Objective To select the optimal monochromatic level for gemstone spectral imaging (GSI) , in order to minimize both the image noise and the beam-hardening artifact (BHA) caused by high concentration of iodine contrast agent. Methods A cylindrical plastic phantom with 9 hard plastic tubes which were filled with different concentrations (100.0, 50. 0, 30. 0, 20. 0, 10. 0, 5. 0, 2. 0, 0. 7, 0. 4 mg/ml) of iodine solution was used, and the tube with highest concentration was in the center, and the other 8 tubes were located clockwise with concentration in outer circle. CT (Discovery CT750 HD) scans with conventional 120 kVp mode and GSI mode were performed, respectively. All data were transferred to Workstation to obtain one set of polychromatic images (120 kVp) and 21 sets of monochromatic images (40-140 keV, interval of 5 keV). Two radiologists reviewed all images respectively and assessed the image quality visually using 4-point scale. The image noise of the most significant affected region was measured. The monochromatic image sets which had the lowest image noise and BHA and the highest subjective score of image quality were analyzed and compared with the polychromatic images with one-way analysis of variance. Results Of the 21 sets of monochromatic images, 85 keV and 90 keV sets had the highest subjective score of image quality, the image noise and BHA of the two sets were the lowest. The image noise with 85 keV and 90 keV sets reduced by 82. 5% and 82. 0% (both P<0. 001) compared with the polychromatic image. For BHA, the reductions were 96. 1% and 95. 7% (both P<0. 001). Both 85 keV and 90 keV sets had significant noise and BHA reduction, but there was no significant difference (P = 0. 232 for noise, and P-0. 817 for BHA). Conclusion Compared with conventional 120 kVp mode, high-definition CT GSI can considerably reduce image noise and BHA of iodine contrast agent, especially at the optimal monochromatic levels of 85 keV and 90 keV.%目的 选择CT能谱成像的最佳单能量水平,以最小化对比剂所致的线束硬化伪影(BHA)和图像噪声.方法 9支含有不同浓度碘溶液(浓度分别为100.0、50.0、30.0、20.0、10.0、5.0、2.0、0.7、0.4 mg/ml)的硬塑料试管,将浓度最高者置于塑料体模的中心,将其余8支按浓度梯度依次等距环形排列于体模周边.分别采用常规120 kVp混合能量模式和能谱(GSI)模式进行扫描,共获得1组120 kVp混合能量和21组单能量图像.由2名放射科医师采用4分量表对图像质量进行主观目测评分,测量并比较图像内指定区域(硬化伪影最明显处)的噪声值(以SD值表示)及BHA.选取噪声及BHA最低、图像主观评分最高的单能量图像,与常规120 kVp混合能量图像进行比较.采用单因素方差分析比较选定组间数据差异.结果 21组单能量图像中,85 keV和90 keV图像的主观评分最高,图像噪声及BHA最低.与120 kVp混合能量图像比较,上述两组单能量图像噪声分别降低了约82.5％和82.0％(P均＜0.001),BHA分别降低了约96.1％和95.7％(P均＜0.001),主观评分提高了1倍.但85 keV及90 keV两组单能量图像间噪声和BHA差异无统计学意义(P＝0.232、0.817).结论 与传统120 kVp混合能量图像相比,CT能谱成像能够提供一系列单能量图像,其中85 keV和90 keV最佳单能量图像能够显著降低图像噪声和对比剂产生的线束BHA,提高图像质量.
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