Magnetostatic focal spot correction for x-ray tubes operating in strong magnetic fields using iterative optimization.
Magnetostatic focal spot correction for x-ray tubes operating in strong magnetic fields using iterative optimization.
复制标题
使用迭代优化对在强磁场中工作的 X 射线管进行静磁焦斑校正。
DOI：
10.1118/1.4742060
复制
发表时间：
2012
期刊：
影响因子：
3.8
通讯作者：
Fahrig,Rebecca
中科院分区：
文献类型：
--
作者：
Lillaney,Prasheel;Shin,Mihye;Conolly,StevenM;Fahrig,Rebecca
目的结合X射线透视和磁共振成像系统指导介入治疗已变得越来越普遍。通过设计一种不受磁共振孔外磁场影响的X射线管，这两个系统可以放置在彼此非常接近的位置。X射线管设计的一个主要障碍是校正磁场对X射线管焦点的影响。一种潜在的解决方案是设计局部抵消焦斑附近磁场的有源屏蔽。方法采用迭代优化算法设计放置在X射线管插件外的阻性有源屏蔽线圈。优化过程试图在满足磁场均匀度约束的同时最小化屏蔽线圈的功率消耗。该算法由相互交错的线性规划步骤和非线性规划步骤组成。用X射线管内电子束的有限元空间电荷模拟验证了线圈的结果。为了减轻对加热的担忧，推导出了一种优化的线圈解决方案，其中包括一个钕永磁体。在对优化线圈进行求解之前，先计算永磁体的任何退磁。结果在磁场强度为88mT的情况下，该算法适用于消耗588A/cm2的线圈。这种特殊的线圈几何形状可以在达到温度故障之前连续运行15分钟。通过在设计中加入钕磁体，电流密度降至337A/cm2，操作时间增加到59min。空间电荷模拟验证了线圈的设计是有效的，但对于倾斜的X射线管几何形状，仍然存在焦斑形状的扭曲以及阳极在径向和周向的约3 mm的偏移量。结论主动屏蔽是一种有吸引力的解决方案，用于校正磁场对X射线焦斑的影响。如果需要极长的透视曝光时间，可以通过在有源屏蔽设计中包括永久磁铁来实现更长的操作时间。
PurposeCombining x‐ray fluoroscopy and MR imaging systems for guidance of interventional procedures has become more commonplace. By designing an x‐ray tube that is immune to the magnetic fields outside of the MR bore, the two systems can be placed in close proximity to each other. A major obstacle to robust x‐ray tube design is correcting for the effects of the magnetic fields on the x‐ray tube focal spot. A potential solution is to design active shielding that locally cancels the magnetic fields near the focal spot.MethodsAn iterative optimization algorithm is implemented to design resistive active shielding coils that will be placed outside the x‐ray tube insert. The optimization procedure attempts to minimize the power consumption of the shielding coils while satisfying magnetic field homogeneity constraints. The algorithm is composed of a linear programming step and a nonlinear programming step that are interleaved with each other. The coil results are verified using a finite element space charge simulation of the electron beam inside the x‐ray tube. To alleviate heating concerns an optimized coil solution is derived that includes a neodymium permanent magnet. Any demagnetization of the permanent magnet is calculated prior to solving for the optimized coils. The temperature dynamics of the coil solutions are calculated using a lumped parameter model, which is used to estimate operation times of the coils before temperature failure.ResultsFor a magnetic field strength of 88 mT, the algorithm solves for coils that consume 588 A/cm2. This specific coil geometry can operate for 15 min continuously before reaching temperature failure. By including a neodymium magnet in the design the current density drops to 337 A/cm2, which increases the operation time to 59 min. Space charge simulations verify that the coil designs are effective, but for oblique x‐ray tube geometries there is still distortion of the focal spot shape along with deflections of approximately 3 mm in the radial and circumferential directions on the anode.ConclusionsActive shielding is an attractive solution for correcting the effects of magnetic fields on the x‐ray focal spot. If extremely long fluoroscopic exposure times are required, longer operation times can be achieved by including a permanent magnet with the active shielding design.