基于自动管电流调节技术的X线剂量对CT图像质量的影响

目的 探讨自动管电流调节(ATCM)技术下不同的X线剂量对CT图像质量的影响。方法 应用GE Discovery HD750 CT机对肺野内含9个直径为5、10、12 mm的磨玻璃密度结节(GGN)的胸部仿真体模进行扫描。采用ATCM技术,设置管电压为80、100及120 kVp,在每个管电压条件下分别应用5个预设噪声指数(NI)级别(10、20、30、40及50)扫描体模,共得到15组图像;计算每组图像的平均管电流,记录每组图像的容积CT剂量指数(CTDIvol)和剂量长度乘积(DLP),计算有效剂量(ED)。测量并比较图像的客观噪声,并由2名放射科医师采用盲法对GGN进行主观评分,用Kappa检验评价观察者间的一致性。结果 3种管电压下的平均管电流值均随着预设NI的增加而减低。相同预设NI、不同管电压下图像的客观噪声及X线剂量基本一致。相同预设NI、不同管电压下GGN的主观评分差异无统计学意义(P>0.05),且2名观察者间的一致性中等或较好(Kappa>0.4)。结论 应用ATCM技术,相同预设NI下管电压的变化对X线剂量和图像质量不会产生明显影响;预设NI为50时,可检出肺尖部直径为5 mm的GGN。

Impact of radiation dose based on an automatic CT tube current modulation system on CT image quality

Objective To evaluate the consequences of different tube voltage and noise index (NI) on radiation dose, as well as image quality with automatic tube current modulation (ATCM) system. Methods The chest phantom containing 9 ground glass nodules (GGNs) with diameter 5, 10 and 12 mm was scanned on GE Discovery HD750 CT scanner. CT data were acquired at varying tube voltage (80, 100, and 120 kVp) and varying NI (10, 20, 30, 40, and 50) with an ATCM system, thus totally 15 groups of images were obtained. The mean tube current of each group was calculated. Volumetric CT dose index (CTDIvol) and dose-length product (DLP) was recorded in the CT scanner, and phantom-based estimates of effective dose (ED) were also calculated. The objective noise of image was measured at three anatomical sections and compared among the groups. Two radiologists blinded to the CT parameters used assessed the identification of GGN using 4-point scale. The degree of interobserve concordance was evaluated using Kappa test. Results Among three varying tube voltage settings, the mean tube current decreased with the NI increasing. There was no significant difference in the radiation dose nor the objective noise among the varying tube voltage settings at the same NI level ranging from 10 to 50. The scales of GGNs had no statistical difference at varying parameter settings ranging from 10 to 50 (P>0.05). The interobserve concordance of GGNs was substantial or almost perfect (Kappa>0.4). Conclusion Varying the tube voltage with ATCM system can provide images with a substantial constancy of the objective noise and the radiation dose at the same NI setting ranging from 10 to 50. GGN of 5 mm in diameter can be identified in apex of the lungs at the NI presetting of 50.