【摘要】 在磁共振成像(MRI)设备中,为了消除目标区域的不均匀磁场,通常有无源和有源两种匀场方法,这两种方法既可以通过磁场点直接优化,又可以先对场点解卷积,得到的各阶不均匀谐波利用谐波法消除.相较于有源,无源匀场不需要额外的电源和电流调控,操作更简单,成本低.然而由于无源匀场的匀场片处在MRI背场的主磁场环境中,不仅发生轴向(z向)磁化,也将发生横向(x和y向)的磁化.由于横向磁化不能饱和,导致不同位置的磁化强度随着匀场片厚度变化,使优化结果无法确定.为此本文提出一种单轴磁化材料用作匀场片,这种材料沿某一个特定方向(z向)极易磁化同时饱和磁化强度远远高于另外两个垂直方向,这样匀场就只需要考虑z向的饱和磁化,大大提高了设计的可靠性.另外以往文献大都侧重于匀场的结果,如最终实现的均匀度效果、匀场垫片空间分布和消耗情况;而针对匀场算法的理论和参数设计,往往基于简化模型和公式,计算结果偏差较大.为此本文通过一种案例设计,详细给出场点法(FM)和谐波法(HM)两种匀场算法的精确理论推导和设计方法,对比其各自的优劣和最终效果.为研究者提供一种精度更高的设计模式,助力于未来更多高场MRI无源匀场技术的突破.更多还原

【Abstract】 There are usually two shimming techniques:passive shimming and active shimming for magnetic resonance imaging(MRI) to eliminate the non-uniform magnetic field in the target area.These two techniques can either be based on Field Point Method(FM) that directly optimize the magnetic field over the DSV surface or firstly perform deconvolution operations of the field points,and then eliminate the non-uniform harmonics of each order obtained by deconvolution using Harmonic Method(HM).Comparing with active shimming,passive shimming does not require additional power supply and current regulation.It is simpler to operate and has a lower cost.However,due to the fact that the passive shims work in the main field environment of the background magnets.It will not only be magnetized axially(in the z-direction),but also laterally(in the x-and y-directions).The transverse magnetization may not be saturated,causing the magnetization intensity at different positions varies with the thickness of the shim sheets,making the entire optimization uncertain.For this reason,this paper proposes to use a uniaxial magnetized material as passive shims.This material is highly magnetized along a specific direction(z-direction),and its saturation magnetization intensity is much higher than that in the other two vertical directions.Thus,shimming only needs to consider the saturated magnetization in the z-direction,which greatly improves the reliability of the design.In addition,most of the previous literature has focused on the results of shimming operations,such as the final homogeneity achieved,the spatial distribution and consumption of shim sets;However,the theoretical and parametric descriptions of the shimming algorithm are often based on simplified models or formulations,prone to causing large calculation deviations.For this reason,this paper conducts a case study to elaborately present the specific theoretical derivations and design methods of the two shimming algorithms:FM and HM.And comparing their respective advantages and disadvantages as well as the final shimming effects.It will provide researchers with a more accurate design framework and contribute to the breakthroughs of more passive shimming techniques for high-field MRI in the future.更多还原