OncoRay-National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany.
Institute of Radiooncology-OncoRay, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany.
Phys Med Biol. 2020 Nov 5;65(21):215014. doi: 10.1088/1361-6560/abb16f.
For the first time, a low-field open magnetic resonance (MR) scanner was combined with a proton pencil beam scanning (PBS) research beamline. The aim of this study was to characterize the magnetic fringe fields produced by the PBS system and measure their effects on MR image quality during simultaneous PBS irradiation and image acquisition. A magnetic field camera measured the change in central resonance frequency (Δf ) and magnetic field homogeneity (ΔMFH) of the B field of the MR scanner during operation of the beam transport and scanning magnets. The beam energy was varied between 70 - 220 MeV and beam scanning was performed along the central horizontal and vertical axis of a 48 × 24 cm radiation field. The time structure of the scanning magnets' fringe fields was simultaneously recorded by a tri-axial Hall probe. MR imaging experiments were conducted using the ACR (American College of Radiology) Small MRI Phantom and a spoiled gradient echo pulse sequence during simultaneous volumetric irradiation. Computer simulations were performed to predict the effects of B field perturbations due to PBS irradiation on MR image formation in k-space. Setting the beam transport magnets, horizontal and vertical scanning magnets resulted in a maximum Δf of 50, 235 and 4 Hz, respectively. The ΔMFH was less than 3 parts per million for all measurements. MR images acquired during beam energy variation and vertical beam scanning showed no visual loss in image quality. However, MR images acquired during horizontal beam scanning showed severe coherent ghosting artefacts in phase encoding direction. Both simulated and measured k-space phase maps prove that these artefacts are caused by phase-offsets. This study shows first experimental evidence that simultaneous in-beam MR imaging during proton PBS irradiation is subject to severe loss of image quality in the absence of magnetic decoupling between the PBS and MR system.
首次将低场开放式磁共振（MR）扫描仪与质子笔束扫描（PBS）研究束线相结合。本研究的目的是表征 PBS 系统产生的磁场边缘场，并测量其在同时进行 PBS 照射和图像采集时对 MR 图像质量的影响。磁场相机测量了 MR 扫描仪的 B 场的中心共振频率（Δf）和磁场均匀度（ΔMFH）在束传输和扫描磁铁运行期间的变化。束能量在 70-220 MeV 之间变化，并且在 48×24 cm 辐射场的中心水平和垂直轴上进行束扫描。三轴霍尔探头同时记录了扫描磁铁边缘场的时间结构。在同时容积照射期间，使用 ACR（美国放射学院）小型 MRI 体模和扰动脉冲梯度回波脉冲序列进行 MR 成像实验。计算机模拟用于预测 PBS 照射引起的 B 场扰动对 k 空间中 MR 图像形成的影响。设置束传输磁铁、水平和垂直扫描磁铁分别导致最大的Δf 为 50、235 和 4 Hz。所有测量的ΔMFH 均小于 3ppm。在束能量变化和垂直束扫描期间采集的 MR 图像没有显示图像质量的视觉损失。然而，在水平束扫描期间采集的 MR 图像在相位编码方向上显示出严重的相干鬼影伪影。模拟和测量的 k 空间相位图均证明这些伪影是由相位偏移引起的。本研究首次提供了实验证据，表明在质子 PBS 照射期间进行同步腔内 MR 成像，如果 PBS 和 MR 系统之间没有磁解耦，图像质量会严重下降。
