Department of Radiation Oncology, Allegheny General Hospital, Pittsburgh, Pennsylvania 15212, USA.
Med Phys. 2013 Aug;40(8):081713. doi: 10.1118/1.4812678.
Removal of the linear accelerator (linac) flattening filter enables a high rate of dose deposition with reduced treatment time. When used for megavoltage imaging, an unflat beam has reduced primary beam scatter resulting in sharper images. In fluoroscopic imaging mode, the unflat beam has higher photon count per image frame yielding higher contrast-to-noise ratio. The authors' goal was to quantify the effects of an unflat beam on the image quality of megavoltage portal and fluoroscopic images.
6 MV projection images were acquired in fluoroscopic and portal modes using an electronic flat-panel imager. The effects of the flattening filter on the relative modulation transfer function (MTF) and contrast-to-noise ratio were quantified using the QC3 phantom. The impact of FF removal on the contrast-to-noise ratio of gold fiducial markers also was studied under various scatter conditions.
The unflat beam had improved contrast resolution, up to 40% increase in MTF contrast at the highest frequency measured (0.75 line pairs/mm). The contrast-to-noise ratio was increased as expected from the increased photon flux. The visualization of fiducial markers was markedly better using the unflat beam under all scatter conditions, enabling visualization of thin gold fiducial markers, the thinnest of which was not visible using the unflat beam.
The removal of the flattening filter from a clinical linac leads to quantifiable improvements in the image quality of megavoltage projection images. These gains enable observers to more easily visualize thin fiducial markers and track their motion on fluoroscopic images.
移除直线加速器（linac）的均整器可实现高剂量沉积率和缩短治疗时间。在用于兆伏成像时，非均匀束流会减少初级束散射线，从而产生更清晰的图像。在透视成像模式下，非均匀束流每帧图像的光子计数更高，从而获得更高的对比度噪声比。作者的目标是量化非均匀束对兆伏端口和透视图像的图像质量的影响。
使用电子平板成像仪在透视和端口模式下获取 6MV 投影图像。使用 QC3 体模定量评估均整器对相对调制传递函数（MTF）和对比度噪声比的影响。还研究了在各种散射条件下，去除均整器对金基准标记对比度噪声比的影响。
非均匀束流提高了对比度分辨率，在测量的最高频率（0.75 线对/毫米）下，MTF 对比度提高了高达 40%。对比度噪声比的增加与增加的光子通量相符。在所有散射条件下，使用非均匀束流可以显著改善基准标记的可视化效果，能够观察到最细的金基准标记，而这些最细的标记使用非均匀束流时是不可见的。
从临床直线加速器中移除均整器可导致兆伏投影图像的图像质量发生可量化的改善。这些增益使观察者能够更轻松地观察到薄基准标记并在透视图像上跟踪其运动。
