Burke Ben, Ghila Andrei, Fallone B G, Rathee Satyapal
Department of Physics, University of Alberta, 11322-89 Avenue, Edmonton, Alberta T6G 2G7, Canada.
Med Phys. 2012 Aug;39(8):5004-14. doi: 10.1118/1.4737097.
In integrated linac-MRI systems, a measurable radiation induced current (RIC) is caused in RF coils by pulsed irradiation. This work (1) tests a buildup method of RIC removal in planar conductors; (2) validates a Monte Carlo method of RIC calculation in metal conductors; and (3) uses the Monte Carlo method to examine the effects of magnetic fields on both planar conductor and practical cylindrical coil geometries.
The RIC was measured in copper and aluminum plates, taken as the RF coil conductor surrogates, as a function of increasing thickness of buildup materials (teflon and copper). Based on the Penelope Monte Carlo code, a method of RIC calculation was implemented and validated against measurements. This method was then used to calculate the RIC in cylindrical coil geometries with various air gaps between the coil conductor and the enclosed water phantom. Magnetic fields, both parallel and perpendicular to the radiation beam direction, were then included in the simulation program. The effect of magnetic fields on the effectiveness of RIC removal with the application of buildup material was examined in both the planar and the cylindrical geometries.
Buildup reduced RIC in metal plate conductors. For copper detector∕copper buildup case, the RIC amplitude was reduced to zero value with 0.15 cm copper buildup. However, when the copper is replaced with teflon as buildup atop the copper conductor, the RIC was only reduced to 80% of its value at zero buildup since the true electronic equilibrium cannot be obtained in this case. For the aluminum detector∕teflon buildup case, the initial amplitude of the RIC was reduced by 90% and 92% in planar aluminum conductor and a surface coil, respectively. In case of cylindrical coils made of aluminum, teflon buildup around the coil's outer surface was generally effective but failed to remove RIC when there was an air gap between the coil and the phantom. Stronger magnetic fields (>0.5 T) perpendicular to the beam direction showed a modest decrease in the RIC for planar conductors with buildup. In the cylindrical geometries, the effect of magnetic fields was very small compared to the effect of introducing air gaps. Loss in signal-to-noise ratio (SNR) due to RIC was reduced from 11% to 5% when a simple buildup was applied to the solenoid in a preliminary experiment.
The RIC in RF coils results from the lack of electronic equilibrium in the coil conductor as the RIC in planar conductor was completely removed by identical buildup of adequate thickness to create electronic equilibrium. The buildup method of RIC removal is effective in cylindrical coil geometry when the coil conductor is in direct contact with the patient. The presence of air makes this method of RIC removal less effective although placing buildup still reduces the RIC by up to 60%. The RIC Monte Carlo simulation is a useful tool for practical coil design where radiation effects must be considered. The SNR is improved in the images obtained concurrently withradiation if buildup is applied to the coil.
在直线加速器 - 磁共振成像（linac - MRI）集成系统中，脉冲辐射会在射频线圈中产生可测量的辐射感应电流（RIC）。本研究（1）测试平面导体中去除RIC的累积方法；（2）验证金属导体中RIC计算的蒙特卡罗方法；（3）使用蒙特卡罗方法研究磁场对平面导体和实际圆柱形线圈几何形状的影响。
在作为射频线圈导体替代物的铜板和铝板中测量RIC，测量其作为累积材料（聚四氟乙烯和铜）厚度增加的函数。基于佩内洛普蒙特卡罗代码，实施了一种RIC计算方法并通过测量进行验证。然后使用该方法计算线圈导体与封闭水模之间具有各种气隙的圆柱形线圈几何形状中的RIC。然后将平行和垂直于辐射束方向的磁场纳入模拟程序。在平面和圆柱形几何形状中都研究了磁场对应用累积材料去除RIC有效性的影响。
累积减少了金属板导体中的RIC。对于铜探测器/铜累积情况，当铜累积厚度为0.15 cm时，RIC幅度降至零值。然而，当在铜导体顶部用聚四氟乙烯替代铜作为累积材料时，由于在这种情况下无法获得真正的电子平衡，RIC仅降至其零累积值的80％。对于铝探测器/聚四氟乙烯累积情况，在平面铝导体和表面线圈中，RIC的初始幅度分别降低了90％和92％。对于由铝制成的圆柱形线圈，线圈外表面周围的聚四氟乙烯累积通常是有效的，但当线圈与模体之间存在气隙时，无法去除RIC。垂直于束方向的更强磁场（>0.5 T）显示，对于有累积的平面导体，RIC略有下降。在圆柱形几何形状中，与引入气隙的影响相比，磁场的影响非常小。在初步实验中，当对螺线管应用简单累积时，由于RIC导致的信噪比（SNR）损失从11％降至5％。
射频线圈中的RIC是由于线圈导体中缺乏电子平衡导致的，因为平面导体中的RIC通过相同厚度的适当累积以产生电子平衡而被完全去除。当线圈导体与患者直接接触时，RIC去除的累积方法在圆柱形线圈几何形状中是有效的。空气的存在使这种RIC去除方法效果较差，尽管放置累积材料仍可将RIC降低多达60％。RIC蒙特卡罗模拟是实际线圈设计中考虑辐射效应时的有用工具。如果对线圈应用累积材料，则在与辐射同时获得的图像中SNR会得到改善。
