Department of Radiology, Stanford University, Stanford, California 94305, USA.
Med Phys. 2011 Jul;38(7):4174-85. doi: 10.1118/1.3600695.
Using magnetic resonance imaging (MRI) for real-time guidance during radiotherapy is an active area of research and development. One aspect of the problem is the influence of the MRI scanner, modeled here as an external magnetic field, on the medical linear accelerator (linac) components. The present work characterizes the behavior of two medical linac electron guns with external magnetic fields for in-line and perpendicular orientations of the linac with respect to the MRI scanner.
Two electron guns, Litton L-2087 and Varian VTC6364, are considered as representative models for this study. Emphasis was placed on the in-line design approach in which case the MRI scanner and the linac axes of symmetry coincide and assumes no magnetic shielding of the linac. For the in-line case, the magnetic field from a 0.5 T open MRI (GE Signa SP) magnet with a 60 cm gap between its poles was computed and used in full three dimensional (3D) space charge simulations, whereas for the perpendicular case the magnetic field was constant.
For the in-line configuration, it is shown that the electron beam is not deflected from the axis of symmetry of the gun and the primary beam current does not vanish even at very high values of the magnetic field, e.g., 0.16 T. As the field strength increases, the primary beam current has an initial plateau of constant value after which its value decreases to a minimum corresponding to a field strength of approximately 0.06 T. After the minimum is reached, the current starts to increase slowly. For the case when the beam current computation is performed at the beam waist position the initial plateau ends at 0.016 T for Litton L-2087 and at 0.012 T for Varian VTC6364. The minimum value of the primary beam current is 27.5% of the initial value for Litton L-2087 and 22.9% of the initial value for Varian VTC6364. The minimum current is reached at 0.06 and 0.062 T for Litton L-2087 and Varian VTC6364, respectively. At 0.16 T the beam current increases to 40.2 and 31.4% from the original value of the current for Litton L-2087 and Varian VTC6364, respectively. In contrast, for the case when the electron gun is perpendicular to the magnetic field, the electron beam is deflected from the axis of symmetry even at small values of the magnetic field. As the strength of the magnetic field increases, so does the beam deflection, leading to a sharp decrease of the primary beam current which vanishes at about 0.007 T for Litton L-2087 and at 0.006 T for Varian VTC6364, respectively. At zero external field, the beam rms emittance computed at beam waist is 1.54 and 1.29n-mm-mrad for Litton L-2087 and Varian VTC6364, respectively. For the inline configuration, there are two particular values of the external field where the beam rms emittance reaches a minimum. Litton L-2087 rms emittance reaches a minimum of 0.72n and 2.01 n-mm-mrad at 0.026 and 0.132 T, respectively. Varian VTC6364 rms emittance reaches a minimum of 0.34n and 0.35n-mm-mrad at 0.028 and 0.14 T, respectively. Beam radius dependence on the external field is shown for the in-line configuration for both electron guns.
3D space charge simulation of two electron guns, Litton L-2087 and Varian VTC6364, were performed for in-line and perpendicular external magnetic fields. A consistent behavior of Pierce guns in external magnetic fields was proven. For the in-line configuration, the primary beam current does not vanish but a large reduction of beam current (up to 77.1%) is observed at higher field strengths; the beam directionality remains unchanged. It was shown that for a perpendicular configuration the current vanishes due to beam bending under the action of the Lorentz force. For in-line configuration it was determined that the rms beam emittance reaches two minima for relatively high values of the external magnetic field.
在放射治疗中使用磁共振成像（MRI）进行实时引导是一个活跃的研究和开发领域。问题的一个方面是 MRI 扫描仪（这里建模为外部磁场）对医用线性加速器（linac）组件的影响。本工作针对两种医用直线加速器电子枪在 MRI 扫描仪与直线加速器之间的同轴和垂直两种情况下的外部磁场行为进行了特性描述。
考虑了 Litton L-2087 和 Varian VTC6364 两种电子枪作为本研究的代表性模型。重点研究了同轴设计方法，其中 MRI 扫描仪和直线加速器对称轴重合，并且不屏蔽直线加速器。对于同轴情况，计算了磁极之间有 60cm 间隙的 0.5T 开放式 MRI（GE Signa SP）磁体的磁场，并在全三维（3D）空间电荷模拟中使用了该磁场，而对于垂直情况，磁场是恒定的。
对于同轴配置，结果表明电子束不会从枪的对称轴偏转，即使在磁场非常高的情况下，主束电流也不会消失，例如 0.16T。随着场强的增加，主束电流在初始的恒定值平台之后会有一个初始平台，然后其值会减小到一个最小值，对应大约 0.06T 的场强。达到最小值后，电流开始缓慢增加。对于在束腰位置进行束流计算的情况，Litton L-2087 的初始平台在 0.016T 结束，而 Varian VTC6364 的初始平台在 0.012T 结束。主束电流的最小值为 Litton L-2087 的初始值的 27.5%，为 Varian VTC6364 的初始值的 22.9%。Litton L-2087 和 Varian VTC6364 的最小电流分别在 0.06 和 0.062T 时达到，在 0.16T 时，束流分别增加到初始值的 40.2%和 31.4%。相比之下，当电子枪垂直于磁场时，即使在磁场较弱的情况下，电子束也会从对称轴偏转。随着磁场强度的增加，束偏转也会增加，导致主束电流急剧下降，Litton L-2087 的电流在大约 0.007T 时消失，而 Varian VTC6364 的电流在大约 0.006T 时消失。在零外部场时，在束腰处计算的束 rms 发射度对于 Litton L-2087 和 Varian VTC6364 分别为 1.54 和 1.29n-mm-mrad。对于同轴配置，在两个特殊的外部场值下，束 rms 发射度达到最小值。Litton L-2087 的 rms 发射度在 0.026 和 0.132T 时分别达到最小值 0.72n 和 2.01 n-mm-mrad，而 Varian VTC6364 的 rms 发射度在 0.028 和 0.14T 时分别达到最小值 0.34n 和 0.35n-mm-mrad。对于同轴配置，对于两个电子枪，都显示了束半径对外部场的依赖性。
对 Litton L-2087 和 Varian VTC6364 两种电子枪进行了同轴和垂直外部磁场的 3D 空间电荷模拟。证明了 Pierce 枪在外部磁场中的一致行为。对于同轴配置，主束电流不会消失，但在较高的场强下，束电流会大幅减小（高达 77.1%）；束的方向性保持不变。结果表明，对于垂直配置，由于洛伦兹力作用下的束弯曲，电流会消失。对于同轴配置，确定了相对较高的外部磁场值下，束 rms 发射度达到两个最小值。
