Smith M F, Jaszczak R J
Department of Radiology, Duke University Medical Center, Durham, NC, USA.
Phys Med Biol. 1998 Apr;43(4):761-75. doi: 10.1088/0031-9155/43/4/006.
Photons penetrate the attenuating material close to the aperture of pinhole collimators in nuclear medicine, broadening the tails of point spread functions (PSFs) and degrading the resolution of planar and SPECT images. An analytic approximation has been developed that models this penetration contribution to the PSF for knife-edge point pinhole apertures. The approximation has the form exp(-gamma r), where r is the distance on the detector surface from the projection of the point source through the pinhole. The rolloff coefficient gamma is a function of the photon energy, point source location and the design parameters of the collimator. There was excellent agreement between measured values of gamma from photon transport simulations of I-131 point sources (364 keV emission only) and theoretical predictions from the analytic formula. Predicted gamma values from the analytic formula averaged 25% greater than measured values from experimental I-131 point source acquisitions. Photon transport simulations were performed that modelled the 364 keV and less abundant 637 and 723 keV emissions and scatter within the scintillation crystal. Measured gamma values from these simulations averaged 12% greater than the experimental values, indicating that about half of the error between the analytic formula and the experimental measurements was due to unmodelled 637 and 723 keV emissions. The remaining error may be due in part to scatter in the pinhole region and backscatter from gamma camera components behind the scintillation crystal. The analytic penetration model was used in designing Metz filters to compensate for penetration blur and these filters were applied to the projection data as part of 3D SPECT image reconstruction. Image resolution and contrast were improved in simulated and experimental I-131 tumour phantom studies. This analytic model of pinhole aperture penetration can be readily incorporated into iterative 3D SPECT pinhole reconstruction algorithms.
在核医学中，光子会穿透靠近针孔准直器孔径的衰减材料，从而使点扩散函数（PSF）的尾部变宽，并降低平面和单光子发射计算机断层扫描（SPECT）图像的分辨率。已开发出一种解析近似方法，用于模拟这种穿透对刀刃形点针孔孔径PSF的贡献。该近似方法具有exp(-γr)的形式，其中r是探测器表面上从点源通过针孔的投影的距离。衰减系数γ是光子能量、点源位置和准直器设计参数的函数。对于I-131点源（仅发射364 keV）的光子传输模拟得到的γ测量值与解析公式的理论预测之间具有极好的一致性。解析公式预测的γ值平均比实验I-131点源采集得到的测量值大25%。进行了光子传输模拟，对364 keV以及丰度较低的637 keV和723 keV发射以及闪烁晶体内的散射进行了建模。这些模拟得到的γ测量值平均比实验值大12%，这表明解析公式与实验测量之间约一半的误差是由于未建模上述637 keV和723 keV发射造成的。其余误差可能部分归因于针孔区域的散射以及闪烁晶体后方γ相机组件的反向散射。解析穿透模型被用于设计梅茨滤波器以补偿穿透模糊，并且这些滤波器作为三维SPECT图像重建的一部分应用于投影数据。在模拟和实验I-131肿瘤模型研究中，图像分辨率和对比度得到了改善。这种针孔孔径穿透的解析模型可以很容易地纳入迭代三维SPECT针孔重建算法中。
