Abstract: Background　Patients diagnosed with gastrointestinal bleeding by multislice-spiral CT（MSCT） received high doses of radiation and contrast agents.The CT value of iodine contrast agent was increased with the decrease of tube voltage.We tried to determine the optimal scanning scheme to reduce the radiation dose and contrast dose simultaneously on the premise of satisfying the diagnostic requirements.Objective　To explore the feasibility of low-concentration contrast medium of MSCT combined with low-dose scanning technology in the diagnosis of gastrointestinal bleeding model scanning，so as to obtain the optimal scanning scheme and provide research basis for clinical application.Methods　CT values of bleeding points of 30 patients with gastrointestinal bleeding diagnosed by MSCT enhanced examination and confirmed by surgery from May 2014 to October 2016 in North China University of Science and Technology Affiliated Hospital were measured.The average value was （212.1±64.5） HU.Taking the measured CT value of the bleeding point as the reference value，the nonionic contrast agent，ioversol（320 mgI/ ml ） was diluted with 0.9% sodium chloride solution and divided into 4 iodine solutions with different concentrations（6.4，5.76，5.12，and 4.48 mgI / ml，respectively）．The corresponding CT values of 120 kV were（214.7±1.6），（185.9±1.4），（162.2±1.4），and （140.5±2.5） HU，respectively.Different doses of CT enhancement scanning were performed on the bleeding model groups.The control group was 120 kV and 300 mAs.Experimental group：Group A：100 kV，300 mAs；Group B：100 kV，400 mAs；Group C：80 kV，400 mAs；Group D：80kV，465 mAs，and the concentration of contrast agent corresponds to the concentration of 4 iodine solutions.Each concentration was scanned 7 times.Two experienced radiologists used double blind method to read the film，recorded CT volume dose index（CTDIvol） and dose length product（DLP），and calculate effective dose （ED）．The objective noise of the image was measured，the signal-to-noise ratio of the image and subjective score of the image quality calculated，the detection of gastrointestinal bleeding in each group of MSCT recorded，and the CT value of bleeding points measured to evaluate the optimal scanning scheme.Results　CTDIvol was 19.6 mGy in control group，12.4 mGy in experimental group A，16.5 mgy in experimental group B，8.2 mGy in experimental group C，and 9.5 mGy in experimental group D.DLP and ED in experimental groups were lower than those in control group（P<0.05 ）．There was no statistically significant difference in image noise，signal-to-noise ratio and subjective score of image quality between experimental group B and control group（P>0.05）．The image noise of groups A，C and D in the experimental group was higher than that of control group，while the signal-to-noise ratio and subjective score of image quality were lower than those of control group（P<0.05）．The average CT values of the bleeding points in all experimental groups were higher than those of the control group at the same concentration of contrast agent（P<0.05）．Control group（6.4 mgI/ml） and experimental group B（6.4 and 5.76 mgI/ml） had the same detection rate of bleeding.There was no statistically significant difference between experimental group B（5.12 mgI/ml） and control group（6.4 mgI/ml） in detection rate（P>0.05），while experimental group B （4.48 mgI / ml） had lower detection rate than control group（P<0.05）．Conclusion　MSCT double low technique is feasible to diagnose hemorrhage in digestive tract hemorrhage model scanning.Experimental group B（100 kV，400 mAs ） is the optimal scanning parameter.Under the condition of simulated hemorrhage rate of 0.5 ml/min，the optimal contrast agent concentration at the bleeding point of the model under the scanning condition is 5.12 mgI/ml，and the corresponding CT value at the bleeding point is about
（99.4±10.1） HU.

Key words: Tomography, Radiation dosage, Low contrast concentration, Gastrointestinal, Active hemorrhage

摘要： 背景 　采用多层螺旋CT（MSCT）诊断消化道出血的辐射剂量及对比剂用量大。碘对比剂的CT值随着管电压的降低而增加，利用碘的这一特性，本研究试图在满足诊断需求的前提下，确定同时降低辐射剂量及对比剂剂量的最优扫描方案。目的　探讨MSCT低浓度对比剂联合低剂量扫描技术在消化道出血模型扫描中诊断出血的可行性，以期获得最优扫描方案，为临床应用提供研究基础。方法　检测2014年5月—2016年10月于华北理工大学附属医院行MSCT增强检查诊断并手术证实的消化道出血患者（30例）出血点CT值，平均值（212.1±64.5）HU。以测得的出血点CT值为参考值，将非离子型对比剂碘佛醇（320 mgI/ml）用0.9%氯化钠溶液稀释成4种不同浓度碘溶液，依次为6.40、5.76、5.12、4.48 mgI/ml，120 kV对应CT值分别为（214.7±1.6）、（185.9±1.4）、（162.2±1.4）、（140.5±2.5） HU。对出血模型分组行不同剂量CT增强扫描，常规组120 kV，300 mAs；低剂量组：A亚组100 kV，300 mAs；B亚组100 kV，400 mAs；C亚组80 kV，400 mAs；D亚组80 kV，465 mAs，对比剂浓度对应4种碘溶液浓度。每种浓度扫描7次。由2位经验丰富的放射医师采用双盲法读片，记录CT容积剂量指数（CTDIvol）、剂量长度乘积（DLP），并计算有效剂量（ED）；测量图像客观噪声、计算图像信噪比、图像质量主观评分，记录每组MSCT对模型消化道出血检出情况，并测量出血点CT值，评价出最优扫描方案。结果　常规组CTDIvol为19.6 mGy，低剂量组A亚组CTDIvol为12.4 mGy，低剂量组B亚组CTDIvol为16.5 mGy，低剂量组C亚组CTDIvol为8.2 mGy，低剂量组D亚组CTDIvol为9.5 mGy。低剂量组各组的DLP、ED低于常规组（P<0.05）。低剂量组B亚组图像噪声、信噪比、图像质量主观评分与常规组比较，差异无统计学意义（P>0.05）；低剂量组A、C、D亚组图像客观噪声高于常规组，图像噪声信噪比、图像质量主观评分低于常规组（P<0.05）。同一对比剂浓度下低剂量组各亚组模型出血点CT值均高于常规组（P<0.05）。常规组6.40 mgI/ml与低剂量组B亚组6.40、5.76 mgI/ml对模型出血检出率相同；常规组6.4 mgI/ml与低剂量组B亚组5.12 mgI/ml对模型出血检出率比较，差异无统计学意义（P>0.05）；低剂量组B亚组4.48 mgI/ml对模型出血检出率低于常规组（P<0.05）。结论　MSCT双低技术在消化道出血模型扫描中诊断出血是可行的，低剂量组B亚组（100 kV，400 mAs）为最优扫描参数组，在模拟出血速率为0.5 ml/min情况下，该扫描条件下显示模型出血点的最优对比剂浓度为5.12 mgI/ml，对应出血点CT值为（99.4±10.1）HU。

关键词: 体层摄影术, 辐射剂量, 低对比剂浓度, 消化道, 活动性出血