CN112304987A - 基于光子计数能谱ct的含能材料等效原子序数测量方法 - Google Patents
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Abstract
本发明提供一种基于光子计数能谱CT的含能材料等效原子序数测量方法。该方法只需利用光子计数能谱CT对待测样品和3种已知材料的标定物进行CT扫描重建,进而由低、高能量区间CT图像的相对比值图像拟合出等效原子序数与相对比值的关系曲线,利用该关系曲线即可进一步计算得到待测样品和标定物的等效原子序数图像。该方法不依赖于光子计数能谱CT的专业知识,实施简便,具有很好的鲁棒性和通用性,可以大大降低现有方法的设备要求和算法复杂度。
Description
技术领域
本发明涉及一种基于光子计数能谱CT的含能材料等效原子序数测量方法,属于工业CT无损检测技术领域。
背景技术
计算机断层成像(Computed Tomography,CT)在医学诊断及工业无损检测中有着广泛的应用。CT图像可看作是材料的线性衰减系数分布,该系数取决于X射线能量和材料特性(质量密度和等效原子序数)。在实际应用中,X射线管产生的光子能谱很宽(这与传统CT理论中X射线源只能发出特定能量光子的假设相违背),积分型探测器对整个能谱的光子进行积分,导致质量密度和等效原子序数均不相同的材料在给定的能谱下可能表现为相同的线性衰减系数,这最终导致传统CT无法有效完成材质的识别。以含能材料检测为例,在实际生产中可以经常发现含能材料样品中含有高线性衰减系数的杂质,其衰减值远高于正常含能材料,甚至高于金属铝。从CT理论分析,高线性衰减系数杂质可能是由于含能材料高聚集造成的(等效原子序数与其他接近,密度远高于正常含能材料),或是由于实际生产过程中混入了高原子序数的杂质。由于传统CT无法有效获得的质量密度和等效原子序数,因此很难判断该类杂质的组成并针对杂质成分改进生产工艺。
作为传统CT的补充,双能CT和能谱CT利用不同能谱的X射线对物体进行CT扫描。结合双能CT和多能CT分解算法,可以获得被扫描物体的有效原子序数和电子密度的分布。但实际应用中上述方法存在能谱测量困难,参考材料标定过程复杂,且可能放大噪声等缺点。据此基于同步辐射以氧气为标准元素的迭代方法被提出,但是该技术设备要求高,难以满足实际需要。后续基于光子计数探测器的标定方法相继被提出,但上述方法必须设置很窄的能量区间(0.5keV或1keV),严重降低了光子计数探测器有效能量区间内接收的光子数,导致CT扫描十分耗时。此外,待测材料的原子序数精度仍可能受重建误差、探测器响应准确性、散射效应等因素影响。
现有的技术缺点主要包括:
(1)双能CT和能谱CT存在能谱测量困难、算法复杂度高、参考材料标定过程复杂,且可能放大噪声等问题。
(2)基于迭代的同步辐射测量方法设备要求高,难以广泛应用。
(3)基于光子计数探测器的标定的方法扫描效率低,难以满足工业需要,且精度易受重建误差、探测器响应准确性、散射效应等因素影响。
综上所述,现有的技术方法存在算法复杂、设备要求高、扫描效率低等问题,难以满足实际含能材料生成中的高效检测需求。
发明内容
为提高实际含能材料生成中等效原子序数的识别精度,并降低设备要求和算法复杂度,本发明提供一种基于光子计数能谱CT的等效原子序数测量方法。
本发明解决其技术问题所采用的技术方案包括以下步骤:
步骤1:获取3个材质组成已知的标定物的等效原子序数Zk,eff,k为标定物序号;
步骤2:利用光子计数能谱CT,将待测样品和3个标定物同时进行CT扫描,并重建低能区CT图像SL(x,y)和高能区CT图像SH(x,y);
步骤3:将SL(x,y)除以SH(x,y),得到待测样品和标定物的相对比值图像SR(x,y);
步骤4:利用标定物的等效原子序数及其相对比值,拟合等效原子序数与相对比值的关系曲线;
步骤5:利用SR(x,y)和步骤4的关系曲线,计算得到待测样品和标定物的等效原子序数图像Zeff(x,y)。
在上述步骤1中,设第k个标定物由原子序数为Z1,Z2,…,Zn的原子组成,根据Glasser公式,其等效原子序数可表示为
其中ai为标定物中第i种原子的质量百分比,Ai为第i种原子的原子质量,ni为第i种原子的原子数目,Zi为第i种原子的原子序数。
在上述步骤3中,当实际图像中的噪声较大时,需要先对SL(x,y)和SH(x,y)进行降噪。为避免异常点,对于SH(x,y)中灰度值小于T的点不进行计算,直接将SR(x,y)中的对应点置为0,其中T为灰度阈值。
在上述步骤4中,利用标定物的等效原子序数及其相对比值,拟合等效原子序数与相对比值的关系曲线为其中Zk,eff是步骤1中获取的标定物等效原子序数,R为相应标定物在SR(x,y)中的相对比值,a、b、c为关系曲线参数。
本发明提供的上述方法,所得到的等效原子序数图像计算公式,在同一光子计数能谱CT和相同扫描参数的情况下可以重复利用,即此时只需对待测样品(无标定物)执行步骤2、步骤3和步骤5,即可得到待测样品的等效原子序数图像。
本发明的有益效果是:该方法在实际应用中不依赖于光子计数能谱CT的专业知识,实施简便,具有很好的鲁棒性和通用性,可以大大降低现有方法的设备要求和算法复杂度,并且所得到的等效原子序数图像计算公式,在同一光子计数能谱CT和相同扫描参数的情况下可以重复利用。
下面结合附图和实施例对本发明做进一步说明。
附图说明
图1为本发明算法示意图。
具体实施方式
利用微焦点光子计数能谱CT(光子计数探测器为XCounter Hydra FX50)应用本发明,执行以下步骤:
步骤1:选择3个材质组成已知的标定物,依次为尼龙6、聚四氟乙烯、铝,获取其等效原子序数Zk,eff,k为标定物序号,根据公式:
可计算得到3种标定物的等效原子序数分别为6.6008、8.6163和13.0000。
步骤2:利用微焦点光子计数能谱CT,选择射线源管电压100kV和管电流70μA,同时扫描标定物和待测样品,扫描几何参数为:射线源到探测器距离420mm,射线源到旋转中心距离175mm,探测器像素尺寸为0.10mm,探测器采集速度为20帧/s,20帧图像求和得到一幅投影;低能量阈值为40keV,高能量阈值为60keV;单幅投影大小为1024×64,投影幅数为900幅,重建矩阵大小为1024×1024×64。利用FDK算法分别重建低能区CT图像SL(x,y)和高能区CT图像SH(x,y)。
步骤3:将SL(x,y)除以SH(x,y),得到待测样品和标定物的相对比值图像SR(x,y)。考虑到实际图像中噪声较大,先对SL(x,y)和SH(x,y)利用BM3D算法进行降噪。为避免异常点,对于SH(x,y)中灰度值小于T的点不进行计算,直接将SR(x,y)中的对应点置为0,其中T为灰度阈值,本实例中T取0.01。
步骤4:根据SR(x,y)获得尼龙6、聚四氟乙烯、铝3种标定物的相对比值分别为1.0912、1.1684和1.4854。获取3种标定物的相对比值时,考虑到SR(x,y)中存在噪声,分别取各标定物的多个像素的灰度平均值。利用标定物的等效原子序数和相对比值拟合关系曲线其中R为相应标定物在SR(x,y)中的相对比值,a、b、c为关系曲线参数,拟合可得关系曲线参数为a=6.375,b=23390,c=125100。
本发明提供的上述方法,所得到的等效原子序数图像计算公式,在同一光子计数能谱CT和相同扫描参数的情况下可以重复利用,即此时只需对待测样品(无标定物)执行步骤2、步骤3和步骤5,即可得到待测样品的等效原子序数图像。
Claims (4)
1.一种基于光子计数能谱CT的含能材料等效原子序数测量方法,其特征在于包括下述步骤:
步骤1:获取3个材质组成已知的标定物的等效原子序数Zk,eff,k为标定物序号;
步骤2:利用光子计数能谱CT,将待测样品和3个标定物同时进行CT扫描,并重建低能区CT图像SL(x,y)和高能区CT图像SH(x,y);
步骤3:将SL(x,y)除以SH(x,y),得到待测样品和标定物的相对比值图像SR(x,y);
步骤4:利用标定物的等效原子序数及其相对比值,拟合等效原子序数与相对比值的关系曲线;
步骤5:利用SR(x,y)和步骤4的关系曲线,计算得到待测样品和标定物的等效原子序数图像Zeff(x,y)。
4.根据权利要求1所述的一种基于光子计数能谱CT的含能材料等效原子序数测量方法,其特征在于:该方法所得到的等效原子序数图像计算公式,在同一光子计数能谱CT和相同扫描参数的情况下可以重复利用,即此时只需对待测样品(无标定物)执行步骤2、步骤3和步骤5,即可得到待测样品的等效原子序数图像。
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