新的超分辨CT扫描模式和图像重建方法

针对通常工业CT转台特点,提出了一种新的基于扇束等距采样的超分辨CT扫描模式和相应的重建算法。分别向左和右偏移CT旋转中心1/4探测器宽度,得到的两组相叠采样的投影数据。通过求解一组具有双对角矩阵的线性方程组,得到高分辨的CT虚拟投影数据,进而由扇束滤波反投影算法重建出高分辨的CT图像。数值模拟结果验证了本文方法的有效性。     

高能数据缺失下的双能CT重建算法实验研究

随着全球恐怖主义事件数量的逐年递增,公共安全问题越来越受到各国的重视,对安检系统的需求也越来越高。具有高精度物质识别特性的双能CT检测系统一直被认为是防范恐怖事件发生的最有效工具之一。但目前最典型的伪双能CT成像系统是由高低能量不同的两层全采样探测器单元组成。大量探测器单元的使用大大提升了系统成本,阻碍了大规模商用。2013年,我们提出了一种基于压缩感知理论的双能CT不完备数据重建算法,该算法仅利用少数高能探测器缺失数据即可实现图像质量相当的重建过程,模拟实验初步验证了算法的有效性。本文进一步使用在双能CT物品机上采集的真实数据验证算法的有效性。实验结果表明,在确保双能CT重建图像质量的前提下,使用基于压缩感知理论的双能CT不完备数据重建算法可以将双能CT探测器成本降低25％。     

首都师范大学检测成像实验室CT研究进展

CT检测成像技术包括数据采集、图像重建、图像应用三个层面。本文简要介绍了近年来首都师范大学检测成像实验室在这三个层面技术研究方面所取得的进展。具体包括多种新的扫描模式和相应重建算法、X射线能量谱的研究、基于GPU的多种解析重建算法和迭代重建算法的加速实现方法、多种伪影形成机理的研究及有效的伪影校正方法、基于GPU实现的体绘制算法、基于这些研究成果的算法平台和软件研发以及成果推广应用情况等。本文目的在于与本领域专家交流相关问题的国内外研究状况及本实验室的研究进展,促进研究的进一步深入。     

FDK算法中一种新的插值方法

针对在FDK算法的反投影过程中，各个体素在探测器上投影分布的特点，本文提出一种新的插值方法。该方法根据体素投影的特点，采用在重建过程中，根据其在不同扫描角度下在各个探测器单元上的投影所占面积的加权和作为反投影值。实际实验结果表明，在FDK算法中这个新的插值方法比传统的插值方法（如：最近邻插值，双线性插值）重建出来的图像边缘清晰，而且能更好地抑制噪声。     

基于光线投射的锥束CT正投影算法研究

计算机断层成像（computed tomography,CT）作为一种现代成像技术已经被广泛应用于医学诊断、工业无损检测等领域。在研究CT重建算法的过程中,正投影模拟数据是必不可少的一部分。本文提出一种基于光线投射算法的圆轨迹锥束扫描模式下的正投影算法,并结合CUDA（compute unified device architecture,统一计算设备架构）技术,实现了GPU（graphics processing unit,图形处理器）加速正投影计算。通过与传统的正投影算法相比,本文算法在投影图像质量上有一定的提高,并具有更高的计算效率。     

锥束三维CT快速连续旋转扫描控制时序优化

快速连续旋转扫描是提高锥束cT扫描速度、降低辐射剂量的有效手段。针对该扫描模式下转台加速减速运动以及扫描控制终端对上位机控制指令响应滞后造成的投影数据冗余问题,提出了优化的运动控制时序,以保证三维重建所需的360°范围内的有效投影数据处于电机匀速旋转状态。同时,以结构相似度系数为控制参数,实现对360°投影位置的准确定位。实验证明,该方法能有效去除快速连续旋转扫描模式造成的冗余投影数据,具有实现简单、精度高、重复性好的优点。     

基于光子计数器能谱响应优化的能谱CT造影剂K边缘分解方法

受到光子计数探测器(PCD)电荷共享、光子逃逸以及散射等效应的影响，在K边缘附近的入射能谱在探测器下会产生失真，降低了能谱CT的重建和材料分解性能。为此，提出了基于能谱恢复优化的投影域K-edge材料分解算法。该方法由两个步骤组成：首先，对探测器的能量响应函数(Energy Response Function, ERF)进行建模，通过MLEM方法恢复入射能谱；然后，基于恢复的投影数据进行能谱CT的K边缘重建和材料分解，得到造影像和组织像的正确分布。碘造影剂的仿真实验表明，能谱恢复较好地实现了造影剂与组织的图像分离，可以有效地提高K边缘重建图像中碘造影剂信号，提升能谱CT的造影剂分辨能力。     

螺旋CT靶扫描诊断孤立性肺结节

目的 探讨靶扫描在孤立性肺结节诊断中的价值.方法 常规扫描采用层厚10mm,扫描视野36 cm左右,标准算法重建,靶扫描采用小扫描视野（16～20 cm）,层厚2～5 mm,P=1-2,重建时重叠50%,用标准算法及骨算法两种重建.对同时具有两种扫描方法并经手术切除,经皮肺穿刺活检病理证实及经抗感染治疗后复查的60例病例进行回顾性分析,比较常规扫描与靶扫描在显示孤立性肺结节征象与诊断准确性上的差异.结果 60例靶扫描显示小结节征、空泡征、钙化、增强后高密度点条征、棘突征、毛刺征、分叶征、胸膜凹陷征、模糊绒毛征,高于常规扫描.靶扫描定性诊断准确率为93.3%,也高于常规扫描的81.7%.结论 靶扫描能提高孤立性肺结节征象的显示率,有助于诊断及鉴别诊断,是肺部结节有效的检查方法.     

ASIR-V重建算法在胸部低剂量CT诊断肺结节中的临床价值

目的:探讨胸部低剂量CT结合基于多模型的迭代重建算法（ASIR-V）在肺结节诊断中的临床价值。方法:收集行Revolution CT胸部平扫的肺结节患者40例。患者首次检查时先采用常规剂量胸部CT扫描,预设噪声指数为14 HU,图像重建采用滤波反投影法（FBP）;若发现肺结节者,于肺结节局部层面进行低剂量靶扫描,预设噪声指数为24 HU,图像采用60%ASIR-V算法进行重建。记录两种扫描模式下肺结节的检出数,测量肺结节的大小、CT值、噪声值以及肺组织CT值和噪声值,并计算肺结节的信噪比（SNR）和对比噪声比（CNR）。两名有经验的放射科医师采用5分法对两种扫描模式下整体图像质量以及部分肺结节的特殊形态学征象进行主观评分。记录两种扫描模式中患者的CT容积剂量指数(CTDIvol)、剂量长度乘积(DLP),并计算有效剂量(ED)。结果:常规剂量扫描ED为(3.20±1.14) mSv,低剂量扫描ED为(1.64±0.29) mSv,辐射剂量减低约48.8%。常规剂量扫描共检出108个肺结节,低剂量扫描共检出107个肺结节,低剂量CT扫描肺结节检出率为99.07%。低剂量60%ASIR-V图像和常规剂量FBP图像在肺结节的大小、CT值、SD值、SNR及CNR方面差异均无统计学意义（P＞0.05）。两名医师对两组图像的主观评分差异亦无统计学意义（P＞0.05）。结论:在低剂量CT扫描中,运用60% ASIR-V算法对于肺结节的检出、肺结节形态学特征的显示与常规剂量FBP图像质量差异不大,但其辐射剂量明显降低。     

满三维图像重建的卷积反投影算法

本文用构造δ序列的方法,给出了一种n维Radon变换反演的卷积反投影算法,证明了该算法在图像的连续点收敛于原图像,当n=2时,就是广泛用于图像重建的卷积反投影算法,由此也给出了卷积反投影方法收敛性的另一种推导。当n=3时,是新的满三维重建的卷积反投影算法,由于三维反演公式具有局部性,该算法对研究三维局部重建有启示。并基于三维情形,利用模拟数据进行了数值仿真,说明该算法是一种有效的三维图像重建算法。     

直线轨迹扫描断层成像中的图像重建与恢复方法

基于直线轨迹扫描的X射线断层成像方式,具有成像速度快、无物体重叠问题且造价较低等优点,在快速行李物品安检、大物体检查等领域具有较大应用潜力。不完备数据(有限角度)的图像重建及恢复是该成像方式的关键难题。本文介绍了一种适用于直线轨迹扫描断层成像的图像重建及恢复方法:在直接解析重建的基础上,利用linogram技术和总变分(Total Variation)正则化准则,实现快速稳定的外插迭代以补偿缺失数据,从而在保持图像边缘的同时,提高了图像像素值准确性,减小了图像伪影。     

Flat-panel cone-beam computed tomography for image-guided radiation therapy

: Geometric uncertainties in the process of radiation planning and delivery constrain dose escalation and induce normal tissue complications. An imaging system has been developed to generate high-resolution, soft-tissue images of the patient at the time of treatment for the purpose of guiding therapy and reducing such uncertainties. The performance of the imaging system is evaluated and the application to image-guided radiation therapy is discussed.

: A kilovoltage imaging system capable of radiography, fluoroscopy, and cone-beam computed tomography (CT) has been integrated with a medical linear accelerator. Kilovoltage X-rays are generated by a conventional X-ray tube mounted on a retractable arm at 90° to the treatment source. A 41 × 41 cm² flat-panel X-ray detector is mounted opposite the kV tube. The entire imaging system operates under computer control, with a single application providing calibration, image acquisition, processing, and cone-beam CT reconstruction. Cone-beam CT imaging involves acquiring multiple kV radiographs as the gantry rotates through 360° of rotation. A filtered back-projection algorithm is employed to reconstruct the volumetric images. Geometric nonidealities in the rotation of the gantry system are measured and corrected during reconstruction. Qualitative evaluation of imaging performance is performed using an anthropomorphic head phantom and a coronal contrast phantom. The influence of geometric nonidealities is examined.

: Images of the head phantom were acquired and illustrate the submillimeter spatial resolution that is achieved with the cone-beam approach. High-resolution sagittal and coronal views demonstrate nearly isotropic spatial resolution. Flex corrections on the order of 0.2 cm were required to compensate gravity-induced flex in the support arms of the source and detector, as well as slight axial movements of the entire gantry structure. Images reconstructed without flex correction suffered from loss of detail, misregistration, and streak artifacts. Reconstructions of the contrast phantom demonstrate the soft-tissue imaging capability of the system. A contrast of 47 Hounsfield units was easily detected in a 0.1-cm-thick reconstruction for an imaging exposure of 1.2 R (in-air, in absence of phantom). The comparison with a conventional CT scan of the phantom further demonstrates the spatial resolution advantages of the cone-beam CT approach.

: A kV cone-beam CT imaging system based on a large-area, flat-panel detector has been successfully adapted to a medical linear accelerator. The system is capable of producing images of soft tissue with excellent spatial resolution at acceptable imaging doses. Integration of this technology with the medical accelerator will result in an ideal platform for high-precision, image-guided radiation therapy.     

CT旋转中心的精确确定方法

CT旋转中心（COR）的精确确定是图像准确重建的一个前提条件,COR的偏差会导致重建图像出现伪影,降低成像质量,本文对现有的COR确定方法进行了分析研究,在此基础上,利用中心投影具有π分割数据一致性的特点,提出了一种利用投影原始数据精确确定CT投影中心的方法。本方法无需专用校正模体,不需知道CT测量的几何参数,不需要全周扫描数据,对于射线源与旋转中心的连线不垂直于探测器的情况同样适用。实验结果证明,本方法适用范围广,抗随机噪声能力强,具有很好的应用前景。     

3D dose reconstruction for narrow beams using ion chamber array measurements

3D dose reconstruction is a verification of the delivered absorbed dose. Our aim was to describe and evaluate a 3D dose reconstruction method applied to phantoms in the context of narrow beams. A solid water phantom and a phantom containing a bone-equivalent material were irradiated on a 6 MV linac. The transmitted dose was measured by using one array of a 2D ion chamber detector. The dose reconstruction was obtained by an iterative algorithm. A phantom set-up error and organ interfraction motion were simulated to test the algorithm sensitivity.In all configurations convergence was obtained within three iterations. A local reconstructed dose agreement of at least 3% / 3 mm with respect to the planned dose was obtained, except in a few points of the penumbra. The reconstructed primary fluences were consistent with the planned ones, which validates the whole reconstruction process. The results validate our method in a simple geometry and for narrow beams. The method is sensitive to a set-up error of a heterogeneous phantom and interfraction heterogeneous organ motion.     

另一种基于弦的锥束CT图像重建的推导方法

A series of papers by our group has layed thefoundation of a very general theory of image reconstruc-tion in cone-beam CT.Initially,Zou and Pan devel-oped a general formula from which a back-projectionfiltration algorithm was derived for yielding the imageon aπ-line segement of a helical source trajectory[1]with a minimum set of projection data.Theπ-line seg-ment is a line segment that connects two points on ahelix separated by no more than a single turn.It wasestablished previously thatπ…     

Comparison of interpolation functions to improve a rebinning-free CT-reconstruction algorithm

The robust algorithm OPED for the reconstruction of images from Radon data has been recently developed. This reconstructs an image from parallel data within a special scanning geometry that does not need rebinning but only a simple re-ordering, so that the acquired fan data can be used directly for the reconstruction. However, if the number of rays per fan view is increased, there appear empty cells in the sinogram. These cells need to be filled by interpolation before the reconstruction can be carried out. The present paper analyzes linear interpolation, cubic splines and parametric (or "damped") splines for the interpolation task. The reconstruction accuracy in the resulting images was measured by the Normalized Mean Square Error (NMSE), the Hilbert Angle, and the Mean Relative Error. The spatial resolution was measured by the Modulation Transfer Function (MTF). Cubic splines were confirmed to be the most recommendable method. The reconstructed images resulting from cubic spline interpolation show a significantly lower NMSE than the ones from linear interpolation and have the largest MTF for all frequencies. Parametric splines proved to be advantageous only for small sinograms (below 50 fan views).