Heijmen B J, Pasma K L, Kroonwijk M, Althof V G, de Boer J C, Visser A G, Huizenga H
Department of Clinical Physics, Dr Daniel den Hoed Cancer Center, Rotterdam, The Netherlands.
Phys Med Biol. 1995 Nov;40(11):1943-55. doi: 10.1088/0031-9155/40/11/012.
Physical characteristics of a commercially available electronic portal imaging device (EPID), relevant to dosimetric applications in high-energy photon beams, have been investigated. The EPID basically consists of a fluorescent screen, mirrors and a CCD camera. Image acquisition for portal dose measurement has been performed with a special procedure, written in the command language that comes with the system. The observed day-to-day variation in local EPID responses, i.e. measured grey scale value (EPID signal) per unit of delivered portal dose, is 0.4% (1 SD); day-to-day variation in relative EPID responses (e.g. normalized to the on-axis response) are within 0.2% (1 SD). Measured grey scale values are linearly proportional to transmitted portal doses with a proportionality constant which is independent of the thickness of a flat, water-equivalent absorber in the beam, but which does significantly depend on the size of the applied x-ray beam. It is shown that the observed increased in EPID response with increasing field size is mainly due to contributions to the EPID signals from scattered light: visible photons produced by the x-ray beam in a point of the fluorescent screen not only generate a grey scale value in the corresponding point of the EPID image, but also lead (due to scatter from components of the EPID structure onto the CCD chip) to an increased grey scale value at all other points of the image. A point spread function, derived from measured data and describing the increase in EPID response at the beam axis due to off-axis irradiation of the fluorescent screen, has been successfully applied to connect portal doses with grey scale values measured with the EPID.
已对一种市售电子射野影像装置（EPID）的物理特性进行了研究，这些特性与高能光子束的剂量学应用相关。该EPID主要由一个荧光屏、反射镜和一个电荷耦合器件（CCD）相机组成。用于射野剂量测量的图像采集是通过用该系统附带的命令语言编写的特殊程序来进行的。观察到的EPID局部响应的逐日变化，即每单位传递的射野剂量所测量的灰度值（EPID信号）为0.4%（1标准差）；EPID相对响应的逐日变化（例如相对于轴上响应进行归一化）在0.2%（1标准差）以内。测量的灰度值与透射的射野剂量呈线性比例关系，比例常数与射束中平坦的水等效吸收体的厚度无关，但确实显著取决于所应用的X射线束的大小。结果表明，观察到的随着射野尺寸增加EPID响应的增加主要是由于散射光对EPID信号的贡献：X射线束在荧光屏某一点产生的可见光子不仅在EPID图像的相应点产生一个灰度值，而且（由于从EPID结构的部件散射到CCD芯片上）导致图像所有其他点的灰度值增加。从测量数据导出的、描述由于荧光屏的离轴照射而在射束轴上EPID响应增加的点扩散函数，已成功用于将射野剂量与用EPID测量的灰度值联系起来。
