OBI系统锥形束CT图像均匀性和噪声的长期稳定性分析

Long-term constancy analysis of image uniformity and noise of cone-beam CT for the OBI system

目的评估瓦里安加速器机载影像(OBI)系统锥形束CT(CBCT)图像均匀性和噪声的长期稳定性.方法对瓦里安Clinac iX加速器OBI系统每月进行质控检测,获取Catphan 504模体在6种扫描协议下的CBCT图像.这6种扫描协议分别是高质量头部(HQH)、低剂量头部(LDH)、盆腔聚光灯(PSL)、标准剂量头部(SDH)、低剂量胸部(LDT)和盆腔(pelvis).对南京医科大学附属南京医院2020年1月至2021年12月共24个月的144套CBCT图像序列进行分析,通过在CTP486模块图像上勾画感兴趣区(ROI)获得图像均匀性和噪声的数据.图像均匀性使用3点钟、6点钟、9点钟和12点钟方向ROI与中心ROI的CT平均值的最大差异的绝对值表示.图像噪声,使用大圆形ROI的CT值的标准差表示.图像均匀性和噪声的数值越小,表明图像质量越好.对数据进行统计描述,得到6种扫描协议下这2种参数的最大值、最小值、平均值和标准差等数据.定义Y为每个方向ROI的平均CT值减去中心ROI的平均CT值得到的差值,用来表示每个月图像质量的变化.结果该OBI系统CBCT的图像均匀性和噪声在2年中总体上比较稳定.对于HQH、LDH、PSL、SDH、LDT、pelvis这6种扫描协议,图像均匀性数值分别为(21.5±1.0)、(21.4±2.2)、(21.3±0.9)、(18.3±1.3)、(3.4±1.9)、(7.2±1.9)HU,图像噪声数值分别为(19.2±0.2)、(41.0±0.3)、(14.2±0.3)、(38.2±0.2)、(16.0±0.9)、(13.5±0.9)HU.其中,LDT的图像均匀性数值的平均值最小,pelvis的图像噪声数值的平均值最小.6种扫描协议下,图像均匀性涉及的中心ROI平均CT值分别为(32.3±2.7)、(24.0±3.3)、(19.3±2.4)、(11.1±2.5)、(67.2±2.1)、(-24.6±1.7)HU,平均CT值数据的稳定性较好.Y的标准差均<3HU,表明每个月图像质量的稳定性较好.结论OBI系统CBCT的图像均匀性和噪声在2年中总体上比较稳定.6种扫描协议中,LDT的图像均匀性最好,pelvis的图像噪声最小.

Objective To evaluate the long-term constancy of image uniformity and noise of cone-beam computed tomography(CBCT)for the on-board imager(OBI)system of a Varian linac.Methods Monthly quality assurance(QA)tests were carried out for the OBI system of a Clinac iX linac,and CBCT images of Catphan 504 phantom were obtained with six scanning protocols.The 6 protocols were high-quality head(HQH),low-dose head(LDH),pelvis spot light(PSL),standard-dose head(SDH),low-dose thorax(LDT)and pelvis,respectively.The 144 series of CBCT images acquired from Nanjing First Hospital,Nanjing Medical University between January 2020 and December 2021 were analyzed,and the data of image uniformity and noise were obtained by delineating regions of interest(ROI)in the images of CTP486 module.Image uniformity was expressed by the absolute value of the maximum difference between the average CT value of the ROI in the 3,6,9 and 12 o'clock directions and the central ROI.Image noise was expressed by the standard deviation of CT value of large circular ROI.The smaller the values of image uniformity and noise,the better the image quality.Statistical description was applied to obtain data of the maximum,minimum,mean and standard deviation for the above mentioned two parameters.Y was defined as the difference between the average CT value of ROI in each direction and the average CT value of central ROI,which was used to represent the monthly changes of image quality.Results The image uniformity and noise of CBCT images of the OBI system were generally stable during 2 years.For the 6 protocols of HQH,LDH,PSL,SDH,LDT and pevlis,the image uniformity was(21.5±1.0),(21.4±2.2),(21.3±0.9),(18.3±1.3),(3.4±1.9)and(7.2±1.9)HU,respectively.The image noise was(19.2±0.2),(41.0±0.3),(14.2±0.3),(38.2±0.2),(16.0±0.9)and(13.5±0.9)HU,respectively.The average image uniformity of LDT was the smallest,and the average image noise of pelvis was the smallest.Under the 6 scanning protocols,the average CT values of central ROI involved in image uniformity were(32.3±2.7),(24.0±3.3),(19.3±2.4),(11.1±2.5),(67.2±2.1),(-24.6±1.7)HU,respectively.The average CT values were stable.The standard deviation of Y was<3 HU,suggesting that the stability of image quality was favorable every month.Conclusions The image uniformity and noise of CBCT images for the OBI system are generally stable in 2 years.Among the 6 scanning protocols,LDT yields the best image uniformity,and pelvis has the lowest image noise.