Li Xiang, Morgan Ashraf G, Liptak Christopher L, Muryn John S, Dong Frank F, Primak Andrew N, Segars W Paul
Medical Physics Graduate Program, Department of Physics, Cleveland State University, Cleveland, Ohio 44115 and Doctoral Program in Applied Biomedical Engineering, Department of Chemical and Biomedical Engineering, Cleveland State University, Cleveland, Ohio 44115.
Section of Medical Physics, Imaging Institute, Cleveland Clinic, Cleveland, Ohio 44115; Medical Physics Graduate Program, Department of Physics, Cleveland State University, Cleveland, Ohio 44115; and Doctoral Program in Applied Biomedical Engineering, Department of Chemical and Biomedical Engineering, Cleveland State University, Cleveland, Ohio 44115.
Med Phys. 2015 Nov;42(11):6258-68. doi: 10.1118/1.4932222.
In CT imaging, a desirable quality assurance dose quantity should account for the dose variability across scan parameters and scanner models. Recently, AAPM Task Group 111 proposed to use equilibrium dose-pitch product (Dˆeq), in place of CT dose index (CTDI100), for scan modes involving table translation. The purpose of this work is to investigate whether this new concept better accounts for the tube voltage (kVp) dependence of organ dose than the conventional CTDI100.
Three extended cardiac-torso patient models were included in this study. They represented normal-weight, overweight, and obese patients with abdomen-pelvis diameters ranging between 23 and 36 cm and body mass indices ranging between 20 and 42. A Monte Carlo program developed and validated for a 128-slice CT system was used to simulate organ dose for abdomen-pelvis scans at five tube voltages (70, 80, 100, 120, 140 kVp) with a pitch of 0.8 and a collimation of 38.4 mm. The same Monte Carlo program was also used to obtain CTDI100 and Dˆeq as well as their volume-averaged values, CTDIvol and Dˆeq,vol.
With other scan parameters kept constant, organ dose itself depended strongly on kVp. For the normal-weight patient model, the coefficient of variation (COV) across the five kVp values ranged between 72% and 75% for nine organs (liver, spleen, stomach, pancreas, kidneys, colon, small intestine, bladder, and ovaries) fully encompassed by the primary radiation beam. The COV generally increased with increasing patient size, ranging between 72%-77% and 76%-81% for the overweight and obese patient models, respectively. One-way analysis of variance for the effect of kVp was highly significant for all patient models (p<2×10(-26)). When organ dose was normalized by CTDIvol, the COV across kVps reduced to 5%-11%, 6%-15%, 12%-22% for the normal-weight, overweight, and obese patients, respectively. The effect of kVp was still highly significant (p=0.0001-0.004). When organ dose was normalized by Dˆeq,vol, the COV further reduced to 1%-8%, 3%-11%, 9%-19%, respectively. The effect of kVp was still significant for the obese patient model (p=0.004), but no longer significant for the normal-weight and overweight patient models (p=0.4 and 0.09, respectively). Finally, if organ dose conversion factors obtained at 120 kVp were used to approximate the values at 70 and 80 kVp, the resulting errors in the estimated organ dose were significantly reduced when the conversion factors were based on Dˆeq,vol instead of CTDIvol.
In adult abdomen-pelvis CT, equilibrium dose-pitch product better accounts for the kVp dependence of organ dose than CTDI100.
在CT成像中，理想的质量保证剂量指标应考虑扫描参数和扫描仪型号之间的剂量差异。最近，美国医学物理师协会第111任务组建议，对于涉及床移动的扫描模式，使用平衡剂量螺距乘积（Dˆeq）代替CT剂量指数（CTDI100）。本研究的目的是探讨这一新概念是否比传统的CTDI100更能体现管电压（kVp）对器官剂量的影响。
本研究纳入了三个扩展的心脏躯干患者模型。它们分别代表体重正常、超重和肥胖患者，腹部-盆腔直径在23至36厘米之间，体重指数在20至42之间。使用一个为128层CT系统开发并验证的蒙特卡罗程序，模拟在五个管电压（70、80、100、120、140 kVp）下，螺距为0.8、准直为38.4毫米的腹部-盆腔扫描的器官剂量。同样的蒙特卡罗程序还用于获取CTDI100和Dˆeq及其体积平均值CTDIvol和Dˆeq,vol。
在其他扫描参数保持不变的情况下，器官剂量本身强烈依赖于kVp。对于体重正常的患者模型，在五个kVp值下，九个完全被初级辐射束覆盖的器官（肝脏、脾脏、胃、胰腺、肾脏、结肠、小肠、膀胱和卵巢）的变异系数（COV）在72%至75%之间。COV通常随着患者体型的增加而增加，超重和肥胖患者模型的COV分别在72%-77%和76%-81%之间。对于所有患者模型，kVp影响的单因素方差分析具有高度显著性（p<2×10(-26)）。当用CTDIvol对器官剂量进行归一化时，体重正常、超重和肥胖患者的kVp之间的COV分别降至5%-11%、6%-15%、12%-22%。kVp的影响仍然具有高度显著性（p=0.0001-0.004）。当用Dˆeq,vol对器官剂量进行归一化时，COV进一步分别降至1%-8%、3%-11%、9%-19%。kVp的影响对于肥胖患者模型仍然显著（p=0.004），但对于体重正常和超重患者模型不再显著（分别为p=0.4和0.09）。最后，如果使用在120 kVp下获得的器官剂量转换因子来近似70和80 kVp时的值，当转换因子基于Dˆeq,vol而不是CTDIvol时，估计器官剂量的误差会显著降低。
在成人腹部-盆腔CT中，平衡剂量螺距乘积比CTDI100更能体现器官剂量对kVp的依赖性。
