多层螺旋CT扫描降低仟伏值对婴幼儿胸部图像质量的影响

目的:比较婴幼儿胸部低仟伏值CT扫描与常规扫描的图像质量,探讨降低仟伏值低剂量扫描应用于婴幼儿胸部检查的可行性。方法:体模测试:用120和90kV条件扫描体模的空间分辨力插件,选择相同位置的4帧图像测量平均空间分辨力;再用相同的方法扫描体模的水层,测量每组4帧图像的平均CT值标准差,作2组的噪声水平比较。病例扫描:选择胸部检查的患儿80例,随机分成2组,每组40例。2组分别用常规剂量(120kV,80mAs)和低剂量(90kV,80mAs)扫描,层厚5mm。比较不同剂量的CT权重剂量指数(CTDIvol)及剂量长度乘积(DLP);测量、比较2组图像降主动脉处的噪声值;并由2位医师按优、合格、不合格的等级盲法评价2组图像质量。结果:90kV扫描的CTDIvol为2.3mGy,DLP为26.4mGy·cm,与常规剂量的比值为43.4%和43.1%。低剂量图像的噪声水平(H=0.66%)较常规组(H=0.44%)高,两者差异有统计学意义(t=11.86,P<0.001)。虽然低剂量组的优质图像所占比率有所降低,但不存在不合格图像,所有低剂量图像都符合临床诊断要求。结论:90kV可以作为多层螺旋CT婴幼儿日常胸部检查的扫描条件。     

早期肺癌低剂量CT检测的技术优化与应用

目的 探讨低剂量螺旋CT扫描对肺结节检测的应用.方法 采用不同扫描参数对TM164A型性能模体和TM320D躯干CT剂量检测模体进行常规剂量及低剂量螺旋CT扫描,记录不同扫描条件下模体辐射剂量及采集图像的空间分辨力、密度分辨力、噪声水平和均匀度结果,对所有数据进行统计学处理,确定螺旋CT胸部低剂量扫描最佳参数;依据优化的低剂量扫描技术参数对肺结节检测进行图像视觉评价.结果 ① TM320D躯干模体CT剂量检测结果:160 mA常规剂量之CT剂量指数(CTDI)与低剂量各组(30 mA、50 mA、70 mA、90 mA)值比较有显著性差异,放射剂量随着管电流的增大而增加,尤其当管电流超过 90 mA时,放射剂量增大的更为显著;②10 mA与30 mA、50 mA、70 mA、90 mA、160 mA各组之间图像噪声有显著性统计学差异,1 mm与3 mm、5 mm、7 mm、10 mm各组之间图像噪声也有显著性统计学差异;图像均匀性与管电流及层厚组间均无显著性差异.③空间分辨率和低对比分辨率随着管电流的增加而提高,当管电流降低至30 mA时,低对比分辨率显著降低.结论 通过对体模的研究,30~50 mA以及3~5 mm重建是低剂量CT肺部扫描的较佳参数组合,既可以有效地减少辐射剂量,又可以较好地保证图像的质量,较适合于早期肺癌筛查.     

X-CT设备低对比度分辨力检测方法研究与分析

目的：检测X—CT设备低对比度分辨力，评价X—CT系统在低对比度条件下区分目标物的能力。方法：利用Catphan体模的CTP515模决进行测试，同样扫描条件重复三次，得到CT设备在低对比度条件下可分辨目标物的最小直径。结果：1％、0．5％和0。3％对比度条件下，A设备三次重复扫描得到的结果分别为(2、4和5mm)、(2、3和3mm)、(2、3和5mm)，螺旋扫描结果为(2、5和6mm)；B设备三次重复扫描得到的结果分别为(3、5和7mm)、(2、6和8mm)、(2、4和6mm)。针对多种对比度检测结果，利用Excel绘制低对比度分辨力变化曲线。结论：利用多种对比度条件评价低对比度分辨力使评价更客观全面。利用低对比度分辨力变化曲线图可对体模不能提供的其他对比度条件下的低对比度分辨力进行估计，而且简单易行。     

16层螺旋CT的z轴空间分辨力测试

目的：采用主客观两种方法评价16层螺旋Cr系统的z轴空间分辨力,分析16层螺旋CT的螺距（Pitch）对z轴空间分辨力的影响。方法：把Catphan500模体的CTP528模块水平放置在检查床面上,使用Somatom Cardiac16层CT机和腹部螺旋扫描模式：准直宽度0．75nma×16,重建间隔0．1mm,重建层厚0．75mm,不同螺距进行扫描,然后重建图像。在后处理工作站对各Pitch的序列图像进行冠状位多平面重组后目测图像上z轴方向可识别的线对测试卡的最大线对数。使用上述扫描条件,对Delta模体进行扫描,制作不同螺距的MTF曲线。结果：采用线对测试卡测试的Pitch分别为0．75、1．00、1．25的z轴空间分辨力是10、9、9LP／cm。采用MTF方法测试的相对信号强度2％的空间分辨力分别是11．31、9．51、9．70LP／cm。结论：Somatom Cardiac16层螺旋CT机具有比较高的z轴空间分辨力。对于该机型,Pitch的变化对z轴空间分辨力的影响非常小。     

MSCT扫描参数对图像质量影响的实验研究

目的:探讨MSCT扫描参数对图像质量的影响。方法:通过固定扫描条件对含有不同密度材料的QCT体模分别进行常规轴面扫描和螺旋扫描,并在体模图像的不同密度材料的中心区域,分别选取一个相同大小的感兴趣区(ROI)进行CT值(-x±SD)测量,观察两种扫描方式下图像的CT值变化;然后在其它条件不变的情况下,改变其中1个扫描条件(层厚,电压,曝光量,螺距)再对体模各扫描3次,测量并比较各条件下各材料的CT值。结果:常规扫描和螺旋扫描对应体模CT图像各材料的x-接近,随材料密度的增加以及螺距的增大,图像的x-和SD增大更明显。在其他条件不变的情况下,随曝光量、层厚的增加,体模CT图像对应各材料的x-无明显变化,而SD明显减小;随着管电压的升高,体模CT图像对应各材料的-x和SD明显减小;随螺距的增大,体模CT图像对应各材料的-x无明显变化,而SD明显增加。结论:CT扫描参数的变化对图像质量有直接的影响,各扫描参数对图像质量影响的大小又有差异。     

螺旋CT胸部低剂量参数优化

目的 探讨螺旋CT胸部低剂量扫描优化参数.方法 通过在不同扫描参数下对检测模体进行常规剂量及低剂量螺旋CT扫描,评定不同扫描条件下模体辐射剂量及采集图像的空间分辨力、密度分辨力、噪声水平和均匀度,选择最佳扫描参数.扫描参数: 120 kV,0.75 s,Pitch 1.0,FOV 360,重建模式RF3(标准滤过),选取不同层厚和不同管电流分别扫描并记录各组结果,对所有数据进行统计学处理.结果 160 mA常规剂量之CTDI与低剂量各组(30 mA、50 mA、70 mA、90 mA)值比较差异有显著性差异,30 mA与50 mA之间、50 mA与70 mA之间CTDI值无显著性差异;层厚1 mm及管电流10 mA的图像噪声与其它各组层厚及管电流参数有非常显著性差异;空间分辨率和低对比分辨率随着管电流的增加而提高,在50 mA处低对比分辨率有较明显的拐点,即当管电流降低至30 mA时,低对比分辨率显著降低.结论 50 mA、层厚5 mm是单螺旋CT胸部低剂量扫描的最佳参数,适用于CT普查和早期肺癌的筛查.     

影响CT值测量的因素及对策的探讨

目的探讨影响CT值测量的因素及对策。方法通过固定扫描条件对机器自带水模进行扫描,然后在其它条件不变的情况下,改变其中1个扫描条件(层厚、曝光量、重建函数、仟伏、FOV、机器)再对水模各扫描1次,测量CT图像一定兴趣区(ROI)的CT值(x±SD)与固定扫描条件的图像相对应ROI的CT值(x±SD)进行比较。结果相对应ROI的平均CT值:层厚为5mm及曝光量为150mAs与10mm、300mAs比较接近;重建函数为Fc80较Fc10略低;仟伏为130kV(因很少用未作校正)较120kV有明显的增加。相对应ROI的SD(噪声):层厚为5mm和曝光量为150mAs比较接近,较10mm、300mAs有明显增加;重建函数为Fc80较Fc10增加更明显;仟伏为130kV较120kV略有降低。在单纯小场水模的小场和中场CT图像中,相对应ROI的平均CT值和噪声:小场都大于中场。在单纯检测不同机器水模的CT图像中,相对应ROI的平均CT值及噪声:东芝旧CT机均高于GE新CT机。结论CT值不是恒定的,在诊断中只能作为参考,而不能作为诊断的依据。     

移动数字X线机后处理模式对密度分辨力影响的研究

目的:探讨移动数字X线机不同后处理模式对密度分辨力的影响。方法:模拟床旁胸片曝光条件对CDRAD2.0对比度细节体模与Multipurpose Chest Phantom N1"LUNGMAN"胸部体模曝光。采用移动DR机所带9种后处理模式分别对每幅图像进行后处理。应用CDRAD2.0型体模配套软件评估系统对9种后处理图像密度分辨力进行客观评价;由三位放射医师采用国际5分法对置入体模心影后的小结节显示进行主观评价。结果:9种后处理模式对图像的密度分辨力客观评价的结果IQFinv分别为2.45、2.13、2.25、2.24、2.17、2.41、2.36、2.82、2.39;降噪模式所得到的密度分辨力(IQFinv=2.82)最佳。采用国际5分法在床旁胸片拍摄条件下,9种后处理方式的图像在心影后小结节显示有差别,降噪模式对心影后小结节显示较其它模式明显清晰;超级图像处理(高对比度及细节处理)对心影后小结节显示能力较弱。结论:移动DR机后处理模式对密度分辨力有较大影响,在实际临床应用时应根据不同病变采用不同的后处理方式。     

不同型号双源CT冠状动脉成像参数及图像质量的对比分析

目的：比较Siemens Definition、Definition Flash及Definition Flash Stellar 3台双源CT回顾性心电门控扫描的辐射剂量及图像质量。方法：在回顾性心电门控扫描条件下，采用不同管电压+恒定管电流和不同管电压+智能调控管电流2种方式对标准Catphan 500模体进行扫描。所得各组数据分别采用不同层厚、不同卷积核参数的滤波反投影算法（FBP）及迭代重建算法（SAFIRE）进行重建。记录各组模体图像的剂量长度乘积（DLP）、CT值、图像噪声（SD），并对空间分辨力及密度分辨力模体模块进行评价。同时，由2名放射科医师对3台CT机的冠状动脉CTA图像进行回顾性分析，记录DLP及SD，并对图像质量进行评价。结果：模体扫描结果显示，辐射剂量与管电压、管电流呈正相关；噪声值与管电压、管电流呈负相关；DLP、CT值及空间分辨力组间差异均无统计学意义（均P>0.05）；噪声值及密度分辨力组间差异均有统计学意义（均P<0.05），管电压越低，密度分辨力差异性越明显。相同管电压条件下，不同层厚、不同卷积核参数的FBP及SAFIRE图像的噪声值大小为0.75If26SAFIRE < 0.75Bf26FBP < 0.6If26SAFIRE < 0.6Bf26FBP < 0.5If26SAFIRE < 0.75If46SAFIRE < 0.75Bf46FBP < 0.6If46SAFIRE < 0.5If46SAFIRE < 0.6Bf46FBP。回顾性分析冠状动脉CTA图像结果显示：DLP在组间差异无统计学意义（P>0.05），噪声值及图像评分在组间差异均有统计学意义（均P<0.05）。结论：Siemens双源CT的辐射剂量、图像质量及重建参数之间存在一定的客观规律；总体辐射剂量在3台Siemens双源CT回顾性心电门控扫描模式中改善不明显，但在相同扫描条件下，辐射剂量越低，图像质量改善越明显；图像质量最易受卷积核参数影响，建议合理选择重建参数，同时开启管电压和管电流调控技术进行扫描。     

肺部容积高分辨CT低剂量扫描方案的体模与临床应用研究

目的 研究肺部容积高分辨CT(volumetric high-resolution CT,VHRCT)的低剂量扫描方案,评价其诊断价值.方法 采用120 kV和10～250 mAs对Catphan 500体模行VHRCT扫描,层厚0.625 mm,记录图像的空间分辨力、密度分辨力、噪声及扫描剂量,制定低剂量VHRCT的扫描方案;105例在本院行常规剂量VHRCT检查的肺弥漫病变患者,复诊时行低剂量VHRCT,比较常规剂量与低剂量VHRCT对于肺弥漫病变的显示情况.结果 体模研究中,管电压120 kV,管电流120～250 mAs时,VHRCT图像的空间分辨力均为9 LP/cm;低于120 mAs时,随着管电流降低,VHRCT图像的空间分辨力和密度分辨力下降而噪声增加.临床研究中,对于肺弥漫病变各种征象的显示,低剂量VHRCT(120 kV,120 mAs)与常规剂量VHRCT(120 kV,250 mAs)比较差异无统计学意义(P＞0.05).扫描剂量较常规VHRCT降低52％.结论 采用120 kV和120 mAs行低剂量VHRCT,可以在保持图像的分辨能力及诊断价值的前提下显著降低放射剂量,其取代常规剂量VHRCT具有可行性.     

Modulation transfer function evaluation of cone beam computed tomography for dental use with the oversampling method

Objectives

The aim was to investigate the possibility of evaluating the modulation transfer function (MTF) of cone beam CT (CBCT) for dental use using the oversampling method.

Methods

The CBCT apparatus (3D Accuitomo) with an image intensifier was used with a 100 μm tungsten wire placed inside the scanner at a slight angle to the plane perpendicular to the plane of interest and scanned. 200 contiguous reconstructed images were used to obtain the oversampling line-spread function (LSF). The MTF curve was obtained by computing the Fourier transformation from the oversampled LSF. Line pair tests were also performed using Catphan®.

Results

The oversampling method provided smooth and reproducible MTF curves. The MTF curves revealed that the spatial resolution in the z-axis direction was significantly higher than that in the axial direction. This result was also confirmed by the line pair test.

Conclusions

MTF analysis was performed successfully using the oversampling method. In addition, this study clarified that the 3D Accuitomo had high spatial resolution, especially in the z-axis direction.     

Spatial variation of resolution and noise in multi-detector row spiral CT

RATIONALE AND OBJECTIVES: The authors performed this study to evaluate an approach for measuring the variations of three-dimensional spatial resolution and image noise throughout a field of view imaged with multi-detector row spiral computed tomographic (CT) scanners. MATERIALS AND METHODS: The authors designed a phantom (diameter, 320 mm) that contained 37 metallic spheres (diameter, approximately 0.8 mm) positioned between two disks made of a material with attenuation being that of water. One sphere was located at the isocenter of the phantom, and the rest were evenly spaced in three concentric rings with diameters of 100, 200, and 300 mm, respectively. The phantom was imaged with two widely used multi-detector row CT scanners by using a standard protocol and four variations of that protocol. Because a recently developed theory holds that image resolution should be proportional to the square root of the trace of the covariance matrix of a point spread function, the authors developed a software package to segment high-attenuation spheres from the CT image volume and compute point spread functions from blurred images of the spheres. Three-dimensional spatial resolution and image noise were calculated as a function of radial distance within the field of view. RESULTS: Resolution and noise were quantified in the resultant CT image volumes and found to be nonisotropic, with worse resolution and less noise occurring at the periphery of the field of view. CONCLUSION: The method enabled measurement of variations in spatial resolution and of their distribution on images obtained with multi-detector row CT scanners. These findings may contribute to the development of an improved algorithm for image reconstruction.     

Spatial resolution measurement for iterative reconstruction by use of image-averaging techniques in computed tomography

The purpose of our study was to investigate the validity of a spatial resolution measuring method that uses a combination of a bar-pattern phantom and an image-averaging technique, and to evaluate the spatial resolution property of iterative reconstruction (IR) images with middle-contrast (50 HU) objects. We used computed tomography (CT) images of the bar-pattern phantom reconstructed by the IR technology Adaptive Iterative Dose Reduction 3D (AIDR 3D), which was installed in the multidetector CT system Aquilion ONE (Toshiba Medical Systems, Otawara, Japan). The contrast of the bar-pattern image was set to 50 HU, which is considered to be a middle contrast that requires higher spatial resolution clinically. We employed an image-averaging technique to eliminate the influence of image noise, and we obtained averaged images of the bar-pattern phantom with sufficiently low noise. Modulation transfer functions (MTFs) were measured from the images. The conventional wire method was also used for comparison; in this method, AIDR 3D showed MTF values equivalent to those of filtered back projection. For the middle-contrast condition, the results showed that the MTF of AIDR 3D decreased with the strength of IR processing. Further, the MTF of AIDR 3D decreased with dose reduction. The image-averaging technique used was effective for correct evaluation of the spatial resolution for middle-contrast objects in IR images. The results obtained by our method clarified that the resolution preservation of AIDR 3D was not sufficient for middle-contrast objects.     

A comparative study for spatial resolution and subjective image characteristics of a multi-slice CT and a cone-beam CT for dental use

Purpose

Multi-slice CT (MSCT) and cone-beam CT (CBCT) are widely used in dental practice. This study compared the spatial resolution of these CT systems to elucidate which CT modalities should be selected for various clinical cases.

Materials and methods

As MSCT and CBCT apparatuses, Somatom Sensation 64 and 3D Accuitomo instruments, respectively, were used. As an objective evaluation of spatial resolution of these CT systems, modulation transfer function (MTF) analysis was performed employing an over-sampling method. The results of MTF analysis were confirmed with a line-pair test using CATPHAN. As a subjective evaluation, a microstructure visualization ability study was performed using a Jcl:SD rat and a head CT phantom.

Results

MTF analysis showed that for the in-plane direction, the z-axis ultrahigh resolution mode (zUHR) of the Sensation 64 and 3D Accuitomo instruments had higher spatial resolutions than the conventional mode (64×) of the Sensation 64, but for the longitudinal direction, the 3D Accuitomo had clearly higher spatial resolution than either mode of the Sensation 64. A line-pair test study and microstructure visualization ability studies confirmed the results for MTF analysis. However, images of the rat and the CT phantom revealed that the 3D Accuitomo demonstrated the failure to visualize the soft tissues along with aliasing and beam-hardening artifacts, which were not observed in the Sensation 64.

Conclusions

This study successfully applied spatial resolution analysis using MSCT and CBCT systems in a comparative manner. These findings could help in deciding which CT modality should be selected for various clinical cases.     

Method of measuring modulation transfer function in computed tomography using slit method with air gap phantom

There are several methods for measuring modulation transfer function (MTF) in computed tomography (CT) images. The aluminum slit method, scanning a phantom consisting of a thin aluminum foil sandwiched by flat plastic slabs, is a standard method for measuring field of view (FOV) in clinical CT scan. But this method requires extreme caution when handling metal foil of high precision. Therefore, we devised a more simple method named air gap slit (AS) method. This new technique is based on the aluminum slit method but use air gap instead of metal foil between phantoms. The MTF was calculated from a reversed profile curve of air slit which indicated minimum CT number. The aim of this study was to investigate a possibility of AS method evaluating MTF. We investigated fluctuation of MTF and FOV in clinical CT scan compared with the aluminum slit method. The result showed that the fluctuation of MTF was caused by statistics noise and is more affected by a bone kernel than standard kernel when reconstructing. Also, the MTF value in AS method was slightly higher than in aluminum slit method and did not correspond with. AS method is a useful method for measurement of MTF in clinical CT scan. When we use this method, we have to take into consideration the noise influence of data.     

Impact of CT parameters on the physical quantities related to image quality for two MDCT scanners using the ACR accreditation phantom: A phantom study

IntroductionTo evaluate the image quality provided by MDCT scanners using an ACR phantom an to find out the relationship between CT parameters and physical quantities related to image quality.MethodsA GE Lightspeed VCT and a GE Lightspeed Pro 16 are used. The ACR phantom consists of four modules for evaluating physical parameters. The image quality parameters, such as CT number, linearity, CNR, image uniformity, SNR and at least spatial resolution using MTF, by different sets of image acquisition protocols (IAPs) are characterized. The influences of the IAPs on the physical quantities are also discussed.ResultsThe CT numbers behaved linearly relative to material density for all tube voltages. The impact of the tube current on the CT numbers is neglectable. However, the variation of the tube current reflects in the CT number uncertainties. The CNR are altered by changing the IAPs. 50% MTF decreases from 6.2 to 3.6 lp/cm and from 6.5 to 3.7 lp/cm using Lung and Soft kernel for the Lightspeed VCT and Lightspeed Pro 16 scanner, respectively.ConclusionThe dependence of the image quality parameters on reconstruction kernels, tube peak voltages, tube currents and the slice thicknesses has been discussed. The tube peak voltage has the most influence on the CT numbers. The results indicate that the reconstruction kernel has the main impact on the spatial resolution. The spatial resolution dependence on the tube voltages, tube currents and slice thicknesses can be ignored.     

Spatial resolution analysis of computed tomographic images

Methods are presented for the quantification of spatial resolution in x-ray computed tomographic (CT) images. Model-dependent methods are derived and compared with model independent methods for computation of the Modulation Transfer Function (MTF). These techniques are applied to phantom images of point, line, edge, and ring discontinuities. The model-dependent methods utilize multiparameter fits of a two-dimensional model function to the image data. Model predictions are compared with results obtained in a model-independent way by numerical transformation of the data. Results of resolution measurements of an Imatron C-100 CT scanner at UCSF and a second experimental scanner at the UCSF Physics Research Laboratory are presented.     

Modulation transfer function-based assessment of temporal resolution: validation for single- and dual-source CT

The purpose of this study was to determine a manufacturer-independent quality assurance measurement for temporal resolution with a three-dimensional cardiac motion robot; validation was with single-source (SS) and dual-source (DS) computed tomography (CT). Image acquisition was performed by using standard cardiac protocols. Image contrast-based modulation transfer function (MTF) was assessed as function of time. For motion frequency of 60 beats per minute, MTF slightly decreased by 14% and 6% for SS CT and DS CT, respectively. For higher frequencies, a stronger decrease of MTF (eg, by 50% [SS CT] and 18% [DS CT] at 120 beats per minute) was detected. Effect of manufacturer's adaptive bisegment algorithm for SS CT and corresponding resonance effects of rotation time and heart rate were quantified. The robot-based approach is a reproducible, objective way to assess temporal resolution; it allows practical measurement of temporal resolution and comparison of CT scanners and protocols.     

64层螺旋CT机架旋转时间对图像质量的影响探讨

目的研究不同机架旋转时间对64层螺旋CT图像质量的影响。方法使用SOMATOM Sensation 64层螺旋CT。把del-ta模体沿Z轴固定于CT机架的中心,使用腹部螺旋扫描模式,进行扫描和重建,然后制作各位置图像序列的层敏感度曲线(SSP),并对各图像序列的SSP进行傅里叶变换得到调制传递函数(MTF)。后用腹部螺旋扫描模式对CT质控模体Catphan500进行扫描和重建,分别记录图像的最大线对数及其能分辨的各级对比度的最小目标的尺寸。结果0.5s组和1s组的MTF曲线几乎重合,5%MTF值分别为11.3LP/cm和11.9LP/cm;其空间分辨率和低对比度分辨率也完全相同。结论机架旋转时间0.5s组和1s组图像质量无差别。为缩短扫描时间,消除呼吸伪影,我们建议在腹部螺旋扫描时,机架旋转时间0.5作为常规应用。