Siddon R L, Chin L M
Med Phys. 1985 Jan-Feb;12(1):77-83. doi: 10.1118/1.595738.
We have developed a new isocentric two-film reconstruction algorithm for brachytherapy seed and needle implants. The algorithm has no requirements that the two films be orthogonal, symmetric, or even be taken in a transverse plane. In addition, there is no requirement that the two films even have the same number of images. We have found removal of these usual constraints useful for head and neck implants where images are often obscured by patient anatomy. The inherent image matching ambiguities associated with traditional two-film techniques are minimized by considering the image end points, rather than just the image centroids. For two films, the new algorithm, which considers all image combinations at one time, matches all the end-point images on one film with those on the other, and then reconstructs the end-point positions of the seeds. The algorithm minimizes the difference between the actual images and the projected images from the reconstructed seeds. The new two-film image matching problem is shown to be equivalent to the well-known assignment problem. For an implant of N seeds, this equivalence allows the two-film problem to be solved by an algorithm (ACM algorithm 548) that scales with a polynomial power of N, rather than N! as is usually assumed. An implant of N seeds can be matched and reconstructed in approximately (N/20)2s on a VAX 11/780.
我们已经开发出一种用于近距离放射治疗种子源和针植入物的新型等中心双片重建算法。该算法不要求两片相互正交、对称，甚至不要求在横向平面内拍摄。此外，也不要求两片具有相同数量的图像。我们发现去除这些通常的限制对于头颈部植入物很有用，因为在头颈部植入中，图像常常被患者的解剖结构遮挡。通过考虑图像端点而非仅仅是图像质心，与传统双片技术相关的固有图像匹配模糊性被最小化。对于两片，新算法一次性考虑所有图像组合，将一片上的所有端点图像与另一片上的端点图像进行匹配，然后重建种子源的端点位置。该算法使实际图像与重建种子源的投影图像之间的差异最小化。新的双片图像匹配问题被证明等同于著名的分配问题。对于有N个种子源的植入物，这种等效性允许通过一种与N的多项式幂成比例缩放的算法（ACM算法548）来解决双片问题，而不是像通常所假设的那样与N!成比例。在VAX 11/780计算机上，一个有N个种子源的植入物大约可以在(N/20)²秒内完成匹配和重建。
