Department of Radiology, Mayo Clinic, 200 First Street Southwest, Rochester, Minnesota 55905, USA.
Med Phys. 2011 Feb;38(2):993-7. doi: 10.1118/1.3547718.
In diagnostic CT imaging, multiple important applications depend on the knowledge of the x-ray spectrum, including Monte Carlo dose calculations and dual-energy material decomposition analysis. Due to the high photon flux involved, it is difficult to directly measure spectra from the x-ray tube of a CT scanner. One potential method for indirect measurement involves estimating the spectrum from transmission measurements. The expectation maximization (EM) method is an accurate and robust method to solve this problem. In this article, this method was evaluated in a commercial CT scanner.
Two step-wedges (polycarbonate and aluminum) were used to produce different attenuation levels. Transmission measurements were performed on the scanner and the measured data from the scanner were exported to an external computer to calculate the spectra. The EM method was applied to solve the equations that represent the attenuation processes of polychromatic x-ray photons. Estimated spectra were compared to the spectra simulated using a software provided by the manufacturer of the scanner. To test the accuracy of the spectra, a verification experiment was performed using a phantom containing different depths of water. The measured transmission data were compared to the transmission values calculated using the estimated spectra.
Spectra of 80, 100, 120, and 140 kVp from a dual-source CT scanner were estimated. The estimated and simulated spectra were well matched. The differences of mean energies were less than 1 keV. In the verification experiment, the measured and calculated transmission values were in excellent agreement.
Spectrum estimation using transmission data and the EM method is a quantitatively accurate and robust technique to estimate the spectrum of a CT system. This method could benefit studies relying on accurate knowledge of the x-ray spectra from CT scanner.
在诊断 CT 成像中，多种重要应用都依赖于对 X 射线光谱的了解，包括蒙特卡罗剂量计算和双能物质分解分析。由于涉及到高光子通量，直接从 CT 扫描仪的 X 射线管测量光谱是困难的。一种潜在的间接测量方法涉及从透射测量中估计光谱。期望最大化 (EM) 方法是解决此问题的准确而强大的方法。本文在商用 CT 扫描仪中评估了该方法。
使用两个步阶（聚碳酸酯和铝）来产生不同的衰减水平。在扫描仪上进行透射测量，并将从扫描仪测量的数据导出到外部计算机以计算光谱。应用 EM 方法求解代表多色 X 射线光子衰减过程的方程。将估计的光谱与使用扫描仪制造商提供的软件模拟的光谱进行比较。为了测试光谱的准确性，使用含有不同水深的体模进行了验证实验。将测量的透射数据与使用估计的光谱计算的透射值进行比较。
从双源 CT 扫描仪估计了 80、100、120 和 140 kVp 的光谱。估计的和模拟的光谱非常匹配。平均能量的差异小于 1 keV。在验证实验中，测量值和计算值的透射值非常吻合。
使用透射数据和 EM 方法进行光谱估计是一种定量准确且强大的技术，可以估计 CT 系统的光谱。该方法可以使依赖于 CT 扫描仪 X 射线光谱准确知识的研究受益。
