不同定位像在管电流自动调制技术CT扫描中对辐射剂量的影响

Effects of different scout images on radiation dose in CT scanning using automatic tube current modulation

目的探讨在应用CT管电流调制技术（ATCM）行头部和胸部CT螺旋扫描时，不同定位像对辐射剂量的影响。方法采用头颈部模体和胸部模体，应用ATCM行头部和胸部CT螺旋扫描。先行定位像扫描，分别采用前后位、后前位、侧位、前后＋侧位、后前＋侧位5种方式，每种定位像扫描3次。每种定位像再进行1次螺旋扫描。头颈部模体在眼眶中心及C5椎体上缘层面选取ROI，胸部模体在肺尖及气管分叉处层面选取ROI，测量记录对比噪声比（CNR）。用热释光剂量计（TLD）测量每次扫描时晶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。结果头颈部模体5种定位像扫描中，后前位定位像晶体累积器官剂量值和CTDIvol最高（分别为18.354、26.43 mGy），侧位定位像上述值最低（分别为11.847、18.20 mGy）；胸部模体定位像扫描中，前后位定位像乳腺累积器官剂量值和CTDIvol最高（分别为6.873、9.42 mGy），侧位定位像乳腺累积器官剂量值最低（4.592 mGy），前后位＋侧位定位像CTDIvol最低（3.94 mGy）。后前位定位像晶体累积器官剂量值比侧位定位像高54.9%（6.507/11.847），CTDIvol值高45.2%（8.23/18.20）；前后位定位像乳腺累积器官剂量值比后前位＋侧位定位像高42.5%（2.051/4.822），CTDIvol值高136.7%（5.44/3.98）。头颈部模体眼眶中心层面、C5椎体上缘层面的CNR分别为102.55~115.89、161.01~204.52；胸部模体肺尖层面、气管分叉处层面的CNR分别为82.74~164.00、83.12~121.49。结论定位像的选择可显著影响CT扫描中辐射剂量和表浅敏感器官剂量。

ObjectiveTo investigate the influence of different scout image on the radiation dose of head and chest spiral scanning in CT using automatic tube current modulation（ATCM）. MethodsCT scanning was performed on the head-neck and chest phantom with ATCM.Five different scout images through the five various positions that include anteroposterior（AP）,posteroanterior（PA）,lateral,AP, lateral,PA and lateral.The phantom was scanned three times for each position.Then each scout image was scanned by spiral technique once more. ROI in orbital center and C5 upper edge level were selected for Head and neck phantom, ROI in the apical and tracheal bifurcation level were selected for chest phantom. The contrast to noise ratio （CNR） was measured and recorded. The organ dose of eye lens and mammary glands were measured by thermoluminescent dosimeters （TLD） in every scanning（the average of 3 measurements）, The cumulative value of scout image and spiral scanning were calculated.The volume CT dose index （CTDIvol） of each scan was recorded, and the cumulative value of CTDIvol was calculated.ResultsIn the five scout images of the head-neck phantom mode, the maximum value of accumulated radiation dose of eye lens and CTDIvol appear on PA scout image（18.354 and 26.43 mGy respectively）,while the minimum value appear on the lateral scout image（11.847 and 18.20 mGy respectively）. In the chest phantom mode, the maximum value of the accumulated radiation dose of the mammary gland and CTDIvol emerge from the AP scout image（6.873 and 9.42 mGy respectively）, while the minimum value of the mammary accumulated radiation dose appear on lateral scout image（4.592 mGy）, the minimum value of CTDIvol appear on AP plus lateral scout images（3.94 mGy）.The accumulated radiation dose of eye lens and CTDIvol value in PA scout image mode were 54.9%（6.507/11.847） and 45.2%（8.23/18.20） higher than those of the lateral scout image mode. The accumulated radiation dose of mammary gland and CTDIvol value in AP scout image mode were 42.5%（2.051/4.822） and 136.7% （5.44/3.98） higher than that of PA plus lateral scout images mode. In the head-neck phantom mode, the CNR value of orbital center and C5 upper edge level were 102.55 to 115.89, 161.01 to 204.52 respectively. In the chest phantom mode, the CNR value of the apical and tracheal bifurcation level were 82.74 to 164.00, 83.12 to 121.49 respectively.