Institute for Research and Development on Advanced Radiation Technologies (radART), Paracelsus Medical University, Salzburg 5020, Austria.
Med Phys. 2013 Mar;40(3):031906. doi: 10.1118/1.4790564.
Solid state flat panel electronic portal imaging devices (EPIDs) are widely used for megavolt (MV) photon imaging applications in radiotherapy. In addition to their original purpose in patient position verification, they are convenient to use in quality assurance and dosimetry to verify beam geometry and dose deposition or to perform linear accelerator (linac) calibration procedures. However, native image frames from amorphous silicon (aSi:H) detectors show a range of artifacts which have to be eliminated by proper correction algorithms. When a panel is operated in free-running frame acquisition mode, moving vertical stripes (periodic synchronization artifacts) are a disturbing feature in image frames. Especially for applications in volumetric intensity modulated arc therapy (VMAT) or motion tracking, the synchronization (sync) artifacts are the limiting factor for potential and accuracy since they become even worse at higher frame rates and at lower dose rates, i.e., linac pulse repetition frequencies (PRFs).
The authors introduced a synchronization correction method which is based on a theoretical model describing the interferences of the panel's readout clocking with the linac's dose pulsing. Depending on the applied PRF, a certain number of dose pulses is captured per frame which is readout columnwise, sequentially. The interference of the PRF with the panel readout is responsible for the period and the different gray value levels of the sync stripes, which can be calculated analytically. Sync artifacts can then be eliminated multiplicatively in precorrected frames without additional information about radiation pulse timing.
For the analysis, three aSi:H EPIDs of various types were investigated with 6 and 15 MV photon beams at varying PRFs of 25, 50, 100, 200, and 400 pulses per second. Applying the sync correction at panels with gadolinium oxysulfide scintillators improved single frame flood field image quality drastically [improvement of the signal-to-noise ratio (SNR) up to 66.1 dB for 6 MV and 66.0 dB for 15 MV]. Also for the EPID with a caesium iodide scintillator, the noise for the lower PRFs could be reduced (SNR at 6 MV of up to 56.3 dB and at 15 MV up to 46.7 dB). However, the simplistic readout interference model fails at higher PRFs, where image lag and ghosting effects due to trapped charges in the thin film transistor and scintillator postglowing require additional corrections.
The presented free-running sync correction method improves SNR of single frames and enables imaging applications, like low-dose rate imaging at increased image frame rates (e.g., to track moving gold fiducials in the lung). Adaptive image guided radiotherapy protocols become even feasible in VMAT plans. Also simultaneous kilovolt and MV imaging applications can benefit from new possibilities of MV scatter removal in x-ray images.
固态平板电子门户成像设备（EPID）广泛用于放射治疗中的兆伏（MV）光子成像应用。除了最初用于患者位置验证的目的外，它们还便于在质量保证和剂量学中使用，以验证束流几何形状和剂量沉积，或执行线性加速器（linac）校准程序。然而，非晶硅（aSi:H）探测器的原始图像帧显示出一系列伪影，必须通过适当的校正算法消除这些伪影。当面板以自由运行帧采集模式运行时，移动的垂直条纹（周期性同步伪影）是图像帧中的一个干扰特征。特别是对于容积调强弧形治疗（VMAT）或运动跟踪应用，同步（sync）伪影是限制因素，因为它们在更高的帧率和更低的剂量率下（即 linac 脉冲重复频率（PRF））会变得更糟。
作者引入了一种同步校正方法，该方法基于描述面板读出时钟与直线加速器剂量脉冲干扰的理论模型。根据应用的 PRF，每个帧中会捕获一定数量的剂量脉冲，这些脉冲按列顺序读出。PRF 与面板读出之间的干扰导致了同步条纹的周期和不同灰度值水平，可以进行分析计算。然后，可以在没有辐射脉冲定时的额外信息的情况下，在预校正帧中以乘法方式消除同步伪影。
为了进行分析，使用了三种不同类型的 aSi:H EPID，分别用 6 和 15 MV 光子束在不同的 PRF（每秒 25、50、100、200 和 400 个脉冲）下进行了研究。在具有硫酸钆闪烁体的面板上应用同步校正后，单个全场洪水图像质量得到了极大改善[6 MV 的信噪比（SNR）提高了 66.1 dB，15 MV 的 SNR 提高了 66.0 dB]。对于碘化铯闪烁体的 EPID，也可以降低较低 PRF 的噪声（6 MV 时的 SNR 高达 56.3 dB，15 MV 时的 SNR 高达 46.7 dB）。然而，简单的读出干扰模型在更高的 PRF 下失效，此时由于薄膜晶体管和闪烁体后辉光中的俘获电荷而导致图像滞后和重像效应需要额外的校正。
本文提出的自由运行同步校正方法提高了单帧的 SNR，并实现了成像应用，例如在增加的图像帧率下进行低剂量率成像（例如，跟踪肺部中的移动金基准点）。自适应图像引导放射治疗方案在 VMAT 计划中甚至变得可行。千伏和 MV 同时成像应用也可以从 X 射线图像中 MV 散射去除的新可能性中受益。
