摘要：
目的 探讨自动管电流调制模式下行头颈部和胸部CT扫描时,管电压的改变对辐射剂量及影像质量的影响.方法 自动管电流和自动管电压模式下,对头颈部和胸部模体进行常规CT扫描.自动管电流模式下,管电压分别手动选择70、80、100、120和140 kV,对头颈部和胸部模体进行常规CT扫描.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5颈椎(C5)椎体上缘层面选取感兴趣区(ROI),胸部模体在肺尖及气管分叉层面选取ROI,测量记录对比噪声比(CNR).用热释光剂量计(TLD)测量每次扫描时眼晶状体和乳腺的器官剂量(取3次测量的平均值),计算定位像和螺旋扫描的累积值.记录每次扫描的容积CT剂量指数(CTDIvol),并计算CTDIvol累积值.最后通过计算品质因数(FOM),找到最优化的管电压值.结果自动管电流和自动管电压模式时,头颈部自动选择120 kV和108 mAs,胸部自动选择80 kV和167 mAs.自动管电流模式时,手动选择70 kV时眼晶状体辐射剂量和CTDIvol值最小(分别为0.779和4.070 mGy),140 kV时眼晶状体辐射剂量和CTDIvol值最大(分别为2.571和25.670 mGy).70 kV时乳腺辐射剂量和CTDIvol值最小(分别为0.698和0.900 mGy),140 kV时乳腺辐射剂量和CTDIvol值最大(分别为3.452和7.400 mGy).CNR值在眼眶和C5椎体上缘层面分别为51.30～118.36和80.78～173.12,在肺尖和气管分叉层面分别为50.15～129.58和49.63～115.40.FOM因子在眼眶层面80 kV最大,在C5椎体上缘层面120 kV最大,在肺尖和气管分叉层面都是70 kV最大.头颈部模体最佳管电压:眼眶层面手动100 kV,颈部层面自动管电压模式(120 kV).胸部模体最佳管电压:手动100 kV.结论 管电压的选择对CT扫描的辐射剂量和影像质量影响较大. 对于常规CT扫描,手动100 kV适合眼眶区域扫描,自动120 kV适合颈部区域扫描,手动100 kV适合胸部扫描.%Objective To investigate the effect of the change of tube voltage on radiation dose and image quality in head-neck and chest scanning under automatic tube current modulation ( ATCM ) . Methods CT scanning was performed on the head-neck and chest phantom with ATCM and automatic tube voltage(CARE kV). The tube voltage was manually selected at 70, 80, 100, 120 and 140 kV separately, and a routine CT scanning of the head-neck and chest with ATCM was performed. The scout was scanned for 3 times and a spiral scanning was performed once at each of tube voltage. The regions of interest( ROIs) were selected in the slices of orbital center and C5 upper edge level for the head-neck phantom, in the slices of apical and tracheal bifurcation level for the chest phantom. The contrast to noise ratios ( CNRs) were measured and recorded. The organ dose of eye lens and mammary are measured with thermoluminescent dosimeters ( TLDs ) for every scanning ( the average of 3 measurements ) . The cumulative dose value of the scout and spiral scanning was calculated. The volume CT dose index ( CTDIvol ) of each scan was recorded, and the cumulative value of CTDIvol was calculated. Finally, the optimized tube voltage was obtained by calculating the FOM ( figure of merit) . Results With ATCM and CARE kV, 120 kV and 108 mAs were chose automatically by system for head-neck phantom, 80 kV and 167 mAs for chest phantom. With ATCM, the radiation dose of eye lens and CTDIvol were minimal with manually selected 70 kV ( 0.779 and 4.070 mGy respectively ) , and maximaum with manually selected 140 kV (2.571 and 25.670 mGy). The radiation dose of the mammary gland and CTDIvol were minimal with manually selected 70 kV ( 0.698 and 0.900 mGy ) , and maximal with manually selected 140 kV (3.452 and 7.400 mGy). The CNR values of orbital center and C5 upper edge level were 51.30-118.36 and 80.78 - 173.12 respectively. The CNR values of the apical and tracheal bifurcation level were 50.15-129.58 and 49.63-115.40, respectively. The optimal FOM was appeared at orbital center slice with 80 kV, at C5 upper edge level slice with 120 kV and at both the apical and tracheal bifurcation level with 70 kV. Optimum tube voltage for head-neck phantom: manual 100 kV at orbital level, CARE kV mode(120 kV) at neck level. Optimal tube voltage for chest phantom: manual 100 kV. Conclusions The selection of tube voltage is responsible for the radiation dose and image quality of CT scanning. For conventional CT scan, manual 100 kV is suitable for orbital scanning, automatic 120 kV is suitable for neck scanning, manual 100 kV is suitable for chest scanning.
