Sydney Medical School, University of Sydney, NSW 2006, Australia.
Med Phys. 2013 Dec;40(12):121705. doi: 10.1118/1.4828792.
To cope with intrafraction tumor motion, integrated MRI-linac systems for real-time image guidance are currently under development. The multileaf collimator (MLC) is a key component in every state-of-the-art radiotherapy treatment system, allowing for accurate field shaping and tumor tracking. This work quantifies the magnetic impact of a widely used MLC on the MRI field homogeneity for such a modality.
The finite element method was employed to model a MRI-linac assembly comprised of a 1.0 T split-bore MRI magnet and the key ferromagnetic components of a Varian Millennium 120 MLC, namely, the leaves and motors. Full 3D magnetic field maps of the system were generated. From these field maps, the peak-to-peak distortion within the MRI imaging volume was evaluated over a 30 cm diameter sphere volume (DSV) around the isocenter and compared to a maximum preshim inhomogeneity of 300 μT. Five parametric studies were performed: (1) The source-to-isocenter distance (SID) was varied from 100 to 200 cm, to span the range of a compact system to that with lower magnetic coupling. (2) The MLC model was changed from leaves only to leaves with motors, to determine the contribution to the total distortion caused by MLC leaves and motors separately. (3) The system was configured in the inline or perpendicular orientation, i.e., the linac treatment beam was oriented parallel or perpendicular to the magnetic field direction. (4) The treatment field size was varied from 0 × 0 to 20×20 cm(2), to span the range of clinical treatment fields. (5) The coil currents were scaled linearly to produce magnetic field strengths B0 of 0.5, 1.0, and 1.5 T, to estimate how the MLC impact changes with B0.
(1) The MLC-induced MRI field distortion fell continuously with increasing SID. (2) MLC leaves and motors were found to contribute to the distortion in approximately equal measure. (3) Due to faster falloff of the fringe field, the field distortion was generally smaller in the perpendicular beam orientation. The peak-to-peak DSV distortion was below 300 μT at SID≥130 cm (perpendicular) and SID≥140 cm (inline) for the 1.0 T design. (4) The simulation of different treatment fields was identified to cause dynamic changes in the field distribution. However, the estimated residual distortion was below 1.2 mm geometric distortion at SID≥120 cm (perpendicular) and SID≥130 cm (inline) for a 10 mT/m frequency-encoding gradient. (5) Due to magnetic saturation of the MLC materials, the field distortion remained constant at B0>1.0 T.
This work shows that the MRI field distortions caused by the MLC cannot be ignored and must be thoroughly investigated for any MRI-linac system. The numeric distortion values obtained for our 1.0 T magnet may vary for other magnet designs with substantially different fringe fields, however the concept of modest increases in the SID to reduce the distortion to a shimmable level is generally applicable.
为了应对分次内肿瘤运动，目前正在开发集成 MRI-直线加速器系统以实现实时图像引导。多叶准直器（MLC）是每一个最先进的放射治疗系统的关键组成部分，能够实现精确的射野成型和肿瘤跟踪。这项工作定量评估了一种广泛使用的 MLC 对这种模式的 MRI 场均匀性的磁影响。
采用有限元法对由 1.0T 分体式 MRI 磁体和瓦里安 Millennium 120 MLC 的关键铁磁部件组成的 MRI-直线加速器组件进行建模，这些关键铁磁部件包括叶片和电机。生成了系统的全 3D 磁场图。从这些磁场图中，评估了在等中心周围 30cm 直径球体体积（DSV）内的峰值到峰值失真，并与最大预失真 300μT 进行了比较。进行了五项参数研究：（1）源到等中心距离（SID）从 100 到 200cm 变化，以涵盖紧凑系统和磁耦合较低的系统范围。（2）改变 MLC 模型，从仅叶片到叶片加电机，以确定 MLC 叶片和电机分别对总失真的贡献。（3）系统配置为直线或垂直方向，即直线加速器治疗束平行或垂直于磁场方向。（4）治疗场大小从 0×0 到 20×20cm²变化，以涵盖临床治疗场的范围。（5）线性缩放线圈电流，产生 B0 为 0.5、1.0 和 1.5T 的磁场强度，以估计 MLC 影响如何随 B0 变化。
（1）MLC 引起的 MRI 场失真随 SID 的增加而连续下降。（2）发现 MLC 叶片和电机对失真的贡献大致相等。（3）由于边缘场的快速衰减，在垂直束方向上，场失真通常较小。在 1.0T 设计中，SID≥130cm（垂直）和 SID≥140cm（直线）时，峰值到峰值 DSV 失真低于 300μT。（4）模拟不同的治疗场被确定会导致场分布的动态变化。然而，对于 10mT/m 频率编码梯度，SID≥120cm（垂直）和 SID≥130cm（直线）时，估计的残余失真低于 1.2mm 几何失真。（5）由于 MLC 材料的磁饱和，磁场失真在 B0>1.0T 时保持不变。
这项工作表明，MLC 引起的 MRI 场失真不容忽视，必须对任何 MRI-直线加速器系统进行彻底研究。对于具有明显不同边缘场的其他磁体设计，我们获得的数值失真值可能会有所不同，但是增加 SID 以将失真降低到可整形水平的概念通常是适用的。
