管电压联合器官剂量调制技术对胸部CT辐射剂量和图像质量影响的模体研究

目的 探讨胸部CT使用器官剂量调制（ODM）技术时，不同管电压对表浅辐射敏感器官辐射剂量和图像质量的影响。方法 以临床胸部CT扫描方案为基准，对胸部模体使用不同管电压（140、120、100和80 kV共4种，其中100 kV为系统推荐值）、在不开启ODM（ODM off）和从扫描起始层至乳腺区开启ODM（ODM part）时对胸部模体进行扫描，在右侧乳腺区域前方固定位置放置长杆电离室（点电离室位于乳腺区域的中心位置），每组参数重复扫描并测量剂量7次，记录容积CT剂量指数（CTDI_vol）和乳腺皮肤剂量测量值（D）；重组冠状位5 mm层厚肺及软组织算法图像，沿z轴方向平均分为8部分，分别测算对比度噪声比（CNR）。对不同ODM扫描方式和不同管电压下CTDI_vol、D，肺及软组织算法图像8区域CNR，进行双因素无重复试验方差分析，组间两两比较采用LSD法。结果 管电压在140至80 kV变化时，CTDI_vol依次降低，在80 kV时最低，差异有统计学意义（F=105.579 5，P<0.05），140~100 kV时乳腺皮肤剂量测量值也依次降低，但管电压降至80 kV时，D反而升高，100 kV时最低，差异有统计学意义（F=27.736，P<0.05）。与ODM off相比，使用ODM part时CTDI_vol和D均下降，差异有统计学意义（F=39.732、81.961，P<0.05）。各种管电压下肺及软组织算法的图像CNR依次下降，差异有统计学意义（F=12.809、11.261，P<0.05），两两比较140~100 kV时CNR差异无统计学意义（P>0.05），80 kV时CNR显著下降，与其他组比较差异有统计学意义（P<0.05）；与ODM off相比，使用ODM part时肺及软组织算法图像CNR下降，差异均无统计学意义（P>0.05）。结论 临床实践中，在不低于系统推荐管电压100 kV时，可在保障图像质量前提下通过降低kV和在射线敏感器官区域联合使用ODM技术有效地降低乳腺辐射剂量。

A phantom study of the effects of tube voltage combined with organ dose modulation on radiation dose and image quality in chest CT

Objective To investigate the effects of different tube voltages on the dose of superficial radiation-sensitive organs and image quality when using organ dose modulation(ODM) in chest CT. Methods Based on clinical chest CT protocol with the sameother parameters, chest phantom was scanned using 140, 120, 100, and 80 kv (100 kV was the recommended by the CT system) without ODM (ODM off) or with ODM from the starting layer to the breast area (ODM part). A long rod ionization chamber was placed iat a fixed position in front of the right breast area. The scans were repeated for 7 times with each group of scanning parameters and dose values were measured for each scanning, the CTDI_vol and breast skin dose measurements(D) were recorded. Coronal images of 5 mm thickness for the lung and soft tissue algorithms were reformatted. The images were divided into 8 parts along the z axis direction, the contrast noise ratios(CNR) for every region were measured. For CTDI_vol, D, CNR for different ODM and tube voltage scanning modes, two factor non-repeat test ANOVA was performed. LSD method was used for comparison among groups. Results The CTDI_vol was lowest at 80 kV, and the breast skin dose measurement was lowest at 100 kV, CTDI_vol decreased in turn from140 to 80 kV (F=105.579 5, P<0.05). The breast skin dose measurement decreased in turn from140 to 100 kV, but increased instead at 80 kV. The difference was statistically significant(F=27.736, P<0.05). Compared with ODM off, the CTDI_vol and D for ODM part both declined and the differences were statistically significant(F=39.732, 81.961, P<0.05). The CNRs of the lung and soft tissue images decreased at every tube voltage(F=12.809, 11.261, P<0.05).The CNRs decreased from140 to 100 kV, but there was no statistical difference(P>0.05), and the difference was significant at 80 kV(P<0.05). Compared with ODM off, the CNRs of lung and soft tissue algorithm images with ODM part decreased, withnot statistically significant differences(P>0.05). Conclusions In clinical practice, with the tube voltage not less than the recommended(100 kV), the optimal reduction of breast radiation dose can be achieved by reducing kV and using ODM on the premise of resonable image quality.