双源变螺距螺旋计算机断层成像重建算法

计算机断层成像血管造影术(Computed tomography angiography, CTA)是诊断血管疾病的重要手段.我们提出了一种适用于CTA的双源变螺距螺旋计算机断层成像(Computed tomography, CT)重建算法,投影数据由双源双多层螺旋CT进行采集,采集过程中,螺旋CT的螺距不再是固定的常数,而是随着时间不断地改变.变螺距的双源螺旋CT的重建算法与定螺距的双源双多层螺旋CT重建算法不同之处主要在于采用了新的更一般的轴内插公式.双源变螺距螺旋CT能够更有效地跟踪造影剂,获得具有更高时间分辨率的重建图像.仿真结果表明了该算法的正确性与有效性.     

64层螺旋CT三维重建活体肝静脉的研究及临床意义

目的:探讨64层螺旋CT应用于正常人活体体肝静脉研究的可行性,观察三维重建肝静脉的一般形态及走行规律.方法:153例正常受试者经肘正中静脉注射造影剂后,使用64层螺旋CT进行上腹部扫描,图像采集后经容积再现(volume rendering,VR)技术重建肝静脉.结果:重建图像清晰,可显示出6～8级血管及与周围组织间的关系.其中153例肝静脉的分型结果如下:①3分支型,占35.3%(54例);②中左共干型,占41.8%(64例);③中左合干型,占20.9%(32例);④中右共干型,占2.0%(3例).结论:64层螺旋CT可以作为研究活体肝静脉形态的有效手段,三维重建能更准确、全方位地显示肝静脉的正常解剖类型和发现变异,而且图像清晰,对于活体肝静脉的研究有较好的临床应用价值.     

64层螺旋CT在活体肝门静脉三维重建中的应用

目的:探讨64层螺旋CT应用于正常人体肝门静脉研究的可行性,观察三维重建肝门静脉的一般形态及变异.方法:153例正常受试者经肘正中静脉注射造影剂后,使用64层螺旋CT进行上腹部扫描,图像采集后经容积再现(VR)技术重建肝门静脉.结果:肝门静脉分5型:a型: 83 7%;b型:7 8%;c型:7 8%;d型:无;e型:0 7%.结论:64层螺旋CT可以作为研究活体肝门静脉形态的有效手段,三维重建能更准确、全方位地显示肝门静脉的正常解剖类型和变异.     

腹腔动脉CT数据三维重建与相关解剖学研究

目的:研究基于CT数据腹腔动脉三维重建方法及相关解剖.方法:利用Mimics软件对120例患者64排螺旋CT数据行腹腔动脉三维重建,评价重建模型的效果并统计各分支的显示率、分支类型、开口位置、走行方向及长度.结果:重建模型形态逼真,能准确显示该血管及其分支的解剖结构,显示率分别为:肝总动脉100%,脾动脉100%,胃左动脉78.3%,胃十二指肠动脉100%,肝固有动脉98.3%,肝左动脉79.2%,肝右动脉95%.胃脾肝共干型分支最多,占89.2%,该型腹腔动脉平均长度为2.76cm.开口位置96.7%位于胸12～腰1之间,83.3%向右走行.结论:利用64排螺旋CT增强扫描数据三维重建腹腔动脉,可真实反应其形态结构并测量相关解剖学数据,对介入插管及虚拟手术研究有较高指导价值,为解剖学教学提供了相关数字化图谱.     

低剂量螺旋CT对不同征像形态肺结节的诊断价值

目的:探析低剂量螺旋CT对不同征像形态肺结节的诊断价值.方法:选取2020年2月至2021年8月我院收治的肺结节病患者100例,随机分为研究组和对照组,每组50例.所有患者均统一接受肺部64排256层螺旋CT扫描,对照组采用常规剂量扫描(螺距设定为1.0,扫描剂量为250 mA),研究组采用低剂量扫描(螺距为2.0,扫...     

MSCT轴向分辨力和图像噪声影响因素分析

目的:探讨多排螺旋CT（MSCT）轴向分辨力或断层灵敏度曲线（SSP）和图像噪声的影响因素。方法:使用临床常 用的腹部扫描模式,采用不同直径的模板,重建层厚、螺距、电压（kV）、重建算法等扫面参数,进行MSCT扫描,对不同参数 对断层图像SSP和图像噪声的影响进行统计学分析。结果:当螺距和准直器宽度保持不变,不同层厚和重建算法得到的 SSP的半高宽FWHM值基本保持不变（P>0.05）;当重建算法和准直器宽度保持不变,不同螺距和层厚得到的SSP的半高 宽FWHM值基本保持不变（P>0.05）;不同准直器宽度,同螺距和层厚得到的SSP 的半高宽FWHM值基本保持不变（P> 0.05）;随着层厚、mAs增加,图像噪声减小（P<0.05）;随着kV增加,图像噪声随之减小,不同重建算法下图像噪声存在明显 差异（P<0.05）。结论:卷积重建算法、螺距和准直器宽度对SSP的影响很小,螺距对图像噪声影响很小,而层厚、重建算 法、mAs、kV对图像噪声影响大。层厚、mAs、kV增大,图像噪声减小;重建算法分辨率越高,图像噪声越大。     

多层螺旋CT冠状动脉造影初步探讨

目的探讨多层螺旋CT冠状动脉造影的可行性及不同心率下冠状动脉的显示能力.方法16例多层螺旋CT冠状动脉造影.东芝AQUILION多层螺旋CT采集原始数据,扫描速度0.32s/270°,层厚2mm,螺距1.0-1.2.对比剂安射力2ml/kg,注射速率3.5ml/s.延迟时间20s,SGI02图像后处理工作站,后处理软件为ALATOVIEW版本Version1.42;采用遮盖容积重建(SVR)、多平面重建(MPR)、曲面重建(CPR)后处理技术.结果心率＜60次/min时冠状动脉主干及主要分支(前降支、回旋支)显示最清晰,心率60～79次/min时冠状动脉主干及主要分支显示较清晰,心率＞80次/min时冠状动脉主要分支显示不清.结论一定心率及扫描参数、对比剂使用合适前提下,多层螺旋CT冠状动脉造影可获得满意的冠状动脉主干及主要分支图像.     

大鼠动脉螺旋血流的荧光成像

目的 建立大鼠动脉螺旋血流的实时荧光成像法。方法 随机将SD大鼠分为静脉造影组(股静脉注射,1 mL/kg)与小剂量动脉造影组(腹主动脉注射,0.1 mL/kg),其中小剂量动脉造影组又分为荧光素钠组、罗丹明B组、FITC-dextran组和量子点组(每组n=7)。通过体式荧光显微镜实时观察大鼠股动脉血液流动现象,用高速摄像机拍摄动态图像,通过图像阈值分割算法及形态学后处理算法标定荧光区域,测算大鼠股动脉螺旋血流的轴向速度与切向速度,并计算其螺距和壁剪切应力。结果 与静脉造影组相比,小剂量动脉造影组可以清晰显示大鼠股动脉的血流呈顺时针螺旋样流动,在FITC-dextran造影剂组可计算出大鼠螺旋血流的轴向速度为(198.4±112.7)mm/s,切向速度为(3.3±0.8)mm/s,螺距为(126.1±76.3)mm,壁剪切应力为(9.5±5.6)Pa。每只大鼠重复3次注射,测得的轴向、切向速度结果无差异。结论 实时荧光成像法可以用于在体观察大鼠动脉螺旋血流并对螺旋血流的血流动力学特征进行定量分析。     

基于螺旋CT扫描图像的人牙齿的计算机三维重建

目的:研究基于螺旋CT扫描图像的人牙齿的计算机三维重建。方法:采用美国PQ6000型螺旋CT扫描机,自下颌颏部下缘开始至鼻骨底做连续横断超薄扫描,得到了人牙齿的二维CT图像。应用图像处理与识别合成软件,读取二维CT图像数据文件,通过灰度分割方法对二维图像进行边缘提取,再利用三维表面重建算法对所截取的二维CT图像进行三维重建,最终获得人牙齿的三维重建影像。结果:获得的人牙齿三维重建影像,细致逼真、可被任意旋转、剖开、透视、截取,从不同角度观察重建影像形态还原性良好、重建效果满意。结论:基于螺旋CT扫描图像的人牙齿的计算机三维重建,可为有限元实体建模提供快捷、方便的方法。     

螺旋CT扫描对结肠癌的诊断价值

目的探讨螺旋CT动态增强扫描对结肠癌的诊断价值。方法对62例结肠癌的患者,行螺旋CT动态增强扫描及图像后处理检查与分析,并与术后病理对照。结果62例中有34例显示肿块型结肠癌,23例显示为浸润型结肠癌;形态不规则和肠腔狭窄的42例,浆膜面模糊32例,淋巴结转移39例;结肠癌术前病灶诊断准确率100%,术前分期与手术后分期符合率为90.2%。结论螺旋CT动态增强扫描及图像后处理对结肠癌的诊断有重要意义。     

Exact reconstruction of volumetric images in reverse helical cone-beam CT

Helical scanning configuration has been used widely in diagnostic cone-beam computed tomography (CBCT) for acquiring data sufficient for exact image reconstruction over an extended volume. In image-guided radiation therapy (IGRT) and other applications of CBCT, it can be difficult, if not impossible, to implement mechanically a multiple-turn helical trajectory on the imaging systems due to hardware constraints. However, imaging systems in these applications often allow for the implementation of a reverse helical trajectory in which the rotation direction changes between two consecutive turns. Because the reverse helical trajectory satisfies Tuy's condition, when projections of the imaged object are nontruncated, it yields data sufficient for exact image reconstruction within the reverse helix volume. The recently developed chord-based algorithms such as the backprojection filtration (BPF) algorithm can readily be applied to reconstructing images on chords of a reverse helical trajectory, and they can thus reconstruct an image within a volume covered by the chords. Conversely, the chord-based algorithms cannot reconstruct images within regions that are not intersected by chords. In a reverse helix volume, as shown below, chordless regions exist in which no images can thus be reconstructed by use of the chord-based algorithms. In this work, based upon Pack-Noo's formula, a shift-invariant filtered backprojection (FBP) algorithm is derived for exact image reconstruction within the reverse helix volume, including the chordless region. Numerical studies have also been conducted to demonstrate the chordless region in a reverse helix volume and to validate the FBP algorithm for image reconstruction within the chordless region. Results of the numerical studies confirm that the FBP algorithm can exactly reconstruct an image within the entire reverse helix volume, including the chordless region. It is relatively straightforward to extend the FBP algorithm to reconstruct images for general trajectories, including reverse helical trajectories with variable pitch, tilted axis, and/or additional segments between turns.     

Modulation transfer function of digitally reconstructed radiographs using helical computed tomography

We have measured the modulation transfer function (MTF), at a distance from CT isocenter, in digitally reconstructed radiographs (DRR) using a bar pattern phantom for axial and helical data acquisitions. Spatial resolution in DRR increases for thinner slice thicknesses (43% at 1.6 lp/cm for 2 mm versus 16% for 5 mm). For the three slice thicknesses studied, the axial scanning mode provided better spatial resolution in DRR than helical scans performed at pitches > or = 1.5 (41% at 1.6 lp/cm for axial, 3 mm slice versus 18% for pitch = 2), but is similar to helical scans with pitch of 1. The reconstruction of overlapping slices from helical acquisitions of pitch = 1.5 results in spatial resolution of the DRR that is similar to that resulting from axial scans with contiguous reconstruction, but also results in the fine streaks known as "zebra" artifacts in the DRR.     

Image reconstruction on PI-lines by use of filtered backprojection in helical cone-beam CT

Recently, we have derived a general formula for image reconstruction from helical cone-beam projections. Based upon this formula, we have also developed an exact algorithm for image reconstruction on PI-line segments from minimum data within the Tam-Danielsson window. This previous algorithm can be referred to as a backprojection-filtration algorithm because it reconstructs an image by first backprojection of the data derivatives and then filtration of the backprojections on PI-line segments. In this work, we propose an alternative algorithm, which reconstructs an image by first filtering the modified data along the cone-beam projections of the PI-lines onto the detector plane and then backprojecting the filtered data onto PI-line segments. Therefore, we refer to this alternative algorithm as the filtered-backprojection algorithm. A preliminary computer-simulation study was performed for validating and demonstrating this new algorithm. Furthermore, we derive a practically useful expression to accurately compute the derivative of the data function for image reconstruction. The proposed filtered-backprojection algorithm can reconstruct the image within any selected ROI inside the helix and thus can handle naturally the long object problem and the super-short scan problem. It can also be generalized to reconstruct images from data acquired with other scanning configurations such as the helical scan with a varying pitch.     

The effect of z overscanning on patient effective dose from multidetector helical computed tomography examinations

z overscanning in multidetector (MD) helical CT scanning is prerequisite for the interpolation of acquired data required during image reconstruction and refers to the exposure of tissues beyond the boundaries of the volume to be imaged. The aim of the present study was to evaluate the effect of z overscanning on the patient effective dose from helical MD CT examinations. The Monte Carlo N-particle radiation transport code was employed in the current study to simulate CT exposure. The validity of the Monte Carlo simulation was verified by (a) a comparison of calculated and measured standard computed tomography dose index (CTDI) dosimetric data, and (b) a comparison of calculated and measured dose profiles along the z axis. CTDI was measured using a pencil ionization chamber and head and body CT phantoms. Dose profiles along the z axis were obtained using thermoluminescence dosimeters. A commercially available mathematical anthropomorphic phantom was used for the estimation of effective doses from four standard CT examinations, i.e., head and neck, chest, abdomen and pelvis, and trunk studies. Data for both axial and helical modes of operation were obtained. In the helical mode, z overscanning was taken into account. The calculated effective dose from a CT exposure was normalized to CTDI(free in air). The percentage differences in the normalized effective dose between contiguous axial and helical scans with pitch = 1, may reach 13.1%, 35.8%, 29.0%, and 21.5%, for head and neck, chest, abdomen and pelvis, and trunk studies, respectively. Given that the same kilovoltage and tube load per rotation were used in both axial and helical scans, the above differences may be attributed to z overscanning. For helical scans with pitch = 1, broader beam collimation is associated with increased z overscanning and consequently higher normalized effective dose value, when other scanning parameters are held constant. For a given beam collimation, the selection of a higher value of reconstructed image slice width increases the normalized effective dose. In conclusion, z overscanning may significantly affect the patient effective dose from CT examinations performed on MD CT scanners. Therefore, an estimation of the patient effective dose from MD helical CT examinations should always take into consideration the effect of z overscanning.     

PI-line-based image reconstruction in helical cone-beam computed tomography with a variable pitch

Current applications of helical cone-beam computed tomography (CT) involve primarily a constant pitch where the translating speed of the table and the rotation speed of the source-detector remain constant. However, situations do exist where it may be more desirable to use a helical scan with a variable translating speed of the table, leading a variable pitch. One of such applications could arise in helical cone-beam CT fluoroscopy for the determination of vascular structures through real-time imaging of contrast bolus arrival. Most of the existing reconstruction algorithms have been developed only for helical cone-beam CT with constant pitch, including the backprojection-filtration (BPF) and filtered-backprojection (FBP) algorithms that we proposed previously. It is possible to generalize some of these algorithms to reconstruct images exactly for helical cone-beam CT with a variable pitch. In this work, we generalize our BPF and FBP algorithms to reconstruct images directly from data acquired in helical cone-beam CT with a variable pitch. We have also performed a preliminary numerical study to demonstrate and verify the generalization of the two algorithms. The results of the study confirm that our generalized BPF and FBP algorithms can yield exact reconstruction in helical cone-beam CT with a variable pitch. It should be pointed out that our generalized BPF algorithm is the only algorithm that is capable of reconstructing exactly region-of-interest image from data containing transverse truncations.     

A rebinned backprojection-filtration algorithm for image reconstruction in helical cone-beam CT

In the last few years, mathematically exact algorithms, including the backprojection-filtration (BPF) algorithm, have been developed for accurate image reconstruction in helical cone-beam CT. The BPF algorithm requires minimum data, and can reconstruct region-of-interest (ROI) images from data containing truncations. However, similar to other existing reconstruction algorithms for helical cone-beam CT, the BPF algorithm involves a backprojection with a spatially varying weighting factor, which is computationally demanding and, more importantly, can lead to undesirable numerical properties in reconstructed images. In this work, we develop a rebinned BPF algorithm in which the backprojection invokes no spatially varying weighting factor for accurate image reconstruction from helical cone-beam projections. This rebinned BPF algorithm is computationally more efficient and numerically more stable than the original BPF algorithm, while it also retains the nice properties of the original BPF algorithm such as minimum data requirement and ROI-image reconstruction from truncated data. We have also performed simulation studies to validate and evaluate the rebinned BPF algorithm.     

High temporal resolution for multislice helical computed tomography

Multislice helical computed tomography (CT) substantially reduces scanning time. However, the temporal resolution of individual images is still insufficient for imaging rapidly moving organs such as the heart and adjacent pulmonary vessels. It may, in some cases, be worse than with current single-slice helical CT. The purpose of this study is to describe a novel image reconstruction algorithm to improve temporal resolution in multislice helical CT, and to evaluate its performance against existing algorithms. The proposed image reconstruction algorithm uses helical interpolation followed by data weighting based on the acquisition time. The temporal resolution, the longitudinal (z-axis) spatial resolution, the image noise, and the in-plane image artifacts created by a moving phantom were compared with those from the basic multislice helical reconstruction (helical filter interpolation, HFI) algorithm and the basic single-slice helical reconstruction algorithm (180 degrees linear interpolation, 180LI) using computer simulations. Computer simulation results were verified with CT examinations of the heart and lung vasculature using a 0.5 second multislice scanner. The temporal resolution of HFI algorithm varies from 0.28 and 0.86 s, depending on helical pitch. The proposed method improves the resolution to a constant value of 0.29 s, independent of pitch, allowing moving objects to be imaged with reduced blurring or motion artifacts. The spatial (z) resolution was slightly worse than with the HFI algorithm; the image noise was worse than with the HFI algorithm but was comparable to axial (step-and-shoot) CT. The proposed method provided sharp images of the moving objects, portraying the anatomy accurately. The proposed algorithm for multislice helical CT allowed us to obtain CT images with high temporal resolution. It may improve the image quality of clinical cardiac, lung, and vascular CT imaging.     

Region-of-interest reconstruction of motion-contaminated data using a weighted backprojection filtration algorithm

The recently developed weighted backprojection filtration (WBPF) algorithm using data redundancy has capabilities that make this algorithm an attractive candidate for reconstructing images from motion-contaminated projection data. First, the WBPF algorithm is capable of reconstructing region-of-interest (ROI) images from reduced-scan fan-beam data, which have less data than the short-scan data required to reconstruct the entire field of view (FOV). Second, this algorithm can reconstruct ROI images from truncated data. Using phantom simulation studies, we demonstrate how these unique capabilities can be exploited to reduce the amount of motion-contaminated data used for reconstruction. In particular, we use examples from cardiac imaging to illustrate how off-center phantom positioning combined with phase-interval ROI reconstruction can result in the suppression of motion artifacts. In terms of temporal resolution, reduced-scan reconstruction with 45% of a full-scan dataset can be used to improve the temporal resolution of a short-scan reconstruction by 25.8% if ungated data are used. For data gated at 66 beats per minute, reduced-scan reconstruction with 45% of a full-scan dataset can be used to improve the temporal resolution of a short-scan reconstruction by 7.9%. As a result of our studies, we believe that the WBPF algorithm demonstrates the potential for reconstructing quality ROI images from motion-contaminated fan-beam data.     

Grangeat-type helical half-scan computerized tomography algorithm for reconstruction of a short object

Currently, cone-beam computerized tomography (CT) and micro-CT scanners are under rapid development for major biomedical applications. Half-scan cone-beam image reconstruction algorithms assume only part of a scanning turn, and are advantageous in terms of temporal resolution and image artifacts. While the existing half-scan cone-beam algorithms are in the Feldkamp framework, we have published a half-scan algorithm in the Grangeat framework for a circular trajectory [Med. Phys. 30, 689-700 (2003)]. In this paper, we extend our previous work to a helical case without data truncation. We modify the Grangeat's formula for utilization and estimation of Radon data. Specifically, we categorize each characteristic point in the Radon space into singly, doubly, triply sampled, and shadow regions, respectively. A smooth weighting strategy is designed to compensate for data redundancy and inconsistency. In the helical half-scan case, the concepts of projected trajectories and transition points on meridian planes are introduced to guide the design of weighting functions. Then, the shadow region is recovered via linear interpolation after smooth weighting. The Shepp-Logan phantom is used to verify the correctness of the formulation, and demonstrate the merits of the Grangeat-type half-scan algorithm. Our Grangeat-type helical half-scan algorithm is not only valuable for quantitative and/or dynamic biomedical applications of CT and micro-CT, but also serves as an intermediate step towards solving the long object problem.     

Exact and approximate algorithms for helical cone-beam CT

This paper concerns image reconstruction for helical x-ray transmission tomography (CT) with multi-row detectors. We introduce two approximate cone-beam (CB) filtered-backprojection (FBP) algorithms of the Feldkamp type, obtained by extending to three dimensions (3D) two recently proposed exact FBP algorithms for 2D fan-beam reconstruction. The new algorithms are similar to the standard Feldkamp-type FBP for helical CT. In particular, they can reconstruct each transaxial slice from data acquired along an arbitrary segment of helix, thereby efficiently exploiting the available data. In contrast to the standard Feldkamp-type algorithm, however, the redundancy weight is applied after filtering, allowing a more efficient numerical implementation. To partially alleviate the CB artefacts, which increase with increasing values of the helical pitch, a frequency-mixing method is proposed. This method reconstructs the high frequency components of the image using the longest possible segment of helix, whereas the low frequencies are reconstructed using a minimal, short-scan, segment of helix to minimize CB artefacts. The performance of the algorithms is illustrated using simulated data.