相同CT剂量指数时管电压对表浅辐射敏感器官剂量的影响

The study on the effect of tube voltage on the dose of superficial radiation sensitive organ in the same CT dose index

目的探讨相同CT容积剂量指数(CTDIvol)时,不同管电压对表浅敏感器官(晶体、甲状腺和乳腺)辐射剂量和图像质量的影响。方法管电压自动模式(CAREkVsemi)下,结合适宜的定位像和扫描中心,对头颈部和胸部模体进行常规CT扫描。在相同的CTDIvol下,管电压分别选取70、80、100、120、140kV,每种管电压下定位像扫描3次,再进行1次螺旋扫描。头颈部模体在眼眶中心及第五颈椎(C5)椎体上缘层面选取ROI,胸部模体在肺尖及气管分叉层面选取ROI,测量并计算对比噪声比(CNR)。头颈部模体和胸部模体分别设置CTDIvol值为15.30、2.60mGy。用热释光剂量计(TLD)测量每次扫描时晶体、甲状腺和乳腺的器官剂量(取3次测量的平均值),计算定位像和螺旋扫描的累积值。最后通过计算品质因子(FOM)因子找到最适宜的管电压。结果头颈部模体CTDIvol为15.30mGy时,管电压为70kV时晶体辐射剂量最小(2.440mGy),140kV时晶体辐射剂量最大(2.823mGy)。管电压100kV时甲状腺辐射剂量最小(4.052mGy),80kV时甲状腺辐射剂量最大(4.914mGy)。CTDIvol为2.60mGy时进行胸部CT扫描,管电压70kV时乳腺辐射剂量最小(2.353mGy),管电压120kV时乳腺辐射剂量最大(2.735mGy)。CNR值在眼眶和第五颈椎椎体上缘层面都是在管电压120kV时最高,其范围分别为82.83~111.67、117.22~162.51;在肺尖和气管分叉层面都是在140kV时CNR值最大,其范围分别为71.26~112.81、76.21~88.52。FOM因子在眼眶层面100kV时最佳(4575.01),C5椎体上缘层面120kV时最佳(6198.94),肺尖层面140kV时最佳(4937.01),气管分叉层面70kV时最佳(3208.01)。结论相同CTDIvol时,管电压的改变对表浅敏感器官剂量和影像质量存在一定的影响。

Objective To investigate the effect of tube voltage on the organ dose of the superficial sensitive organs (lens, thyroid and mammary gland) and the image quality under the same volume CT dose index (CTDIvol). Methods Combining the appropriate scout image and scanning center, the head-neck and chest phantom were scanned by CT under CARE kV semi mode. Under the same CTDIvol, the tube voltage was set by 70,80,100,120,140 kV respectively, and the scout image was scanned three times under each tube voltage. Then each tube voltage was used for spiral scan. For head-neck phantom ROIs were selected in orbital center and C5 upper edge level, for chest phantom ROIs were selected in the apical and tracheal bifurcation level.The contrast noise ratio (CNR) was measured and recorded.The CTDIvol values of the head-neck and chest phantom were set by 15.30 mGy, 2.60 mGy respectively. The organ dose of lens, thyroid and mammary were measured by thermoluminescent dosimeters (TLD) in every scanning(take the average of 3 measurements),calculating the cumulative value of scout image and spiral scanning. Finally, the appropriate tube voltage was sought by calculating the FOM (figure of merit). Results When CTDIvol was 15.30 mGy, the lens radiation dose was the smallest at 70 kV(2.440 mGy), the lens radiation dose was the largest at 140 kV(2.823 mGy), the thyroid radiation dose was the smallest at 100 kV(4.052 mGy), and the largest at 80 kV(4.914 mGy). When CTDIvol was 2.60 mGy, the mammary radiation dose was the smallest at 70 kV(2.353 mGy), the largest at 120 kV(2.735 mGy). When the tube voltage was 120 kV, the CNR value was the highest at the orbital center and C5 upper edge level, and the range was 82.83 to 111.67 and 117.22 to 162.51 respectively. When the tube voltage was 140 kV, the CNR value was the highest at the apical and tracheal bifurcation level, and the range was 71.26 to 112.81 and 76.21 to 88.52 respectively. The optimal FOM factor was 100 kV at orbital center and 120 kV at C5 upper edge level (4 575.01, 6 198.94 respectively). The optimal FOM factor was 140 kV at the apical and 70 kV at tracheal bifurcation level (4 937.01, 3 208.01 respectively). Conclusion The change of the tube voltage has a certain effect on the dose of the superficial sensitive organs and the image quality under the premise of the same CTDIvol.