呼吸时相和模体位置对4D-CT图像均匀性的影响

目的:通过分析Catphan模体CTP486模块的四维CT（4D-CT）图像，研究呼吸时相和模体位置对4D-CT图像均匀性的影响。方法:使用西门子Sensation Open CT模拟机和瓦里安RPM系统，获取Catphan 504模体CTP486模块的4D-CT图像。对3种模体位置情形进行研究。情形A:模体悬空放置；情形T:模体下有一个碳纤维CT平板床；情形B+T:模体下有一个碳纤维固定底板和一个CT平板床。每种情形重复3次测量。使用DoseLab放疗验证软件分析4D-CT图像，得到图像均匀性值（U）。对每一套4D-CT重建10个呼吸时相（0%, 10%,[ ?], 90%）的CT图像序列。对U值进行统计分析和比较研究。结果:4D-CT所有CT序列的U值均小于5 HU。每种情形得到30个U值数据，情形A、情形T和情形B+T 3种情形U值分别为（1.44±0.79）、（1.91±0.91）和（1.99±0.77） HU。统计U值对应的4个边缘感兴趣区（ROI）出现的次数和比例，情形T中ROI 22（9点钟方向）出现13次（占比43.33%），情形B+T中ROI 23（12点钟方向）出现13次（占比43.33%）。结论:本研究中4D-CT的图像均匀性满足使用要求，不同呼吸时相4D-CT的图像均匀性值不同，模体摆放位置对4D-CT的图像均匀性有一定影响，碳纤维CT平板床和固定底板的存在使4D-CT 的图像均匀性变差。

Effects of respiratory phase and phantom position on uniformity of 4D-CT image

Objective To investigate the effects of respiratory phase and phantom position on the uniformity of four-dimensional computed tomography(4D-CT)images by analyzing the 4D-CT image of the CTP486 module in Catphan phantom.Methods Siemens Sensation Open CT simulator and Varian RPM system were used to obtain the 4D-CT images of CTP486 module in Catphan 504 phantom.The following 3 cases in which the phantom position was different were utilized to carry out this study.In Case A,the Catphan phantom was hung in the air;in Case T,a carbon fiber CT flat table was placed under the phantom;and in Case B+T,a carbon fiber immobilization baseplate and the CT flat table were placed under the phantom.Three measurements were implemented for each case.DoseLab radiotherapy verification software was used to analyze the 4D-CT images,and the image uniformity(U values)were obtained.Ten series of CT were reconstructed for 10 respiratory phases(0%,10%,⋯,90%)for each 4D-CT measurement.Statistical analysis and comparative study were conducted on U values.Results All the U values of 4D-CT were smaller than 5 HU.For each case,30 U values were obtained.The U values in CaseA,Case T and Case B+T were(1.44±0.79),(1.91±0.91)and(1.99±0.77)HU,respectively.The numbers of times and proportions of the occurrence of 4 peripheral regions of interest which were corresponding to U values were analyzed.There were 13 ROI 22(9 o'clock position)in case T,with a proportion of 43.33%,and 13 ROI 23(12 o'clock position)in Case B+T,with a proportion of 43.33%.Conclusion The uniformity of 4D-CT images meets the requirements.The uniformity values of 4D-CT images for different respiratory phases are different.The phantom position has certain effects on the uniformity of 4D-CT images,and the existence of carbon fiber CT flat table and immobilization baseplate reduces the uniformity of 4D-CT images.