不同扫描参数对宽体探测器CT散射线的影响研究

Effect of different scanning parameters on scattered radiation from wide-detector CT

目的探讨宽体探测器CT在使用不同扫描模式、不同探测器宽度时在z轴方向上散射线的分布特点。方法使用美国GE Revolution 16 cm宽体探测器CT,在机架扫描孔洞中心轴(z轴)上,以一定间隔布放热释光剂量计(TLD),分别在逐层扫描模式下使用4、8、16cm和螺旋扫描模式下4、8cm探测器宽度,对CT标准剂量模体进行扫描,扫描条件:管电压为120kV,有效管电流为200mAs,扫描长度为16cm,螺旋扫描时螺距分别为0.984∶1、0.516∶1。所有扫描重复4次,曝光后将所有TLD测量值除以4,并对数据进行统计学分析。结果z轴方向上,人体头侧散射线剂量值均高于人体足侧(Z=-2.366、-2.197、-2.366、-2.371、-2.028、-2.236、-2.028,P<0.05)。逐层扫描时,不同探测器宽度的散射线分布差异有统计学意义(χ^2=28.000,P<0.05),均为探测器4cm时最大,16cm时最小,最大差值为67.5μGy。螺旋扫描时,不同探测器宽度的散射线分布差异有统计学意义(Z=-3.233、-2.982,P<0.05),均为探测器8cm时最大,4cm时最小,其中螺距0.516∶1时最大差值为97.67μGy。螺旋扫描相同探测器宽度及有效管电流条件下,螺距为0.516∶1时高于螺距为0.984∶1的散射线,差异有统计学意义(Z=-3.296、-3.296,P<0.05),其中探测器宽度为8cm时最大差值为49.95μGy。结论宽体探测器CT不同探测器宽度的选择,可显著影响辐射场的分布和辐射值,应根据具体的临床需求选择合理的探测器宽度和相关参数,从而降低受检者、近台操作医务人员以及陪护人员的辐射剂量。

Objective To explore the distribution characteristics on z-axis of scattered radiation from a wide-detector CT with different scan modes and detector widths. Methods The CT standard-dose phantom was scanned using a 16 cm wide detector Revolution CT. Thermoluminescent dosimeters (TLDs) were placed on the central axis (z-axis) of the scan hole at given intervals. As scan modes, both axial scan mode (using detector with width in 4, 8 or 16 cm) and the helical scan mode (using detector with width in 4 and 8 cm) are used. The scan parameters were as follows: tube voltage 120 kV;effective tube current 200 mAs;scan length 16 cm;pitch (for helical scan): 0.984∶1 and 0.516∶1, and all scans were repeated for 4 times. All TLDs were measured, after exposure, and divided by four for further analysis. Results The scattered radiation on z-axis was higher at the direction of human head than at the direction of human foot (Z=-2.366, -2.197, -2.366, -2.371, -2.028, -2.236, -2.028, P<0.05). Under the axial scan, the difference in distribution of scattered radiation with different detector widths was statistically significant. The maximum increase for detector width of 4 cm and 16 cm was 67.5 μGy(χ^2=28.000, P<0.05). Under the helical scan, the difference in distribution of scattered radiation with different detector widths was statistically significant (Z=-3.233, -2.982, P<0.05). The largest distribution of scattered radiation was found when the detector width was 8 cm and the smallest at the detector width is 4 cm. The maximum increase for detector width of 8 and 4 cm was 97.67 μGy at a pitch of 0.516∶1. Furthermore, when the detector width and effective mAs were the same, the scattered radiation at a pitch of 0.516∶1 was greater than that at a pitch of 0.984∶1, with the statistically significant difference(Z=-3.296, -3.296, P<0.05). The maximum increase was 49.95 μGy when the detector width was 8 cm. Conclusions In a 16 cm wide-detector CT, the selection of different detector widths can significantly influence the distribution of radiation field and related radiation values. Suitable detector width and relevant parameters shall be chosen according to the specific clinical requirements so as to reduce the radiation doses to workers, patients and carers.