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中文摘要：
目的 观察C型臂锥形束CT成像偏离正中心点对图像噪声及影像变形程度的影响。方法 采用Siemens Axiom Artis Zee血管机，DynaCT成像模式，设置成像时间为20 s。①标准水模实验：对CT标准质控水模进行成像，选择水模图像密度均匀且伪影少的20个层面，分别测量前（A）、后（P）、左（L）、右（R）方向上正中心点和偏离正中心点2、4、6、8、10 cm位置图像的X线衰减值及其标准差；以X线衰减值标准差为图像噪声，采用单因素方差分析法进行客观评价。②钢球模型实验：分别将直径为17.20 mm钢球分别放于前后（A-P）、左右（L-R）及头足（H-F）方向正中心和偏离正中心2、4、6 cm处进行成像；在Leonardo工作站上重建三维图像，测量每个钢球在A-P、L-R及H-F方向上的直径，计算在各方向上钢球影像的变形度。结果 C型臂锥形束CT成像中，在A、P、L、R各方向上，随偏离正中心点距离增大，噪声值逐渐减小（P均〈0.05）。在A、P、L、R、H、F方向上，随着偏离正中心点距离增加，不同方向钢球直径变形度有所增加；H-F方向上偏离正中心点时，钢球H、F方向直径变形度相对较小（变形度绝对值均〈0.2%）；P方向上，以A-P方向偏离正中心点时，钢球直径变形度最小。结论 C型臂锥形束CT成像中，在偏离正中心点的各个方向的不同位置，图像噪声和影像变形程度均存在一定差异。
英文摘要：
Objective To investigate the impact of offsetting isocenter point on image noise and deformations in C-arm cone beam CT. Methods C-arm cone-beam CT was performed on QA phantom and steel ball using DynaCT imaging mode and setting the acquisition time to 20 s by Siemens Axiom Artis Zee DSA. ①QA phantom experiment： Twenty imaging slices with uniformed density and less image artifacts were selected. Attenuation values and their standard deviations were measured in the center point and off-center 2, 4, 6, 8, 10 cm on the anterior （A）, posterior （P）, left （L） and right （R） direction, respectively. The measured standard deviations of attenuation values were calculated as image noise levels to evaluate the image quality. One-way ANOVA analysis of variance was used to assess the differences among noise levels. ②Steel ball experiment： The steel balls were placed on the center point and off-center 2, 4, 6 cm on the anterior-posterior （A-P）, left-right （L-R）, head-foot （H-F） direction, respectively. Three-dimensional images were reconstructed in Leonardo workstation, and the diameters of the steel balls in A-P, L-R and H-F direction were measured and used to calculate the degrees of deformations. Results As the offsetting center distance increased in A, P, L, R direction, image noise of C arm cone beam CT imaging decreased （all P〈0.05）. C arm cone beam CT imaging of the balls would get deformative to some extent when placed offsetting center in A, P, L, R, H, F direction, and as the distances increased, the degree of the diameter deformations in different direction increased. The diameter in H-F direction had relatively small degree of deformation （the absolute value less than 0.2%） when offset center point in H and F direction. When offset center point in P direction,the degree of diameter deformations in A-P direction was the least. Conclusion Image noise and deformations are statistically different when offset center point in different direction and different distances in C ar
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