Comparison of Image Quality between Virtual Monochromatic Images and Conventional CT Images
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摘要: 目的：对比双源双能CT的虚拟单能图像与常规CT图像的图像质量。方法：采用西门子SOMATOM Force双源双能CT扫描图像质量控制体模Catphan 500的高对比度分辨率CTP 528组件和低对比度分辨率CTP 515组件，分别采用双球管扫描方案重建低、中、高3档虚拟单能能谱图像（A组：40 keV，B组：70 keV，C组：100 keV）和一种单球管扫描方案图像（D组：120 kVp）。两名放射科医师分别测量CTP 528的高对比分辨率（最佳线对数）和CTP 515的低对比度分辨率（1％ 对比度浓度下的最小孔径）；随后测量各组图像的平均噪声、信噪比（SNR）及对比噪声比（CNR）。结果：两名医师的图像一致性评价良好（高/低对比度分辨率：Kappa系数为0.667；噪声参数：Spearman相关系数r=0.920。各组图像中，D组高对比度分辨率最佳（线对6 lp/cm）；A、B组图像低对比度分辨率最佳（孔径均为4 mm）。C组图像平均噪声最低（2.333±0.289），SNR表现最佳（37.944±3.949）；噪声和SNR在各组图像间差异有统计学意义。B组图像CNR最高（3.361±0.875），但各组图像间CNR差异不具有统计学意义。结论：双能CT虚拟单能图像相比常规CT图像可以在保持低对比度分辨率的基础上，改善图像噪声；随着虚拟单能能级的增加，图像噪声减小，信噪比增加。Abstract: Objective: To compare the image quality between virtual monochromatic images (VMI) of dual-source dual-energy CT (DECT) and conventional CT images. Siemens SOMATOM Force dual source dual energy CT was adopted in the scanning of the high contrast resolution CTP 528 component and low contrast resolution CTP 515 component of image quality control phantom catphan 500. Dual-tube dual-energy scanning scheme was used to reconstruct low, medium and high-level VMI (group A: 40 keV, group B: 70 keV, group C: 100 keV) and a single tube scanning scheme was used to reconstruct conventional images (Group D: 120 kVp). Two radiologiss carried out the measurement of high resolution by the smallest line pair one could distinguish and low-contrast sensitivity was measured by the smallest diameter under 1% contrast. Mean noise, signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were also evaluated for each group. Results: The image consistency of the two radiologists turned out well (high reolution and low-contrast sensitivity: Kappa coefficient=0.667; noise parameter: Spearman r = 0.920). Among all images, D group performed the best in high resolution (6 lp/cm) and A and B groups showed the best low-contrast sensitivity (diameter of 4 mm). C group had the lowest noise (2.333±0.289) and highest SNR (37.944±3.949). Mean noise and SNR were found significantly difference among the four groups. B group showed the highest CNR (3.361±0.875), but no statistical significance was found among the groups. Conclusion: Compared with conventional CT, VMI images from DECT could improve image quality while retain low-contrast sensitivity; image noise decreases while SNR increases as VMI elevates.
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Keywords:
- dual-energy CT /
- virtual monochromatic image /
- phantom
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图 2 三档虚拟单能图像的示例图
（a）～（c）分别代表40、70和100 keV图像；（a1）、（b1）和（c1）分别代表相同窗宽窗位下（400，0）40、70和100 keV的图像对比；（a2）和（a3）、（b2）和（b3）、（c2）和（c3）分别代表相应虚拟单能图像的CTP 515低对比度分辨率和CTP 528高对比度分辨率模块图像。
Figure 2. Schematic diagram for the three series of virtual monochromatic images
表 1 四组图像高对比度分辨率和低对比度分辨率比较
Table 1 Comparison of high-contrast resolution and low-contrast resolution of four groups of images
对比度分辨率 A组
（40 keV）B组
（70 keV）C组
（100 keV）D组
（120 kVp）高对比对分辨率（线对/（lp/cm）） 5 5 5 6 低对比度分辨率（孔径直径/mm） 4 4 5 5 
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表 2 四组图像图噪声参数评价结果比较
Table 2 Comparison of the evaluation results of the noise parameters of the five groups of images
组别 SD/HU SNR CNR A组（40 keV） 4.167±0.289 10.942±0.465 2.383±0.336 B组（70 keV） 2.500±0.500 24.028±5.000 3.361±0.875 C组（100 keV） 2.333±0.289 37.944±3.949 3.139±0.337 D组（120 kVp） 2.500±0.000 30.806±1.398 2.778±0.241 $P $ <0.001 <0.001 0.179
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