锥束CT图像矩形包围盒快速重建

针对传统锥束CT重建存在大量冗余计算的问题,建立了锥束CT图像矩形包围盒快速重建方法。该方法将重建空间限定为三维矩形包围盒,大大减少了待重建体素的数量,接着将Z线优先算法反投影部分的计算移植到GPU上加速重建。实验结果表明:该方法显著提高了锥束CT图像的重建速度,可推广应用于采用类似算法重建的CT系统。     

基于硬件实现的锥束CT图像重建系统的存储机制设计

为了实现三维锥束CT图像重建加速系统的小型化,建立了基于FPGA的三维图像重建系统。并对该系统中所采用的FDK重建算法所需的数据存储量和数据传输量以及由SDRAM、SRAM和FPGA内部缓存组成的存储系统的数据吞吐率进行了研究。首先,根据FDK算法的滤波与反投影两个步骤介绍了三维锥束CT图像重建系统的数据规模。接着,介绍了一种以SDRAM为外部主存,以SRAM为外部缓存和以FPGA内部SRAM资源作为内部高速缓存的存储机制。然后,介绍了该存储机制的实现方法以及测试方法。最后对该三维图像重建系统的数据吞吐能力进行了测试,并将之与FDK算法所需的数据传输量进行了对比分析。试验结果表明:该存储机制的数据连续存取速度为151.9MB/s,数据随机存取速度为100MB/s,基本满足小规模的三维图像重建的数据存储与传输带宽的要求。     

锥束CT重建FDK算法的两级并行计算研究

三维锥束CT图像的FDK算法重建由于运算量大,在重建高分辨率的图像时,重建所需时间通常难以满足实际应用的需求,集群并行计算是解决上述问题的常用方法之一.在一个SMP集群系统上,采用MPI和Pthreads两种模型相结合的方法,通过节点之间的消息传递和节点内部的共享内存,实现了FDK算法的两级并行.     

三维锥束精确重建的完全条件和源点轨迹

本文从理论上分析推导了３Ｄ锥束精确重建的完全条件，并结合一些具体的源点轨迹进行了讨论。     

基于FDK算法的锥束CT重建近似算法性能比较

相比通常的扇束和平行束CT重建,锥束CT有诸多优点(如空间分辨率高,扫描速度快等)。近似锥束重建算法理论结构简单,重建速度快,易于实际应用。对新出现的几种近似锥束重建算法作了简单介绍,并利用这些算法对3D Shepp-logan模型的仿真模拟对这些算法的特点作了比较和讨论。     

中间函数法CT图像重建算法研究

CT自从被发明以来其技术已发生了很大的变化.这些变化不仅体现在计算机技术、探测器技术和X线技术方面,同时CT的图像重建算法也在不断地发展.本文提出的中间函数重建算法,是一种可以应用于扇束和锥束扫描的图像重建算法.研究它的目的是为将来的锥束扫描提供直接重建算法.     

锥束CT的改进FDK算法及加速实现

锥束CT成像中,FDK重建算法得到广泛应用。提出一种可以扩大锥束CT旋转轴方向重建体积的FDK方法。在反投影步骤中,对重建物体的每个体素点被反投影的总次数进行计算,之后进行权重计算,最终得到三维重建图像。同时,采用CUDA技术,利用GPU对改进的FDK算法进行并行计算加速实现,提高重建效率。实验结果表明,改进的FDK算法可以增大重建体积,重建更多物体信息,采用CUDA技术后,速度大幅度提升。     

锥束CT扫描方式对ART算法图像重建的影响研究

平板探测器锥束CT的扫描方式对ART(Algebraic Reconstruction Techniques)算法图像重建会产生影响.本文用128×128×128三维Shepp-Logan头部模型作为仿真对象,探讨了不同源-扫描中心距离、投影数、缺角度下扫描方式对图像重建的影响.结果表明,源-扫描中心距离与扫描中心-探...     

一种CT成像散射抑制方法

在X射线CT(Computed Tomography)成像尤其是锥束成像过程中,散射是不可忽略的影响因素。探测信号中散射信号的存在导致CT图像中的伪影。依据散射信号的特征属性,提出了一种散射的估计方法。该方法利用简单的解析公式估计散射,简单易操作。实际的实验数据验证了方法的有效性。     

均匀介质中γ成像三维图像重建

将γ相机的小孔成像模型类比成平行束模型,在代数重建法(ART)的基础上,加入衰减补偿,实现了均匀介质γ成像的三维图像重建。对比加入衰减补偿和未加入衰减补偿所重建放射源的位置和强度分布,得出介质对γ相机成像三维图像重建的影响:屏蔽介质只会影响重建后放射源的强度分布,不会影响放射源的位置分布。通过MC模拟数据的重建结果表明,本方案是切实可行的。     

Apparent Linear Attenuation Coefficients in Phase Contrast X-Ray Tomography

In the inline phase contrast X-ray tomography the reconstructed apparent linear attenuation coefficient values may be greatly larger than sample’s linear attenuation coefficients or even be negative. In this work we present a general formula to quantitatively relate the apparent linear attenuation coefficient values in cone-beam phase contrast tomography to sample’s linear attenuation coefficients and refractive indices. This formula overcomes the gross inaccuracy of the existing formula in the literature in analyzing high-resolution phase contrast tomography, and it will be useful for correctly interpreting and quantifying the apparent linear attenuation coefficients in cone-beam X-ray phase contrast tomography.     

Investigation of optimal scanning protocol for X-ray computed tomography polymer gel dosimetry

X-ray computed tomography is one of the potential tool used to evaluate the polymer gel dosimeters in three dimensions. The purpose of this study is to investigate the factors which affect the image noise for X-ray CT polymer gel dosimetry. A cylindrical water filled phantom was imaged with single slice Siemens Somatom Emotion CT scanner. The imaging parameters like tube voltage, tube current, slice scan time, slice thickness and reconstruction algorithm were varied independently to study the dependence of noise on each other. Reductions of noise with number of images to be averaged and spatial uniformity of the image were also investigated. Normoxic polymer gel PAGAT was manufactured and irradiated using Siemens Primus linear accelerator. The radiation induced change in CT number was evaluated using X-ray CT scanner. From this study it is clear that image noise is reduced with increase in tube voltage, tube current, slice scan time, slice thickness and also reduced with increasing the number of images averaged. However to reduce the tube load and total scan time, it was concluded that tube voltage of 130 kV, tube current of 200 mA, scan time of 1.5 s, slice thickness of 3 mm for high dose gradient and 5 mm for low dose gradient were optimal scanning protocols for this scanner. Optimum number of images to be averaged was concluded to be 25 for X-ray CT polymer gel dosimetry. Choice of reconstruction algorithm was also critical. From the study it is also clear that CT number increase with imaging tube voltage and shows the energy dependency of polymer gel dosimeter. Hence for evaluation of polymer gel dosimeters with X-ray CT scanner needs the optimization of scanning protocols to reduce the image noise.     

A rotational calibration method using thermoluminescent dosemeters for dose determination in computed tomography beams

A procedure for calibration of thermoluminescent dosimeters, called rotational calibration, was developed to create a procedure more adequate for dose procedures used in computed tomography. Thermoluminescent dosimeters were rotated during irradiation to simulate the set-up used in dosimetric procedures in computed tomography equipment. Three commercial types of thermoluminescent dosimeters were calibrated using this methodology. The results showed that the measured values were lower when the dosimeters were irradiated with rotation than in a static geometry. Although the reading differences were not very large, they are not negligible, and they contribute to underestimating the dose.     

Determination of within-rod caesium and europium isotopic distributions in high burnup fuel rods through computerised gamma-ray emission tomography

Computerised gamma-ray emission tomography has been applied to single PWR UO₂ fuel rods, with pellet averaged burnups of 52, 71, 91 and 126 GWd/t respectively, for the determination of ¹³⁴Cs, ¹³⁷Cs and ¹⁵⁴Eu internal radial distributions. State-of-the-art image reconstruction techniques, analytical and iterative, have been applied, evaluated and compared using test phantoms first and, in a second step, the actual measured data. Further, linear attenuation maps, previously derived on the same samples by means of gamma-ray transmission tomography, have been used to correct for density inhomogeneities. The final results have indicated large central depressions in the caesium distributions, but of varying extent from sample to sample. Particularly interesting is the case of the 126 GWd/t sample, showing a very deep central depression (periphery-to-centre ratios of ∼2.5 for ¹³⁷Cs and ∼3 for ¹³⁴Cs). In addition, a difference in the relative activity distributions of ¹³⁷Cs and ¹³⁴Cs has been observed for all the samples. In contrast, the europium shows an almost flat distribution.     

Application of gamma ray computed tomography to non-destructive testing

A gamma ray computed tomography (ψ-ray CT) system has been developed for applications to large specimens of high density materials.The system features a wide fan beam of ψ-rays, extendable up to 20 degrees, 20 channel ψ-ray scintillation detectors using bismuth germanate oxide with a counting rate being up to 10⁵ per second for reducing the mechanical scan time. Performance of the ψ-ray CT system is confirmed by measuring a test piece and a stepping motor (diameter: 5 cm; main material component: iron), for instance. It can represent these cross-sectional images with a spatial resolution of 1.5 mm. About 10 min are required to obtain these images using a 100 mCi ¹³⁷Cs source.     

X射线数字分层成像的非线性方法研究

对薄层结构的复合材料及多层电路板进行密度分布情况及分层成像的研究，是无损检测技术的一个重要应用领域，通过实验可观察到，在接近焦平面时图像的灰度值有一定的非线性，提出了一种新的成像方法，可以提高灵敏度，非线性的层面合成可以仅仅从很少的投影中重建物体，使测量进间大大减少。     

Airborne Scan-Converter System

A method of converting very high bandwidth, single-transient information in a rocket-borne experiment to a much lower bandwidth is described. Application of the scan conversion technique to single-transient information of nanosecond rise time permits conversion to bandwidths commensurate with present day telemetry systems. Signal-storage rise time capability is 1 to 10 ns depending on the type of storage tube used. Of special interest is the unique transistorized linear sweep. Its delay between trigger and start of the sweep is less than 6 ns and the sweep voltage is plus and minus 400 volts. Other transistorized circuitry, such as readout sweep, time base generator, and signal amplifier, is also discussed.     

Line-Scanning Proportional Counter Camera

A new method was developed for imaging spatial distributions of photons, charged particles, or neutrons over large areas by a line-scanning camera. This camera uses a linear position-sensitive proportional counter (PSPC) based on resistance-capacitance (RC) position encoding. 1-6 Applications of this camera include low-dose, medium-resolution radiography of large objects (>2 by 2 m); and, in nuclear medicine, low-dose, whole-body radionuclide imaging and radiography with low energy photons (<150keV). A prototypic camera was built and tested to scan an area of 60 by 100 cm. The spatial resolution is 1 by 1 mm fwhm for radiography with 60-keV photons and 3 by 3 mm fwhm for radionuclide imaging with 50-keV photons (limited by collimator resolution). Compared to point-by-point scanning, the line-scanning method reduces the mechanical complexity of the camera and eliminates problems encountered in construction of large-area, high pressure PSPCs. This method is superior to the area PSPC for imaging objects >60 by 60cm.     

Tomography of nuclear fuel experiments with an electronic line scan camera

A line scan camera has been developed, which may be applied for tomography in the non-destructive examination of nuclear fuel experiments. The camera has been tested with different radiation sources like 400 kV X-ray tubes, an 18 mV betatron and a thermal neutron beam. The camera uses silicon photo-diode-arrays with an integrated shielding suppressing most of the radiation coming from elsewhere than from the source. Tomograms of preirradiated fuel pins were generated showing the fuel debris distribution after an over-power test. Various phantoms have been tomographed, which demonstrate the feasibility of imaging cross sections of fuel bundles nondestructively. The system is able to measure density distributions in a 0.3 mm thick cross section with about 0.3 mm spatial resolution. The camera itself is small and the control computer of standard industry type. Thus the tomography system is transportable and may be integrated in existing nondestructive facilities.     

Calculation of the rotational centers in computed tomographysinograms

An efficient method for accurately calculating the center-of-rotation, or projection center, for parallel computed tomography projection data, or sinograms, is described. This method uses all the data in the sinogram to estimate the center by a least-squares technique and requires no previous calibration scan. The method also finds the object's center-of-mass without reconstructing its image. Since the method uses the measured data, it is sensitive to noise in the measurements, but that sensitivity is relatively small compared to other techniques. Examples of its use on simulated and actual data are included. For fan-beam data over 360°, two related methods are described to find the center in the presence or absence of a midline offset