数字乳腺X线摄影中调节密度曝光档位对图像质量和辐射剂量影响的体模研究

A modal study of the effect of adjusting density exposure steps on image quality and radiation dose in digital mammography

目的探讨数字乳腺X线摄影中调节密度曝光档位对图像质量与辐射剂量的影响。方法使用数字乳腺X线摄影机的自动曝光控制(AEC)模式,在RMI-156模体下附加0、1、2、3、4块PMMA有机玻璃板模拟4.3、5.3、6.3、7.3、8.3 cm共5种不同的腺体厚度,每种厚度下分别调节密度曝光档位为-3、-2、-1、0、1、2、3、4档,记录每个档位及厚度下的靶/滤过组合、管电压、管电流量、入射体表剂量(ESD)、入射表面空气比释动能(ESAK)、半值层(HVL)和设备显示的平均腺体剂量(显示AGD),计算图像信噪比(SNR)、对比噪声比(CNR)、品质因子(FOM)和经计算所得的平均腺体剂量(计算AGD),对模体内模拟纤维、模拟钙化和模拟肿块的显示效果进行主观评分,分析不同档位下图像质量和辐射剂量的变化,比较ESD与ESAK、显示AGD与计算AGD、显示AGD/ESD与计算AGD/ESAK的关系。对档位与SNR、CNR、FOM采用线性拟合,档位与mAs、ESAK、计算AGD采用指数型函数曲线拟合,对于ESD与ESAK、显示AGD与计算AGD、显示AGD/ESD与计算AGD/ESAK的差异采用配对样本t检验分析。结果每增减1个档位,图像CNR与SNR升降约8%。图像模拟纤维、模拟钙化、模拟肿块的评分随档位的增加整体呈上升趋势,但存在升高档位评分却下降的情况。FOM在各档位下变化幅度为97%~104%,差异性小。衡量辐射剂量的ESD、ESAK、显示AGD、计算AGD随档位均呈增幅越来越快的指数型函数增长趋势,变化幅度为63%~165%。ESD、ESAK分别为15.14±10.08、16.66±11.07,两者差异具有统计学意义(t=-9.61,P=0.001)。显示AGD与计算AGD分别为3.66±2.18、3.61±1.99,两者差异无统计学意义(t=1.20,P=0.240)。结论调节密度曝光档位能使图像质量呈线性改变,使辐射剂量呈增幅越来越快的指数型函数改变,调节方式和幅度恰当稳定,具有较高的应用价值。

Objective To investigate the effect of adjusting density exposure steps on image quality and radiation dose in digital mammography.Methods Using the automatic exposure control(AEC)mode of the digital mammography machine,five different gland thicknesses of 4.3,5.3,6.3,7.3,and 8.3 cm were simulated by attaching 0,1,2,3,and 4 PMMA plexiglass plates under the RMI-156 modal body,and the density exposure steps were adjusted to-3,-2,-1,0,1,2,3,and 4 for each thickness.The target/filter combination,tube voltage,tube current,incident body surface dose(ESD),incident surface air kerma(ESAK),half-value layer(HVL)and the average glandular dose displayed by the device(displayed AGD)were recorded at each step and thickness,and the signal-to-noise ratio(SNR),contrast-to-noise ratio(CNR),figure of merit(FOM)and the calculated average glandular dose(calculated AGD)were calculated.Then,the display effects of simulated fibers,simulated calcifications and simulated masses within the modal body were scored subjectively,and the changes in image quality and radiation dose at different steps were analyzed,and the relationships between ESD and ESAK,displayed AGD and calculated AGD,and displayed AGD/ESD and calculated AGD/ESAK were analyzed.A linear fit was used for the steps with SNR,CNR,and FOM,and an exponential function curve fit was used for the steps with mAs,ESAK,and calculated AGD.The differences between ESD and ESAK,displayed AGD and calculated AGD,and displayed AGD/ESD and calculated AGD/ESAK were analyzed by paired-samples t test.Results The CNR and SNR of mammographic images rose and fell by about 8%with each increase or decrease of one step.The scores of image simulated fibers,simulated calcifications,and simulated masses showed an overall upward trend with increasing steps,but there were still cases where the scores decreased with increasing grades.FOM varied from 97%to 104%at each grade with little variability.ESD,ESAK,displayed AGD,and calculated AGD,which could measure radiation dose,showed an exponential trend of increasing function with increasing steps,with a variation of about 63%to 165%.There were statistically significant differences(t=-9.61,P=0.001)between ESD(15.14±10.08)and ESAK(16.66±11.07).However,there were no statistically significant differences(t=1.20,P=0.240)between displayed AGD and calculated AGD,which were 3.66±2.18 and 3.61±1.99,respectively.Conclusions The adjustment of density exposure steps can make the image quality change linearly and the radiation dose change exponentially with increasing speed,and the mode and magnitude of the adjustment are appropriately stable with high application value.