In prostate brachytherapy, a transrectal ultrasound (TRUS) will show the prostate boundary but not all the
implanted seeds, while fluoroscopy will show all the seeds clearly but not the boundary. We propose an intensity-based
registration between TRUS images and the implant reconstructed from fluoroscopy as a means of achieving
accurate intra-operative dosimetry. The TRUS images are first filtered and compounded, and then registered to
the fluoroscopy model via mutual information. A training phantom was implanted with 48 seeds and imaged.
Various ultrasound filtering techniques were analyzed, and the best results were achieved with the Bayesian
combination of adaptive thresholding, phase congruency, and compensation for the non-uniform ultrasound
beam profile in the elevation and lateral directions. The average registration error between corresponding seeds
relative to the ground truth was 0.78 mm. The effect of false positives and false negatives in ultrasound were
investigated by masking true seeds in the fluoroscopy volume or adding false seeds. The registration error
remained below 1.01 mm when the false positive rate was 31%, and 0.96 mm when the false negative rate was
31%. This fully automated method delivers excellent registration accuracy and robustness in phantom studies,
and promises to demonstrate clinically adequate performance on human data as well.