利用MTF法分析Varian 23 EX锥形束CT图像空间分辨率

Evaluation of Spatial Resolution Characteristic of Varian 23 EX Cone-beam CT with MTF

目的:研究在扫描范围内不同空间位置处锥形束CT图像的空间分辨率。方法:使用安装在Varian 23 EX直线加速器上的锥形束CT,重复扫描在不同位置处的Catphan 504模体。模体依次放置于加速器等中心、偏心3 cm、偏心6 cm、偏心12 cm以及不同Z轴位置。扫描采用标准头部模式,重建层厚为2.5 mm。然后利用Matlab 7.0编写程序,测量不同位置处的点扩散函数(PSF),并对PSF进行二维傅里叶变换得到调制传输函数(MTF)。结果:锥形束CT在等中心平面内的空间分辨率最可高达50%MTF=8.15 Lp/cm,并沿着X、Y轴方向偏离等中心时,分辨率逐渐下降。在X=±6 cm、Y=±12 cm时,空间分辨率分别降为50%MTF=5.24 Lp/cm和50%MTF=0.64 Lp/cm;同样的趋势也出现在Z轴方向,当Z=±5 cm时50%MTF=7.16 Lp/cm,Z=±8 cm时50%MTF=4.09 Lp/cm;在Z轴方向上空间分辨率这种变化显然没有X、Y轴方向明显。结论:对于附加在Varian 23 EX直线加速器上的锥形束CT,其图像空间分辨率因扫描野中不同位置而有所变化,因此建议在患者复位时尽量将靶区中心放在等中心左右前后10 cm的空间区域内,在此范围内边缘和中心点可获得接近一致的空间分辨率。

Objective This research is mainly about the spatial resolution of the cone-beam CT images at the different spatial locations in the scanning area.Methods The Catphan 504 phantom at the different locations is scanned repeatedly by using the Cone-beam CT fixed on the Varian 23 EX linear accelerator.The phantom is located the isocenter point,eccentric 3 cm,eccentric 6 cm,eccentric 12 cm and different points on the Z-axis successively on the accelerator.The scanned mode is the standard head mode.The reconstructive thickness is 2.5 cm.The PSFs on the different locations are measured via the program composed by Matlab 7.0 and then done the Fourier transformation to the MTF.Results the maximum spatial resolution on the isocenter flat can reach 50% MTF=8.15 Lp/cm,and the spatial resolution reduces gradually while the location deviates from the isocenter point at the direction of X-axis and Y-axis.The spatial resolutions on the X=±6 cm and the Y=±12 cm reduce to 50% MTF=5.24 Lp/cm and 50% MTF=0.64 Lp/cm respectively.Such trend is expressed on the Z-axis.The spatial resolutions on the Z=±5 cm and the Z=±8 cm reduce to 50% MTF=7.16 Lp/cm and 50% MTF=4.09 Lp/cm respectively.However the reduction of spatial resolution on the Z-axis is less notable than the other two axis.Conclusion The images spatial resolution of the cone-beam CT fixed on the Varian 23 EX linear accelerator are varied while the scanning locations are changed.Therefore,we suggest that it is better to place the target center in the spherical area around the isocenter,the diameter of which must be less than 10 cm.In such area,the spatial resolution at the fringe is almost similar with that at the centre point.