Brachytherapy seed localization via iterative forward projection matching (IFPM) algorithm using intraoperative cone-beam-CT sinogram projections

Purpose: To experimentally validate a new algorithm for reconstructing the 3D positions of implanted brachytherapy seeds from intraoperatively acquired 2D cone-beam CT sinogram projections. Methods and materials: The iterative forward projection matching (IFPM) consists of finding the 3D seed geometry that minimizes the sum-of-squared intensity differences between computed projections of the candidate seed configuration and experimentally acquired auto-segmented projections of the implanted seeds. Projections are convolved by 2D Gaussian kernel to provide gradient, necessary for convergence. Four Pd¹⁰³ post-implant patients were scanned using an Acuity digital-simulator with a full 660 projections conebeam-CT for post-implant dosimetry. 3-10 x-ray images were selected from the CBCT sinogram and were preprocessed to create the binary seed images. The IFPM accuracy was quantified both at projections and 3D. Results: For all example cases, the mean registration error was found to be about 1.2mm; the IFPM converged in 16-21 iterations and in 4-7 iterations following reduction of Gaussian blurring function width with computation time of about 1.9-2.3 minutes/ iteration on a 2 GHz processor. Discussion: The IFPM algorithm avoids the need for matching corresponding seeds images on each projection as required by standard back-projection methods and has a potential to accommodate incomplete projection data. The method has been found to work robustly with unknown number of seeds and uncertainties in the imaging viewpoints.