4层螺旋CT有效层厚的理论分析与实验测定

目的 通过建立数学模型,分析4层螺旋CT有效层厚的计算方法 ,并与理论计算值比较.方法 假设螺旋层灵敏度曲线(SSP)为高斯分布,SSP的半高宽(FWHM)为有效层厚,通过分析推导出采用180°线性插值法4层螺旋CT有效层厚的计算公式.使用Philips Mx 8000 4层螺旋CT对Catphan 500 CT模体的CTP 528模块进行扫描,扫描参数:管电压120 kV,管电流量150 mAs,探测器宽度0.5 mm,FOV 180 mm,重建矩阵512×512,180°线性插值,分别使用不同的螺距和探测器宽度进行扫描和重建.对于每一图像序列进行冠状位多平面重组,在经过CTP 528模块的钨质小球中心的MPR影像上测量出有效层厚.结果 采用螺距0.500、0.625、0.875、1.000、1.250、1.500、1.750测得的有效层厚分别是0.7、0.7、0.7、0.8、0.7、0.7、0.7 mm;采用探测器宽度0.5、1.0、5.0 mm测得的有效层厚分别是0.7、1.2、6.7 mm.有效层厚的实验测定值均大于理论计算值.结论 4层螺旋CT有效层厚与探测器宽度成正比;与螺距呈复杂的曲线变化关系,存在极大值和极小值.

Theoretical analysis and experimental mensuration of effective slice width for 4-slice spiral CT

Objective To establish the mathematical model and perform experimental mensuration on effective slice width for 4-slice spiral CT. Methods First, assuming the spiral slice sensitivity profile (SSP) was Gauss distribution, a formula about effective slice width, defined as the full width at half maximum (FWHM) of SSP, was get. Then, using Philips Mx8000 4-slice spiral CT system and the same CT technique as those used for clinical CT, CTP 528 of Phantom Catphan 500 was scanned with parameters as follows: 120 kV, 150 mAs, 180 mm FOV, 512×512 Matrix, 180° linear interpolation, different pitch and 0.5 mm detector width. Finally, FWHM was got from the coronal multiplanar reconstruction image passing by the center of tungsten ball in CTP528 for each sequence. Results With pitch of 0.500, 0.625, 0.875, 1.000, 1.250, 1.500 and 1.750, the measured FWHM was 0.7, 0.7, 0.7, 0.8, 0.7, 0.7 and 0.7 mm, respectively. With detector width of 0.5, 1.0 and 5.0 mm, the measured FWHM was 0.7, 1.2, and 6.7 mm, respectively. All the measured effective slices were higher than those of theoretical values. Conclusion The relativity between effective slice width and detector width is direct proportion, while that between effective slice width and pitch is non-linear.