数字血管减影中kVp及mAs选择对患者剂量及低对比度血管检出能力的影响

Effect of kVp and mAs settings on patient exposure dose and the detecting sensitivity for low contrast blood vessel in digital subtraction angiography

目的分析数字血管减影中kVp及mAs设置对患者皮肤剂量、有效剂量及低对比度血管检出能力的影响。方法以不同浓度碘造影剂自制低对比度血管减影体模，保持影像增强器输入端X射线比释动能为1．1μGy／帧，改变不同kVp设置，测量体模表面X射线入射剂量ESD，观察影像，确定可检测的最低碘造影剂浓度。管电压保持70kV，改变mAs设置，测量影像增强器输入端每帧影像所需X射线空气比释动能、体模ESD剂量及可检测的最低碘造影及浓度。以Monte Carlo模拟程序PCXMCV1．5模拟计算有效剂量。结果体模ESD及患者有效剂量随mAs呈线性变化。保持影像增强器输入端空气比释动能1．1μGy／帧，kVp由50变化到100，体模DSA影像可分辨的最低碘造影剂浓度线性增大；保持管电压70kV，mAs由6．4变化到64，体模DSA影像可分辨的最低碘造影剂浓度按指数规律减小。当曝光量（mAs）超过40后，其对低对比度血管检出的影响不明显。结论对DSA系统而言，降低X射线机管电压要比增大X射线机输出强度更能有效改善其低对比度血管的检测能力。

Objective To investigate the effectiveness of kVp, mAs on patient exposure dose and the detecting sensitivity for low contrast blood vessel in digital subtraction angiography. Methods A DSA phantom with 1.5 mm diameter vessels containing iodine at various concentrations was made. The air kerma 1.1 μGy/frame was kept at the entrance of 1. 1, with kVp changed to measure ESD and to determine the detectable threshold iodine concentration （DTIC） of the phantom on image displayer. X-ray tube voltage was kept at 70 kV, adjusted mAs and measured air kerma at entrance of 1.1, ESD and DTIC. The effect dose was calculated with Monte Carlo simulation program PCXMC V 1.5. Results As kVp varied from 50 to 100 kV,while the entrance air kerma of 1.1 was kept at 1.1 μGy/frame, the ESD and effect dose of patient varied linearly according to mAs. As mAs changed from 6.4 to 64 less than 70 kV, the DTIC decreased exponentially, while as the exposure exceed 40 mAs, the improvement of DTIC become modest. Conclusions Reduction of the X-ray tube voltage instead of increase of the exposure level would improve the DSA system performance. As the entrance air kerma of 1.1 exceed 2.5 μGy/frame,increasing exposure may not improve the image quality.