64层螺旋CT的自动管电流调制技术(ATCM)的控制参数和辐射剂量的胸部模体实验研究

The study of control parameters and radiation dose of the automatic 64-MSCT tube current modulation system with chest phantom

目的通过胸部模体扫描探讨影响自动管电流技术的控制参数及影响因素对管电流mA调制效果和辐射剂量以及图像噪声的影响。方法使用GE lightspeed VCT对模拟胸部模体进行扫描,实验组使用不同的噪声指数(noise index,NI),调制方法,扫描层厚,定位像,管电压等组合扫描,并记录CTDIvol值以及测量图像的中心,右侧和体前空气的感兴趣区的标准差(standard deviation,SD)值。对照组使用固定管电流200mA扫描,比较各种组合的辐射剂量和成像噪声。结果对照组的CTDIvol为10.07mGy,实验组A,B,C,D,E的CTDIvol分别为18.58mGy,24.56mGy,14.98mGy,12.46mGy,11.49mGy均大于对照组。使用smartmA进行角度调制的C组比A组的CTDIvol降低约15%;而使用侧位定位像扫描的实验组D的辐射剂量也与使用正位像的实验组C辐射剂量不同;使用相同的NI值扫描,层厚越薄,调制的管电流越大,辐射剂量越大。图像中心区域的噪声(6.12±0.85)HU的变化要比体侧和体前的噪声(6.73±1.78)HU,(7.29±1.23)HU的变化小,各层的一致性要好于体侧和体前的噪声。但由于有最大mA调制800mA的限制,在实验组B的肩部几乎都是最大mA扫描,而最后的CTDIvol为31.76mGy。使用联合调制后,各层的平均mA明显减少,而图像噪声的没有统计学改变;NI值升高,辐射剂量减少;管电压改变,辐射剂量改变,但图像噪声不变。结论ATCM技术可以通过实时调整管电流获得各层图像一致的噪声水平。联合调制比Z轴调制更有效的控制辐射剂量而不会带来图像质量的损失。定位像的选择,NI值的设定,扫描层厚的选择,最大mA的设定会影响辐射剂量。

Objective The purpose of this study was to discuss the consequences of different choices of acquisition and control parameters on the actual image noise and on the patient dose with an automatic tube current modulation system with chest phantom .Methods The chest phantom was performed CT scan with different parameters in groups .The pa‐rameters included the Noise Index ,the modulation method ,the scan slice thickness ,the scout image ,the tube voltage . The radiation dose was evaluated with CTDIvol and the noise of center ,right and air beyond body was measured with the standard deviation of regain of interest .The contrast group had the 200 mA tube current .The radiation dose and noise were compared between the different groups .Results The CTDIvol of constant tube current group was 10 .07 mGy ,and the radiation dose of group A ,B ,C ,D and E （18 .58 mGy ,24 .56 mGy ,14 .98 mGy ,12 .46 mGy ,11 .49 mGy） were all higher than 10 .07 mGy .The CTDIvol of group C with smartmA angle modulation technology was 15% lower than that of group A .The CTDIvol was different between group D with lateral scout and group C with frontal scout .With the same scan NI value ,the scan thickness was more thinner and the tube current was modulated more higher and the radiation was more higher .The wave and consistency of noise of different slice at the center （6 .12 ± 0 .85） HU was better than that at the body side （6 .73 ± 1 .78） HU and the air beyond body （7 .29 ± 1 .23） HU .The tube current reach the max current set‐ting （800 mA） of group B on the body shoulder ,and the CTDIvol was 31 .76 mGy .The average modulated tube current was distinctly reduced after the compound modulation used but the image noise was not statistical losing .NI value in‐creased ,the radiation reduced ;the tube voltage changed ,the radiation dose changed ,but the image noise kept invariabili‐ty .Conclusion The constant noise level of all slice images can be obtained using ATCM technology with tube current modulating in time during tube rotation around the body .The compound modulation technology is better effective on radia‐tion control without the image quality losing .The acquisition parameters including the scout image ,the NI value setting , the scan slice thickness and the max mA setting all can influence the radiation dose .