Dept. of Radio Eng., Beijing Univ. of Posts and Telecommun.
IEEE Trans Med Imaging. 1993;12(3):555-9. doi: 10.1109/42.241883.
A two-dimensional (2-D) search flow scheme is proposed for the incremental backprojection algorithm. In this scheme, the searching direction in a beam is determined only by the distance from the current pixel to the corresponding ray; no pixels outside the beam are involved in the procedure. In addition, the geometrical relationships between the image grids and the projection rays in different views are fully exploited to further reduce the search operations. It is found that backprojection of four views can be performed at the same time, and the search operation is only required for one of them if the number of views is even. Thus, backprojection is accomplished on a quartet-by-quartet of views basis as well as on beam-by-beam basis. These approaches not only minimize the number of search operations but also possess a simple algorithm structure. Implementation results obtained with an AST-386 computer for the incremental backprojection algorithm using this search flow scheme and the relationship among projection views show that the processing time can be reduced by a factor of about 2.
提出了一种二维（2-D）搜索流程方案，用于增量反向投影算法。在该方案中，仅根据当前像素与相应射线之间的距离来确定波束中的搜索方向；不涉及波束外的像素。此外，还充分利用了不同视图中图像网格和投影射线之间的几何关系，以进一步减少搜索操作。结果发现，如果视图数为偶数，则可以同时进行四视图的反向投影，并且仅需要对其中之一进行搜索操作。因此，反向投影是基于 quartet-by-quartet 的视图和 beam-by-beam 的基础上进行的。这些方法不仅最小化了搜索操作的数量，而且还具有简单的算法结构。使用 AST-386 计算机实现该搜索流程方案和投影视图之间的关系的增量反向投影算法的实现结果表明，处理时间可以减少约 2 倍。
