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目的 研究扫描液体电离室型电子射野影像装置 (EPID)的剂量响应特性及其各种影响因素 ,如机架角、照射野大小、图像获取模式 ,以便进一步利用EPID进行剂量验证方面的研究。方法 所有实验均在装备有PortalVisionTMMK2型电子射野影像装置的Varian 6 0 0C/D加速器上实现。为了得到剂量响应曲线 ,需要建立入射到探头的射线强度与EPID像素值之间的关系。首先 ,通过改变源到探头电离室的距离得到不同的射线强度。其次 ,针对任一剂量率条件 ,用EPID拍摄 3幅数字射野图像取平均 ,取射野中心轴附近 11× 11个像素点的平均值作为EPID响应。最后 ,根据相同条件下测得的剂量率和对应的像素值 ,绘制剂量响应曲线。改变机架角、照射野大小和图像获取模式 ,得到一系列剂量响应曲线。结果 EPID输出像素值与入射剂量率之间并非线性关系。EPID剂量响应曲线与图像获取模式关系密切 ,在离轴点略受机架角的影响 ,但不受射野大小的影响。结论 由于射野图像获取模式明显地影响EPID剂量响应曲线的形状 ,所以对不同获取模式应该分别刻度。机架角的影响可通过在不同机架角下刻度加以消除。EPID剂量响应与射野大小无关的事实为日常剂量响应刻度提供了便利 ,即用一种射野条件刻度就可以准确地应用于其它射野条件
Objective To study the dose-response characteristics of scanning liquid ionization chamber electronic imaging device (EPID) and its various influencing factors, such as frame angle, irradiation field size and image acquisition mode, in order to further study the dosage verification of EPID. Methods All experiments were performed on a Varian 600C / D accelerator equipped with a PortalVisionTMMK2 electron field imaging device. In order to obtain a dose response curve, it is necessary to establish the relationship between the intensity of the radiation incident on the probe and the EPID pixel value. First, different ray intensities are obtained by changing the distance of the source to the ionization chamber of the probe. Secondly, for any dose rate conditions, three digital field images were taken with EPID and averaged. The average of 11 × 11 pixels near the central axis of the field was taken as the EPID response. Finally, the dose response curve is plotted based on the measured dose rate and the corresponding pixel value under the same conditions. Changing the gantry angle, exposure field size, and image acquisition mode yielded a series of dose response curves. Results There is a non-linear relationship between the EPID output pixel value and the incident dose rate. The EPID dose response curve is closely related to the image acquisition mode, slightly affected by the gantry angle at the off-axis point, but not by the size of the field. Conclusions Since the field acquisition pattern significantly affects the shape of the EPID dose response curve, the different acquisition patterns should be individually scaled. The effect of rack angle can be eliminated by ticking at different rack angles. The fact that the EPID dose response has nothing to do with the size of the field facilitates the daily dose response scale, that is, it can be accurately applied to other field conditions using a field condition scale