Department of Medical Physics, Cross Cancer Institute, 11560 University Avenue, AB, T6G 1Z2, Canada.
Department of Oncology, Medical Physics Division, University of Alberta, 11560 University Avenue, Edmonton, AB, T6G 1Z2, Canada.
Med Phys. 2019 Feb;46(2):822-838. doi: 10.1002/mp.13309. Epub 2018 Dec 18.
Real-time magnetic resonance (MR) guidance is of interest to various groups globally because the superior soft tissue contrast MR images offer over other x-ray-based imaging modalities. Because of the precision required in proton therapy, proton therapy treatments rely heavily on image guidance. Integrating a magnetic resonance imaging (MRI) into a proton therapy treatment is a challenge. The charged particles (protons) used in proton therapy experience magnetic forces when travelling through the MRI magnetic fields. Given that it is desired that proton beams can be delivered with submillimeter accuracy, it is important that all potential sources of beam displacement are well modeled and understood. This study investigated the behavior of monoenergetic proton beams in the presence of a simulated set of realistic three-dimensional (3D) vector magnetic gradient fields required for spatial localization during imaging. This deflecting source has not been previously investigated.
Three-dimensional magnetic vector fields from a superconducting 0.5 T open bore MRI magnet model (previously developed in-house) and 3D magnetic fields from an in-house gradient coil model were applied to two types of computer simulations. In all simulations, monoenergetic proton pencil beams (from 80 to 250 MeV) were used. The initial directions of proton beams were varied. In all simulations, the orientation of the B field coincided with the positive z-axis in the simulation geometry. The first type of simulation is based on an analytic magnetic force equation (analytic simulations) while the second type is a full Monte Carlo (MC) simulation. The analytic simulations were limited to propagating the proton beams in vacuum but could be rapidly calculated in a desktop computer while the MC simulations were calculated in a cluster computer. The proton beam locations and dose profiles at the central plane (z = 0 cm) with or without magnetic fields were extracted and used to quantify the effect of the presence of the different magnetic fields on the proton beam.
The analytic simulations agree with MC results within 0.025 mm, thus acting as the verification of MC calculations. The presence of the B field caused the beam to follow a helical trajectory which resulted in angular offsets of 4.9 , 3.6 , and 2.8 for the 80, 150, and 250 MeV, respectively. Magnetic field deflections caused by a rapid MRI sequence (bSSFP, with maximum gradient strength of 40 mT/m) show a pattern of distortion which remained spatially invariant in the MR's field of view. For the 80 MeV beam, this pattern shows a maximum ranged in the y direction of 1.5 mm. The presence of the B field during the bSSFP simulations adds the same beam rotation to the observed during the B only simulations.
This investigation reveals that time-varying gradient magnetic fields required for image generation can cause a small spread in the proton beams used in the study which are independent of the effects arising from the B field. Further, studies where clinical beam kernels were convolved with this spread show that these magnetic fields are expected to have an insignificant impact on the beam's entrance dose.
实时磁共振（MR）引导引起了全球各领域的关注，因为 MR 图像提供的软组织对比优于其他基于 X 射线的成像方式。由于质子治疗需要精度，因此质子治疗治疗严重依赖图像引导。将磁共振成像（MRI）集成到质子治疗中是一个挑战。在质子治疗中使用的带电荷的粒子（质子）在穿过 MRI 磁场时会受到磁力的影响。鉴于希望质子束能够以亚毫米的精度传递，因此重要的是要很好地建模和理解所有潜在的束位移源。本研究调查了在存在模拟的三维（3D）矢量磁场梯度场的情况下单能质子束的行为，这些梯度场是在成像期间进行空间定位所需的。这个偏转源以前没有被研究过。
从先前内部开发的超导 0.5 T 开式磁体模型获得的三维磁场矢量和内部梯度线圈模型获得的三维磁场应用于两种类型的计算机模拟。在所有模拟中，都使用了单能质子铅笔束（80 至 250 MeV）。质子束的初始方向有所不同。在所有模拟中，B 场的方向与模拟几何形状中的正 z 轴一致。第一种模拟基于解析磁力方程（解析模拟），而第二种是全蒙特卡罗（MC）模拟。解析模拟仅限于在真空中传播质子束，但可以在台式计算机上快速计算，而 MC 模拟则在计算机群上计算。提取有或没有磁场时中央平面（z = 0 cm）上的质子束位置和剂量分布，并用于量化不同磁场对质子束的影响。
解析模拟与 MC 结果的误差在 0.025 mm 以内，因此可以作为 MC 计算的验证。B 场的存在导致束沿螺旋轨迹传播，分别导致 80、150 和 250 MeV 的角度偏移 4.9°、3.6°和 2.8°。快速 MRI 序列（bSSFP，最大梯度强度为 40 mT/m）引起的磁场偏转显示出在 MR 视野中空间不变的失真模式。对于 80 MeV 的束，在 y 方向上的最大范围为 1.5 mm。在 bSSFP 模拟期间存在 B 场会将在仅存在 B 场的模拟期间观察到的相同的束旋转添加到观察到的旋转中。
这项研究表明，用于图像生成的时变梯度磁场会导致研究中使用的质子束发生小的扩展，这种扩展与 B 场引起的影响无关。此外，对与临床束核卷积的这些磁场进行的研究表明，这些磁场预计对束入口剂量没有重大影响。
