扫描液体电离室型电子射野影像装置的剂量响应研究  

Study on the dose response characteristics of a scanning liquid ion-chamber electronic portal imaging device

作　　者：马少刚[1] 胡逸民[1] 宋一昕[1] 

机构地区：[1]中国协和医科大学中国医学科学院肿瘤研究所肿瘤医院放射治疗科,北京100021

出　　处：《中华放射肿瘤学杂志》2002年第1期45-49,共5页Chinese Journal of Radiation Oncology

摘　　要：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 。Objective To study the dose response characteristics and the influence factors such as gantry angle, field size and acquisition mode on the dosimetric response curves, when using a scanning liquid ion chamber electronic portal imaging device (EPID) for dose verification.Methods All experiments were carried out on a Varian 600C/D accelerator (6?MV X ray) equipped with a Varian PortalVision TM MK2 type EPID . To obtain the dose response curve, the relationship between the incident radiation intensity to the detector and the pixel value output from the EPID were established. Firstly, the different dose rates of 6?MV X rays were obtained by varying SSD. Secondly, three digital portal images were acquired for each dose rate using the EPID and averaged to avoid the influence of the dose rate flunctuations of the accelerator. The pixel values of all images were read using self designed image analysis software,and an average for a region consisting of 11×11 pixels around the center was taken as the response of the EPID. Thirdly, a dosimetric response curve was drawn by the pixel values against their corresponding dose rates. A set of similar dosimetric response curves were obtained by changing gantry angle, field size and acquisition mode in the same way described above. Results The relationship between the dose rates and pixel values was not linear, so all the characteristic curves had been fitted using polynomial functions. The dose response was strongly influenced by image acquisition mode including pulse repitition frequency and EPID sampling mode, and slightly varied with gantry angle position to the points away from the central axis but kept unchanged at the central axis , while the dosimetric response curves were found to be independent of radiation field sizes. Conclusions For different image acquisition modes, calibrations must be done separately as the acquisition mode obviously affects the shape of dosimetric response curve. Gantry angle effects can be limited by doing calibrations at differ

关 键 词：电子射野影像装置 剂量验证 剂量效应关系 图像获取模式 EPID 

分 类 号：R14[医药卫生—公共卫生与预防医学]