Bowsher J E, Tornai M P, Peter J, González Trotter D E, Krol A, Gilland D R, Jaszczak R J
Duke University Medical Center, Durham, NC 27710, USA.
IEEE Trans Med Imaging. 2002 Mar;21(3):200-15. doi: 10.1109/42.996339.
Reconstruction algorithms for transmission tomography have generally assumed that the photons reaching a particular detector bin at a particular angle originate from a single point source. In this paper, we highlight several cases of extended transmission sources, in which it may be useful to approach the estimation of attenuation coefficients as a problem involving multiple transmission point sources. Examined in detail is the case of a fixed transmission line source with a fan-beam collimator. This geometry can result in attenuation images that have significant axial blur. Herein it is also shown, empirically, that extended transmission sources can result in biased estimates of the average attenuation, and an explanation is proposed. The finite axial resolution of the transmission line source configuration is modeled within iterative reconstruction using an expectation-maximization algorithm that was previously derived for estimating attenuation coefficients from single photon emission computed tomography (SPECT) emission data. The same algorithm is applicable to both problems because both can be thought of as involving multiple transmission sources. It is shown that modeling axial blur within reconstruction removes the bias in the average estimated attenuation and substantially improves the axial resolution of attenuation images.
透射断层成像的重建算法通常假定，在特定角度到达特定探测器单元的光子源自单个点源。在本文中，我们着重介绍了几种扩展透射源的情况，在这些情况下，将衰减系数估计视为一个涉及多个透射点源的问题可能会很有用。详细研究了带有扇形束准直器的固定透射线源的情况。这种几何结构会导致衰减图像出现明显的轴向模糊。本文还通过实验表明，扩展透射源会导致平均衰减的估计出现偏差，并提出了一种解释。使用先前从单光子发射计算机断层扫描（SPECT）发射数据中推导出来的期望最大化算法，在迭代重建过程中对透射线源配置的有限轴向分辨率进行建模。该算法适用于这两个问题，因为两者都可被视为涉及多个透射源。结果表明，在重建过程中对轴向模糊进行建模可以消除平均估计衰减中的偏差，并显著提高衰减图像的轴向分辨率。
