King B, Greer P
University of Newcastle, Newcastle, NSW, AU.
Calvary Mater Newcastle Hospital, Newcastle, NSW, AU.
Med Phys. 2012 Jun;39(6Part24):3911. doi: 10.1118/1.4735962.
To develop a method for removing the effect of support arm backscatter from Varian electronic portal imaging devices (EPIDs), improving the dosimetric abilities of the imager.
A physical, kernel based model of the backscatter signal produced during an exposure was developed. The model parameters were determined through an optimization process, comparing measured images without arm backscatter (EPID removed from arm) to measured images that include arm backscatter. The backscatter model was used to develop a backscatter correction process that removes the support arm backscatter from measured EPID images. The correction process was tested by applying the method to measured images of 17 rectangular asymmetric fields and comparing the Result to off-arm images. The same process was repeated with 42 IMRT fields.
The backscatter removal process was able to effectively remove the arm backscatter from all of the measured images and accurately predict the measured off-arm images. Comparing the corrected images to the measured off-arm images, the mean absolute difference at the centre of each rectangular field was 0.29% (standard deviation 0.18%). This is an improvement over the uncorrected images which gave a mean difference of 1.01% (standard deviation 0.73%). The largest discrepancy observed with the corrected images was 0.6%, compared to 2.8% for the uncorrected images. Comparing the corrected IMRT images to the measured off-arm images, an overall mean gamma value of 0.28 (standard deviation 0.04) was found using 2%, 2mm criteria. Comparison of the uncorrected images to the measured off-arm images resulted in an overall mean gamma of 0.40 (standard deviation 0.10).
A method for accurately and reliably removing the effect of support arm backscatter from EPID images has been developed and extensively tested. The method can be applied to any measured EPID image and does not require any additional information about the exposure.
开发一种从瓦里安电子射野影像装置（EPID）中去除支撑臂背散射影响的方法，以提高成像仪的剂量测定能力。
建立了曝光期间产生的背散射信号的基于物理内核的模型。通过优化过程确定模型参数，将无臂背散射的测量图像（从臂上移除的EPID）与包含臂背散射的测量图像进行比较。背散射模型用于开发一种背散射校正过程，从测量的EPID图像中去除支撑臂背散射。通过将该方法应用于17个矩形不对称射野的测量图像，并将结果与无臂图像进行比较，对校正过程进行了测试。对42个调强放疗（IMRT）射野重复相同过程。
背散射去除过程能够有效地从所有测量图像中去除臂背散射，并准确预测测量的无臂图像。将校正后的图像与测量的无臂图像进行比较，每个矩形射野中心的平均绝对差值为0.29%（标准差0.18%）。这比未校正图像有所改进，未校正图像的平均差值为1.01%（标准差0.73%）。校正图像观察到的最大差异为0.6%，而未校正图像为2.8%。将校正后的IMRT图像与测量的无臂图像进行比较，使用2%、2毫米标准时，总体平均伽马值为0.28（标准差0.04）。未校正图像与测量的无臂图像比较，总体平均伽马值为0.40（标准差0.10）。
已开发并广泛测试了一种从EPID图像中准确可靠地去除支撑臂背散射影响的方法。该方法可应用于任何测量的EPID图像，且不需要任何关于曝光的额外信息。
