI systems could enable novel integrated medical devices such as MRI-Linacs, MRI-xray-angiography systems, and MRI-proton therapy systems. This work aimed to investigate the feasibility of rotating actively shielded superconducting MRI magnets in the presence of environmental steel-in particular, construction steel in the floor of the installation site. Two magnets were investigated: a 1.0 T split bore magnet, and a 1.5 T closed bore magnet. Each magnet was scaled to emulate field strengths of 0.5, 1.0, and 1.5 T. Finite Element Modeling was used to simulate these magnets in the presence of a 3 × 4 m steel plate located 1250 mm or 1400 mm below the isocenter. There are two possible rotation directions: around the longitudinal (z) axis or around the transverse (x) axis. Each model was solved for rotation angles between 0 and 360° in 30° intervals around each of these axes. For each simulation, a 300 mm DSV was extracted and decomposed into spherical harmonics. For the closed-bore magnet, total induced perturbation for the zero degree rotation angle was 223, 432, and 562 μT peak-to-peak (pk-pk) for the 0.5, 1.0, and 1.5 T models respectively (steel at 1250 mm). For the split-bore magnet, the same numbers were 1477, 16747, and 1766 μT. The substantially higher perturbation for the split-bore magnet can be traced to its larger fringe field. For rotation around the z-axis, total perturbation does not change as a function of angle but is exchanged between different harmonics. For rotation around the x-axis, total perturbation is different at each rotation angle. For the closed bore magnet, maximum perturbations occurred for a 90° rotation around the transverse axis. For the split-bore magnet, the opposite was observed, with the same 90° rotation yielding total perturbation lower than the conventional position. In all cases, at least 95% of the total perturbation was composed of 1st and 2nd order harmonics. The presence of environmental steel poses a major challenge to the realization of an actively shielded rotating superconducting MRI system, requiring some novel form of shimming. Possible shimming strategies are discussed at length.
旋转 MRI 系统可以实现新型的集成医疗设备，例如 MRI-直线加速器、MRI-X 射线血管造影系统和 MRI-质子治疗系统。本研究旨在研究在存在环境钢（尤其是安装现场地板中的结构钢）的情况下主动屏蔽超导 MRI 磁体旋转的可行性。研究了两种磁铁：1.0 T 分体式磁体和 1.5 T 封闭式磁体。每个磁体的比例均模拟为 0.5、1.0 和 1.5 T 的场强。有限元建模用于模拟在位于等中心下方 1250 毫米或 1400 毫米处的 3×4 米钢板存在的情况下的这些磁铁。有两种可能的旋转方向：绕纵轴（z 轴）或横轴（x 轴）旋转。对于每个模型，围绕这些轴以 30°的间隔旋转 0 到 360°的角度进行了求解。对于每个模拟，提取了一个 300 毫米的 DSV 并将其分解为球谐函数。对于封闭式磁体，零角度旋转时的总感应干扰为 223、432 和 562 μT 峰峰值（pk-pk），对于 0.5、1.0 和 1.5 T 模型分别为 562 μT（钢在 1250 毫米处）。对于分体式磁体，相同的数字为 1477、16747 和 1766 μT。分体式磁体的感应干扰高得多，这可归因于其较大的边缘场。绕 z 轴旋转时，总干扰不会随角度而变化，但会在不同的谐次之间交换。绕 x 轴旋转时，总干扰在每个旋转角度都不同。对于封闭式磁体，横向轴 90°旋转时会产生最大干扰。对于分体式磁体，情况则相反，相同的 90°旋转产生的总干扰低于常规位置。在所有情况下，总干扰的至少 95％由 1 阶和 2 阶谐次组成。环境钢的存在对主动屏蔽旋转超导 MRI 系统的实现构成了重大挑战，需要某种新颖的匀场形式。详细讨论了可能的匀场策略。
