Fluoroscopy to ultrasound image registration using implanted seeds as fiducials during permanent prostate brachytherapy

A method using implanted seeds as fiducials to register ultrasound (US) images with fluoroscopic images for prostate brachytherapy dose analysis is proposed. In a simulation study, transformed point clouds with 154 points were sampled at different sampling rates with different levels of noise applied and then registered with the original imaging data. Superior performance in comparison to conventional four point fiducial registration was demonstrated. The root-mean-squared-distance at registration was 0.962mm when 25% of the points were used as fiducials and with noise level at 3mm. A phantom with 64 implanted seeds was scanned by CT at 1.5mm intervals and by step-section US at 2.5mm intervals. Fluoroscopic images of the phantom were also taken at several different projection angles. Coordinates of implanted seeds were determined for each imaging modality. CT-US and fluoroscopy-US registration were then carried out using the implanted seeds as fiducials. Over 90% overlap between the segmented CT prostate volume and US prostate volume was observed at registration, and the distance between the centers of the registered volumes was 3mm. The mean distance between the seed coordinates at registration was 2.5mm for CT and US, and 3mm for fluoroscopy and US. These results suggest that registration of fluoroscopic images with US images of the prostate can be effectively accomplished by using implanted seeds as fiducials. Consequently, accurate US-fluoroscopic image registration should facilitate intraoperative radiation dosimetry for permanent prostate brachytherapy.