猪结肠息肉模型MDCT低剂量扫描实验研究

The low dose MDCT study for colon polyps screening:basic experiment of simulated polyps in pig colon

目的:利用猪结肠息肉模型,通过改变噪声指数,观察不同扫描剂量对结肠息肉的显示,在降低扫描剂量的前提下,寻找能同时有效显示结肠息肉及腹部实质脏器的最大噪声指数及相应的肝脏SD值。方法:①结肠息肉性病变模型的制备。取新鲜处死的猪,游离回盲部以上结肠约15cm,清除肠内容物,在肠粘膜面用细缝线扎出不同大小的息肉6枚,直径分别为3mm,5mm,7mm,10mm,12mm,15mm。充气后结扎,将断端重新缝合,关闭猪腹腔,准备扫描;②扫描设备及条件:GEBrightspeedBT99-OCO,120kV,自动毫安,层厚7.5mm,螺距1.375:1,DFOV29.8cm,扫描图像进行薄层重建,重建层厚1.25mm,层间隔1.25mm,重建方式Standard,重建图像传至AW4.3-04工作站,用CTC软件重建。扫描的噪声指数范围自4至22,共分十组,每组间隔为2。记录不同噪声指数时相应的肝脏SD值、辐射剂量,并对不同大小息肉及肝脏图像质量进行评分,由两名有经验的放射科医师采用盲法读片。猪结肠息肉评分标准:不能清晰,显示1分;息肉显示,但边界欠清晰2分,息肉显示,边界清晰3分。肝脏评分标准:图像不能接受,1分;亚标准,2分;图像可接受3分,高于平均水平,4分;优,5分。测量常规扫描时肝脏的SD值,与本次实验的SD值进行比较,观察在不同噪声指数时扫描剂量减少的程度。用SPSS10.0软件进行统计分析。结果:不同的噪声指数能发现所有的病灶。但对小于5mm的病灶,在噪声指数较大时(大于10),图像质量下降。辐射剂量随着噪声指数的增加而减少,辐射剂量范围0.91～22.27mGy,平均5.70±2.17mGy。本次研究中,按照判断标准,评分3分及以上的肝脏图像质量可接受,也即说明在进行低剂量扫描时噪声指数不能高于14(即肝脏SD不大于15.42),否则将影响腹部实质性脏器图像的观察。该条件下的辐射剂量与常规扫描相比,存在显著性差异。结论:在辐射剂量低于常规扫描的情况下,单纯观察猪结肠息肉性病变,在最大噪声指数22,即在极低扫描剂量时,仍能满足诊断要求。在兼顾肝脏图像观察时,扫描时噪声指数不应高于14,即相应肝脏SD不大于15.42。

Objective： Our purpose was to find maximal noise index and standard deviation（SD） of liver by which we could decrease radiation dose, find simulated polyps in pig colon and obtain the acceptable image of abdominal parenehymatous organ. We evaluated the effects on lesion detection of noise index. Methods： Approximately 15cm-long segments of ascending pig colon were reseeted and cleansed. Sessile polyps with diameters of 3, 5, 7, 10, 12, 15 mm were created by inverting the colon segment, puckering the mueosa between forceps and seeming. The colon was reinverted and filled air before the two ends of the colon were double tied. Then put it into the abdomen and dose it for seanning. Scanning was performed at 120kV and auto mA （Bright.Speed Elite GE Medical Systems）. Images were reconstructed at 1.25 mm thickness, 1.25 mm intervals with standard algorithm. Images were obtained with noise index of 4 to 22, interval 2. SD of liver, radiation doses was recorded, the polyp and the image quality of liver was graded by two experienced radiologists who was blinded to the diagnosis. The polyp scoring criteria： 1 polyp invisible; 2 polyp visible, edge sharpness; 3 polyp visible, edge less sharpness. The liver score eriterian： 1 ,image unacceptable; 2,subefitefian; 3,image acceptable; 4,above the average level; 5,excellent. Compare the liver SD of routine scanning and using different noise index in order to detennine the decrease of radiation dose. The data was statistic by SPSS 10.0.Results：All polyps were detected on every noise index group. But for polyps below 5mm-diameter, image quality distinctly deereased when the noise index was larger than 10. Radiation dose was reduced as noise index became larger and lager. The dose ranged from 0.91 to 22.27 mGy, averaging 5.70 ± 2.17 mGy. In our research, ff the noise index was larger than 14, image quality of liver would be tmaeeeptable. In such situation, the liver image score was 3 and its SD was 15.42. Compared with normal scanning,the radiation dose had been obviously reduced （ P 〈 0.05）. Conclusion：in low-dose CT scanning, colon polyps could be correedy detected regardless of noise index. When concerned of image quality of liver, the noise index was no larger than 14 and the liver SD was no larger than 15.42.