Incoherent-scatter computed tomography with monochromaticsynchrotron x ray: feasibility of multi-CT imaging system forsimultaneous measurement-of fluorescent and incoherent scatter x rays

We describe a new system of incoherent scatter computed tomography (ISCT) using monochromatic synchrotron X rays, and we discuss its potential to be used in in vivo imaging for medical use. The system operates on the basis of computed tomography (CT) of the first generation. The reconstruction method for ISCT uses the least squares method with singular value decomposition. The research was carried out at the BLNE-5A bending magnet beam line of the Tristan Accumulation Ring in KEK, Japan. An acrylic cylindrical phantom of 20-mm diameter containing a cross-shaped channel was imaged. The channel was filled with a diluted iodine solution with a concentration of 200 μgI/ml. Spectra obtained with the system's high purity germanium (HPGe) detector separated the incoherent X-ray line from the other notable peaks, i.e., the iK_α and K_β1 X-ray fluorescent lines and the coherent scattering peak. CT images were reconstructed from projections generated by integrating the counts In the energy window centering around the incoherent scattering peak and whose width was approximately 2 keV. The reconstruction routine employed an X-ray attenuation correction algorithm. The resulting image showed more homogeneity than one without the attenuation correction     

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%.     

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     

Low-dose exposure energy-resolved X-ray computed tomography using a contrast agent with a high-energy K-edge

X-ray computed tomography (CT) with iodine contrast agent is widely employed to locate cancers. However, this method has shortcomings such as high-radiation dose exposure, iodine side effects, and a beam hardening effect. We have been working on the energy-resolved CT measurement method using a novel X-ray detection system, the “transXend” detector, which measures X-rays as electric currents and gives the energy distribution of incident X-rays after analysis. In the present study, we propose a method for low-dose exposure CT that involves the combination of the energy-resolved CT method, which is free from the beam hardening effect, and a harmless contrast agent with high-energy K-edge absorption, such as gold nanoparticles expected as a future contrast agent. Comparisons of radiation dose exposures as functions of aluminum filter thickness at the exit aperture of an X-ray tube and the K-edge energies of contrast agents are described.     

Positron emission tomography of large rock samples using amultiring PET instrument

The technique of positron emission tomography (PET) is well established in the field of medical imaging. Non-medical applications have also been shown to be valuable in the measurement of dynamic chemical processes, specifically in the determination of the characteristics of small rock samples. Since rock is more dense than tissue, the problems of attenuation and scatter are accentuated. However, we are able to show that measurement of porosity in a large rock sample (21.5 cm diameter) is indeed practicable using three-dimensional (3-D) acquisition techniques. Due to multiple scattering of the photons in the rock and the cylindrical symmetry of the experiment, we measured a scatter distribution which is approximately homogeneous, allowing a correction for scattered radiation with a simple method. The problems of determining attenuation coefficients and applying a scatter correction makes absolute quantisation difficult. However, relative changers in porosity within the sample can be measured with a spatial resolution not appreciably different from that in water     

Extraction algorithm of pulmonary fissures from thin-section CTimages based on linear feature detector method

Describes a new automatic extraction algorithm of the pulmonary major and minor fissures from three-dimensional (3-D) chest thin-section images of helical computed tomography (CT). These fissures are used for the analysis of pulmonary conformation and the diagnosis of lung cancer. This algorithm consists mainly of the correction and the emphasis of a 2-D linear shadow. The authors applied the proposed algorithm to 25 sets of CT examinations of 12 patients. The results showed that major and minor fissures can be extracted by the proposed algorithm, without reference to streak artifacts on axial CT images by the beam hardening effect, and the motion artifacts by the cardiac beat     

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.     

Comparison of three applications of ConTraSPECT

Today, the trend in SPECT attenuation correction is to acquire a transmission data set and reconstruct the corresponding attenuation map, for incorporating into the emission reconstruction. Recently, some techniques have been developed to determine the attenuation map directly from the emission data, by exploiting the fact that in the absence of noise only some specific transmission data can be consistent with given emission data. However it is necessary to include some a priori information so that the consistency conditions can be used in an efficient manner. Here, three types of priors are used: a uniform ellipse, a uniform spline curve, and a uniform spline curve knowing the untruncated part of the transmission sinogram. Once the corresponding attenuation map has been computed, it is used to correct for attenuation in the emission image reconstruction process. The three methods have been tested on both simulations and experimentally acquired data. The results show that the two first methods give equally acceptable results, while the third method provides reconstructions which are closer to reconstructions obtained when the attenuation map is computed from untruncated transmission data     

Evaluation of the effect of scatter correction on lesion detectionin hepatic SPECT imaging

The effect of scatter correction on the accuracy of lesion detection in single-photon emission computed tomography (SPECT) imaging requires analysis of observer performance. Experiments were designed to evaluate the class of correction methods that subtract counts. Simulations were used to approximate liver imaging with labeled antibodies. The lesion was a 2.5-cm-diameter, spherical, “cold” tumor. Ramp-filtered backprojection and noniterative Chang attenuation compensation were used to approximate clinical practice. Perfect scatter rejection, defined as images containing only primary (nonscattered) photons, was selected as the ideal case. These images were compared with uncorrected images for conditions of both low- and high-scatter fractions (SF). The dual photopeak window (DPW) method was also tested to evaluate a practical subtraction correction. Receiver operating characteristic (ROC) experiments were conducted under signal-known-exactly (SKE) conditions, using the area under the curve as the index of accuracy. A statistically significant difference in detection was found only in a few experiments when scatter rejection was compared with no correction. The results suggest that scatter correction does not necessarily assure improved detection accuracy at a statistically significant level. However, corrections that produce conditions similar to ideal scatter rejection may offer such improvement in detection, particularly for cases of high SF's     

局部喷血分数的计算机定量法

平衡法放射性核素心血管造影局部室壁运动的定性分析有助于对室壁运动异常作出评价。但由于评价结果主要由观察者依据心室舒张末期及收缩末期这二个时相的边界叠加影象来判断,因而主观因素不可避免。特别是对冠心病例,其室壁运动“正常”与“运动低下”之间的鉴别尤为困难,对室间隔的活动也往往难于估价。为此,本文研究了局部室壁运动的计算 平衡法放射性核素心血管造影局部室壁运动的定性分析有助于对室壁运动异常作出评价。但由于评价结果主要由观察者依据心室舒张末期及收缩末期这二个时相的边界叠加影象来判断,因而主观因素不可避免。特别是对冠心病例,其室壁运动“正常”与“运动低下”之间的鉴别尤为困难,对室间隔的活动也往往难于估价。为此,本文研究了局部室壁运动的计算     

SPECT的衰减校正

综述了常见的ＳＰＥＣＴ衰减校正方法，包括前、后校正法，本征法和迭代法。简要介绍了各种方法的基本原理并对各种方法的特点作了比较分析。从校正后图像的噪声特性、失真度、方法的运算速度有适用于何种脏器的重建等方面作了有关阐述。有助于系统理解不同的衰减校正方法，也对ＳＰＥＣＴ临床应用选择适当的衰减校正方法有所帮助。     

工业CT散射校正方法的研究

在工业CT的扫描重建过程中，散射效应使得重建图像出现模糊，各点灰度值同实际物体的吸收系数之间出现偏差，需要进行校正。如果采用一般的卷积方法，则需要迭代多次才能收敛，所以在校正时间上不能达到要求。采用一种结合散射模型的Ordered Subsets Convex方法对散射效果进行校正，在校正速度和质量上都得到了很好的结果。     

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     

CT image correction for beam hardening using simulated projectiondata

A general beam-hardening correction technique is presented. Knowledge of the CT (computerized tomography) scanner X-ray spectrum is necessary. This postreconstruction method does not require the original projection data. Simulated projections through an uncorrected reconstructed image are used to correct for beam hardening. Errors in the mean linear attenuation coefficient are decreased from 30% to 5% with virtual elimination of the visual streaking artifact. The theoretical image improvement equals that of projection linearization postreconstruction methods using the original projection data. The correction is limited to cases where the material causing the beam hardening is contained within the reconstruction space     

符合线路断层显像原理及其应用

符合线路断层显像主要由可变角双探头SPECT系统，符合探测线路和衰减校正等组成。对医用核仪器尤其是近年来在国内外刚开始广泛应用的双和／或三探头符合线路断层显像仪的工作原理和特点以及临床应用进行了综述。     

An Efficient Algorithm for Reconsiruction of SPECT Images in the Presence of Spatially Varying Attenuation

An algorithm is presented which permits the reconstruction of SPECT images in the presence of spatially varying attenuation. The algorithm considers the spatially variant attenuation as a perturbation of the constant attenuation case and computes a reconstructed image and a correction image to estimate the effects of this perturbation. The corrected image will be computed from these two images and is of comparable quality both visually and quantitatively to those simulated for zero or constant attenuation taken as standard reference images. In addition, the algorithm is time efficient, in that the time required is approximately 2.5 times that for a standard convolution-back projection algorithm.     

A PIXE micro-tomography experiment using MLEM algorithm

A new approach for PIXE tomography has recently been introduced by our group. The reconstruction program is based on the maximum likelihood expectation maximization (MLEM) algorithm. Since the algorithm is built upon the actual stochastic model of X-ray production and propagation, attenuation correction as well as ion energy loss are readily incorporated into the reconstruction. The resultant procedures have been previously tested by using simulated X-ray yields and the results were given in an earlier paper. In this work, we present the results of the tomographic reconstruction of a realistic sample done in order to study the effectiveness of the algorithm.     

基于基图像TV模型的CT射束硬化校正方法

X射线CT中,X射束硬化导致重建图像中出现伪影,严重影响了图像质量。论文深入分析了射束硬化对原始投影数据的影响,并提出了一种基于基图像TV模型的射束硬化校正方法。该方法首先依据射束硬化的物理特征建立了带有可调参数的初步校正模型;其次,在不同的可调参数条件下,原始投影数据经该模型预处理变换得到多组预处理投影序列;再次,分别对预处理投影序列进行重建得到一系列校正基图像;最后,将基图像的线性组合作为最终重建图像。其中,加权系数的最优解是以最终重建图像的全变分(Total Variation,TV)函数作为目标函数,通过迭代法求得。为验证该算法,对真实的铝柱体模和工业检测件进行了实验,结果表明,该算法对射束硬化引起的杯状和条状伪影均有显著的抑制效果。     

Iterative restoration of SPECT projection images

Photon attenuation and the limited nonstationary spatial resolution of the detector can reduce both qualitative and quantitative image quality in single photon emission computed tomography (SPECT). In this paper, a reconstruction approach is described which can compensate for both of these degradations. The approach involves processing the projection data with Bellini's method for attenuation compensation followed by an iterative deconvolution technique which uses the frequency distance principle (FDP) to model the distance-dependent camera blur. Modeling of the camera blur with the FDP allows an efficient implementation using fast Fourier transform (FFT) methods. After processing of the projection data, reconstruction is performed using filtered backprojection. Simulation studies using two different brain phantoms show that this approach gives reconstructions with a favorable bias versus noise tradeoff, provides no visually undesirable noise artifacts, and requires a low computational load