16层螺旋CT中混叠假影的分析

Analysis of aliasing artifacts in 16-slice helical CT

目的认识CT图像中混叠假影(aliasingartifacts)的表现,并探讨成像参数对它的影响。方法使用PHILIPSBrilliance16SCT机器,在不同的扫描和重建参数条件下,扫描浸入水中的成人干颅,对所得原始横断图像在假影方面作出比较评价。结果大部分方案图像可见混叠假影,表现为在纵轴方向上有着径线急剧变化的高密度物体边缘发出的轮辐状假影。探测器组合16×1·5、重建层厚2mm时,螺距为0·3、0·6和0·9mm的图像均见混叠假影,其程度随螺距增大而加重;探测器组合16×0·75、螺距0·6时,重建层厚为0·8和1mm的图像均有混叠假影,且0·8mm图像较1mm图像严重,2mm图像未见混叠假影;螺距0·6、重建层厚2mm时,探测器组合16×0·75的图像未见混叠假影,16×1·5的图像存在该假影;重建间隔和管电流的变化对混叠假影无影响。结论采样不足引起混叠假影,可酌情选择薄的采集层厚、小的螺距和稍宽的重建层厚来抑制或消除这种假影。

Objective To recognize the features of aliasing artifacts on CT images, and to investigate the effects of imaging parameters on the magnitude of this artifacts. Methods An adult dry skull was placed in a plastic water-filled container and scanned with a PHILIPS 16-slice helical CT. All the acquired transaxial images by using several different acquisition or reconstruction parameters were examined for comparative assessment of the aliasing artifacts. Results The aliasing artifacts could be seen in most instances and characterized as the spokewisc patterns emanating from the edges of high contrast structure as its radius varies sharply in the longitudinal direction. The images that scanned with pitch of 0.3, 0.6 and 0.9, respectively, showed aliasing artifacts, and its severities increased with pitches escalated （detector combination 16×1.5, reconstruction thickness 2 ram）; There were more significant aliasing artifacts on the images reconstructed with 0.8 mm slice width compared with 1-mm slice width, and no aliasing artifacts were observed on the images reconstructed with 2-mm slice width （detector combination 16 × 0.75, pitch 0.6）; No artifacts were perceived on the images scanned with detector combination 16 × 0.75, while presented evidently with the use of detector combination 16 × 1.5 （pitch 0.6, reconstruction thickness 2 mm） ; The degrees of aliasing artifacts were unaltered when reconstruction interval and tube current changed. Conclusions Aliasing artifacts are caused by undersampling. When the operator choose the thinner sampling thickness, lower pitch and a much wider reconstruction thickness judiciously, aliasing artifacts could be effectively mitigated or suppressed.