胸部能谱成像模式与常规CT扫描的辐射剂量及图像质量的仿真体模研究

目的 通过仿真胸部体模研究胸部能谱CT不同方案成像模式与常规胸部CT扫描的辐射剂量、图像质量、对比噪声比及主观评分对比，获得最佳能谱扫描参数。方法 对仿真胸部体模分别进行常规胸部CT扫描以及3种不同扫描方式的能谱CT成像。3种能谱模式为宝石能谱CT （GSI）Assist模式（方案A）、管电流平均值时的GSI模式（方案B）及管电流最大值时的GSI模式（方案C）。所有扫描方式分别在噪声指数（NI）=9和11时，螺距0.984∶1，依次扫描。记录辐射剂量，同时测量感兴趣区（ROIs）5个不同层面水平的脂肪和肌肉组织的图像噪声值（SD）以评价图像质量。所有扫描序列由两位资深放射医师对肺窗肺纹理及分支5分制主观评分。结果 NI=9和11时，常规CT平扫与方案A、B、C的有效剂量（E）值分别为（8.0、8.5、6.2、10.4）和（5.3、5.1、4.3、6.2）mSv。NI=9时，常规胸部CT平扫与方案A、C的SD值差异有统计学意义（F=4.496，P<0.05）；NI=11时，方案A、B、C与常规胸部CT平扫SD值差异有统计学意义（F=8.425，P<0.05）；常规胸部CT扫描中，NI分别为9和11时SD值差异有统计学意义（t=－2.570，P<0.05）；在相同能谱扫描模式中，NI不同，SD值差异均无统计学意义（P>0.05）。NI相同扫描模式不同及扫描模式相同NI不同时，CNR及主观评分差异均无统计学意义（P>0.05）。结论 合理的能谱扫描模式与常规扫描的辐射剂量没有明显差异，但能谱扫描模式可以获得较高的图像质量。此外，选择合适的噪声指数在图像质量相仿的同时可以明显降低辐射剂量。综合辐射剂量及图像质量，能谱CT智能模式可以达到辐射剂量及图像质量的双向平衡。

A study on radiation dose and image quality of chest spectral imaging and conventional CT in anthropomorphic phantom

Objective To obtain the parameters of optimal spectral scanning mode in chest spectral CT imaging, radiation doses, image quality,CNR and subjective scores are compared in spectral and conventional scan modes with anthropomorphic chest phantom. Methods An anthropomorphic chest phantom underwent both conventional scan and spectral scan using three different protocols:GSI assist mode (protocol A); GSI mode with mean tube current (protocol B);GSI mode with maximum tube current(protocol C). All above scans were performed with the noise index (NI) as 9 and 11 respectively. The radiation dose was recorded, and the SD values of adipose tissue and muscle tissue of 5 different sections were measured in regions of interest (ROIs) to evaluate the image quality. All scans were subjectively scored in 5 score system by two senior radiologists on lung markings and branches with the lung window technique. Results The effective dose (E)values of conventional CT scan and protocols A, B and C were 8.0, 8.5, 6.2, 10.4 and 5.3, 5.1, 4.3, 6.2 mSv. When NI=9, the difference of SD values had statistically significance between conventional scan and protocols A, C (F=4.496, P<0.05). When NI=11, there was statistically significant difference of SD value between conventional scan and protocols A, B, C (F=8.425, P<0.05). In conventional chest scan,the difference of SD values was statistically significant between NI=9 and 11 (t=-2.570, P<0.05), while other protocols had no statistically significant difference(P>0.05). The difference of CNR and subjective score had no statistically significance in either the same NI different scanning modes or the same scanning mode with different NIs. Conclusions Appropriate spectral scanning mode had no significant difference with conventional CT scanning in radiation dose, yet higher image quality can be obtained. Furthermore, appropriate NI can reduce the radiation dose obviously while the similar image quality was achieved. Considering the two factors, GSI assist mode can achieve balance between radiation dose and image quality.