Filter-based energy-resolved X-ray computed tomography with a clinical imager

Filter-based energy-resolved X-ray computed tomography (CT) is an approach for implementing energy-resolved CT imaging using a flat-panel-detector-based cone-beam system. In this study, we performed experiments with a 20-cm-diameter phantom on a clinical X-ray imager. The material density results showed good agreement with the ideal values. We also propose an improved method for obtaining the detector response function and the X-ray spectrum, which requires fewer measurements and will be practical for future clinical use. Issues such as scatter and image noise remain to be addressed.     

Response function estimation with fine energy bins for the energy-resolved computed tomography using a transXend detector

A new ‘transXend’ detector system has been developed for energy-resolved computed tomography (CT). It consists of several segmented detectors that measure X-rays as electric currents. Response functions of segmented detectors are estimated using component materials of a body under inspection to unfold X-ray spectra. To avoid material-dependent measurements, response functions inherent to segmented detectors are evaluated by Monte Carlo calculations. CT was performed for a phantom consisting of five resins and was analyzed by the estimated response functions. The linear attenuation coefficients for the five resins have excellent agreement with database values.     

Iterative reconstruction with attenuation compensation fromcone-beam projections acquired via nonplanar orbits

Single photon emission computed tomography (SPECT) imaging with cone-beam collimators provides improved sensitivity and spatial resolution for imaging small objects with large field-of-view detectors. It is known that Tuy's cone-beam data sufficiency condition must be met to obtain artifact-free reconstructions. Even though Tuy's condition was derived for an attenuation-free situation, the authors hypothesize that an artifact-free reconstruction can be obtained even if the cone-beam data are attenuated, provided the imaging orbit satisfies Tuy's condition and the exact attenuation map is known. In the authors' studies, emission data are acquired using nonplanar circle-and-line orbits to acquire cone-beam data for tomographic reconstructions. An extended iterative ML-EM (maximum likelihood-expectation maximization) reconstruction algorithm is derived and used to reconstruct projection data with either a pre-acquired or assumed attenuation map. Quantitative accuracy of the attenuation corrected emission reconstruction is significantly improved     

A method to determine the gamma-ray linear attenuation coefficient

In this work, gamma-ray linear attenuation coefficients of the materials such as Pb, Fe, Cu and Al were determined by means of traditional energy method and the developed timing detection technique. 1.33 MeV-energy radiation emitted from ⁶⁰Co radioisotope was used in the experiments. The gamma-ray linear attenuation coefficient values of the materials were calculated by using Xcom code, and the experimental results were compared with the theoretical ones.     

蒙特卡罗模拟确定γ射线衰减系数函数及参数

在中低密度样品中,γ射线的线衰减系数主要由γ射线能量和样品密度决定,采用MCNP(Monte Carlo N Particle Transport Code)程序模拟计算了多种γ射线能量和多种样品密度条件下的线衰减系数,对线衰减系数模拟值进行多元非线性回归,确定了以γ射线能量和样品密度为因变量的线衰减系数函数及参数。实验测定了三种能量γ射线在6种不同密度样品中的线衰减系数值,并与模拟所得函数值进行比较分析。结果表明,所得函数值与实验值的相对误差均在7%以内,蒙特卡罗程序计算所得函数值与实验测量值较为吻合,所采用的函数模型准确验证了线衰减系数与γ射线能量、样品密度之间的关系特征。     

Practical use of the modified Bronnikov algorithm in micro-CT

Through the years, the resolution of X-ray Computed Tomography (CT) systems has increased rapidly, in particular for the newer micro- and nano-CT systems. With this increasing resolution, the limits of absorption contrast CT are being reached. At the same time, a new type of contrast becomes visible: phase contrast. Mainly for low-absorbing objects such as insects and wood, phase contrast can lead to a new type of CT reconstruction using the modified Bronnikov algorithm (MBA) [A. Groso, R. Abela, M. Stampanoni, Implementation of a fast method for high 297 resolution phase contrast tomography, Opt. Express 14 (18) (2006) 8103.] Despite it’s theoretical limitation to pure phase objects, the algorithm has some clear advantages with respect to filtered back-projection (FBP). The MBA is therefore commonly used at the Centre for X-ray Tomography of the Ghent University (UGCT) to obtain additional information for optimal scanning results.     

Energy-dependent systematic errors in dual-energy X-ray CT

Dual-energy X-ray computed tomography (DECT) is a technique which is designed to allow the determination of energy-independent material properties. In this study, results of a computer simulation show that energy-dependent systematic errors exist in the values of attenuation coefficients synthesized using the basis material decomposition technique with acrylic and aluminum as the basis materials, especially when a high atomic number element such as iodine (e.g., from radiographic contrast media) is present in the body. The errors are reduced when an acrylic and an iodine-water mixture are used as the basis materials. The authors propose a simple theoretical model for the calculation of energy-dependent systematic errors using effective energies at the lower and higher energy windows of the X-ray spectrum used in the DECT system. The calculated errors agree well with the errors observed in the simulation. These results suggest that the observed systematic errors are predominantly due to the energy dependence of the basis material coefficients     

Measurements of linear attenuation coefficients of irregular shaped samples by two media method

The linear attenuation coefficient values of regular and irregular shaped flyash materials have been measured without knowing the thickness of a sample using a new technique namely “two media method”. These values have also been measured with a standard gamma ray transmission method and obtained theoretically with winXCOM computer code. From the comparison it is reported that the two media method has given accurate results of attenuation coefficients of flyash materials.     

Attenuation correction of polychromatic L-shell X-ray fluorescence computed tomography imaging

Polychromatic L-shell X-ray fluorescence computed tomography is a promising imaging technique to explore the element distribution of the sample. However, the image quality of large samples would be decreased dramatically due to the present of the self-absorption. In this investigation, an attenuation correction algorithm based on theories related to X-ray fluorescence was proposed. With this method, the attenuation coefficients at the incident energies were expressed as a function of known X-ray energies and unknown platinum concentrations. Then the attenuation coefficients were calculated based on the theories and added in the contribution value of the pixel in Maximum Likelihood Expectation Maximization reconstruction method. The element distributions can be got through continuous iterations. Finally, the feasibility of this method was tested by Monte Carlo simulation. The results indicated that, for a 4-mm diameter phantom containing an object of 2 mm in diameter with 0.10 wt% platinum solutions, the CNR of reconstructed images increased from 53.5 to 203.8 after attenuation correction while the relative error decreases from 97% to 2%.     

非准直点源透射法测量体源样品的线性衰减系数

体源样品的线性衰减系数是计算自吸收修正因子最重要的参数。本文针对未知体源样品的线性衰减系数测量问题,提出了非准直点源透射法。该方法直接把点源置于样品盒上方一定距离或直接放置在样品盒上方进行透射,对蒙特卡罗模拟透射率和实验测量透射率进行比较,反推待测体源样品的线性衰减系数。本文详细介绍了该方法的基本原理和计算过程,选取了5种不同密度的物质作为待测对象,采用3种不同能量的点源,对该方法进行了实验验证。研究结果表明,不同能量的γ射线下,该方法的实验结果和XCOM平台理论计算结果吻合程度好（＜8%）,能方便计算出体源样品的线性衰减系数。该方法不受准直器以及探测器和样品尺寸、形状的限制,简单易行,便于各实验室推广使用。     

Determination of radiation attenuation coefficients of heavyweight- and normal-weight concretes containing colemanite and barite for 0.663 MeV γ-rays

Accurate measurements have been made to determine radiation transmission of concretes produced with barite, colemanite and normal aggregate by using beam transmission method for 0.663 MeV γ-rays energy of ¹³⁷Cs radioactive isotopes by using NaI(Tl) scintillation detector. Linear and mass attenuation coefficients of thirteen heavy- and four normal-weight concretes were calculated. It was determined that the linear attenuation coefficient (μ, cm⁻¹) decreased with colemanite concentration and increased with barite concentration in both type of the concretes. Mass attenuation coefficient values of our concretes were compared with the values proposed by the United States National Institute of Standards and Technology (NIST).     

用多个能量探测方法校正层析γ扫描透射图像重建中射线线衰减系数

对采用多个能量探测方法来校正层析γ扫描透射图像重建中射线线衰减系数进行了初步探讨.应用蒙特卡洛方法计算了18种材料在不同能量下的γ射线线衰减系数,并拟合得出当物质材料和密度确定时γ射线的线衰减系数与能量的关系,从而重建出不同能量下的透射图像.     

Influence of resonant Raman scattering in the elemental analysis using X-ray emission based techniques

A tabulation of characteristic X-ray energies across the periodic table are provided where those X-rays are expected to result in a significant fractional resonant Raman scattering (RRS) contribution to the X-ray attenuation from a particular shell/subshell of the same or another element. The tabulations can be considered as guideline so as to know what can be expected due to RRS in typical photon- and particle-induced X-ray emission spectrometry. The RRS contribution is not included in the available theoretical attenuation coefficients, which are generally used in estimation of the matrix corrections in routine quantitative elemental analysis based on various X-ray emission techniques. The radiative RRS peaks can also interfere with normal X-ray spectrum and influence the elemental analysis. The RRS cross-section depends upon the energy difference of the X-ray energy and the shell/subshell ionization threshold taken in the units of the shell/subshell energy width, density of available states near the Fermi level, and the band structure in case the element is in the solid form. Some aspects of the dependence of the RRS contribution on the chemical forms of the elements are also discussed.     

Evaluation of tissue-equivalent materials to be used as human brain tissue substitute in dosimetry for diagnostic radiology

Tissue-equivalent materials to be used as substitutes for human brain tissue in dosimetry for diagnostic radiology have been investigated in terms of calculated total mass attenuation coefficient (μ/ρ), calculated mass energy-absorption coefficient (μ_en/ρ) and absorbed dose. Measured linear attenuation coefficients (μ) have been used for benchmarking the calculated total mass attenuation coefficient (μ/ρ). The materials examined were bolus, nylon®, orange articulation wax, red articulation wax, PMMA (polymethylmethacrylate), bees wax, paraffin I, paraffin II, pitch and water. The results show that water is the best substitute for brain among the materials investigated. The average percentage differences between the calculated μ/ρ and μ_en/ρ coefficients for water and those for brain were 1.0% and 2.5%, respectively. Absorbed doses determined by Monte Carlo methods confirm water as being the best brain substitute to be used in dosimetry for diagnostic radiology, showing maximum difference of 0.01%. Additionally this study showed that PMMA, a material often used for the manufacturing of head phantoms for computed tomography, cannot be considered to be a suitable substitute for human brain tissue in dosimetry.     

波长色散X射线荧光光谱法快速测定铂铑合金中铂和铑的含量

采用波长色散X射线荧光(X-ray fluorescence,XRF)光谱法对铂铑合金中Pt和Rh的含量进行测定,建立了一种Pt和Rh的分析测量方法。实验设定波长色散XRF光谱仪激发电压为60 k V、电流为50 m A,采用100mm的黄铜滤光片、间距300mm的准直器、PX10分光晶体和闪烁探测器对Pt的La线和Rh的Ka线进行分析测量;为消除样品杯罩产生的干扰使用直径27 mm的准直器面罩。应用经验系数法对基体效应进行了校正,建立了Pt和Rh的校正曲线,其K值和RMS值(均方根偏差)较小,线性相关性较好。分析结果表明,Pt和Rh的相对误差分别小于0.09%和0.54%,相对标准偏差分别为0.11%和0.17%(n=10),检出限分别为208mg·g~(-1)和37mg·g~(-1),该方法能准确可靠地测定铂铑合金中Pt和Rh的含量,单样分析时间仅需74 s。将该方法应用到铂铑合金配制生产过程中,Pt和Rh的测定结果与配方值的相对误差分别小于0.06%和0.20%。在铂铑合金稀释熔炼中,对稀释后样品的Rh含量进行测量,其测量结果与计算值的相对误差低于0.29%。该方法能够满足大批量铂铑合金样品的分析需求,为生产实践提供准确可靠的科学数据。     

X射线工业CT散射修正

X射线工业CT（ICT）依赖准直器压低散射光子存在实际困难和不足。为解决这一问题，需采用相应的修正软件加以校正。文章从理想的模型出发，近似推导透射式ICT中散射X光子的理论计算公式，并把计算结果与实验结果进行比较。比较结果表明，两者吻合较好。据此，文章给出了散射光子软件修正的可行性方案。      

Approximate reconstruction of attenuation map in SPECT imaging

The influence of linear attenuation presents certain difficulties for quantitative single-photon emission computed tomography (SPECT) in medical imaging. To apply an attenuation correction algorithm, one has first to estimate the attenuation map from some additional measurements or by the use of SPECT data. The determination of the attenuation map from SPECT-type data may also be considered as a special problem of nondestructive evaluation of materials. A new technique for obtaining the attenuation map from SPECT data is proposed. Unlike other methods, the approach suggested demands no a priori knowledge of the problem. The method has been derived on the basis of the approximate linear relation between SPECT data and the attenuation map. The approach permits local changes of the attenuation coefficient to be reconstructed rather than its actual value. In this way, such details of the attenuation map as spots, curves and edges can be localized. Limitations of application of the suggested technique to real data are considered. Results of numerical evaluation and real data processing are presented     

巨大长型物体的双直线扫描断层成像

本文从实际应用背景出发，对直线源轨迹的扇束重建公式进行了简单推导，给出了相应的计算机模拟结果，同时根据实际情况提出了双直线轨迹的图像重建公式，最后将此算法推广到三维锥束图像重建，克服了传统三维图像重建中在锥角处误差很大的缺点。     

Comparison between beam-stop and beam-hole array scatter correction techniques for industrial X-ray cone-beam CT

In industrial X-ray cone-beam computed tomography, the inspection of large-scale samples is important because of increasing demands on their quality and long-term mechanical resilience. Large-scale samples, for example made of aluminum or iron, are strongly scattering X-rays. Scattered radiation leads to artifacts such as cupping, streaks, and a reduction in contrast in the reconstructed CT-volume. We propose a scatter correction method based on sampling primary signals by employing a beam-hole array (BHA). In this indirect method, a scatter estimate is calculated by subtraction of the sampled primary signal from the total signal, the latter taken from an image where the BHA is absent. This technique is considered complementary to the better known beam-stop array (BSA) method. The two scatter estimation methods are compared here with respect to geometric effects, scatter-to-total ratio and practicability. Scatter estimation with the BHA method yields more accurate scatter estimates in off-centered regions, and a lower scatter-to-total ratio in critical image regions where the primary signal is very low. Scatter correction with the proposed BHA method is then applied to a ceramic specimen from power generation technologies. In the reconstructed CT volume, cupping almost completely vanishes and contrast is enhanced significantly.     

Development of precise energy spectrum measurement technique for high-power pulsed X-ray sources for industrial use

It is difficult to obtain an accurate energy spectrum by measurement for X-rays which are produced by linear accelerators because so many photons are emitted within a very short period and the photons have energies in a wide range. In order to solve the problem, we developed a new technique utilizing the Bayesian estimation method combined with attenuation curve measurement using a step-shaped attenuation material. We experimentally confirmed the validity of the presented technique for an X-ray computed tomography (CT) system. By using the accurate energy information of emitted X-rays obtained by the developed technique, the quality of non-destructive inspection images can be expected to be improved for industrial X-ray radiography, X-ray CT and so on, using high-power X-ray sources.