Seed-based transrectal ultrasound-fluoroscopy registration method for intraoperative dosimetry analysis of prostate brachytherapy

Received 28 March 2007; revised 27 December 2007; accepted for publication 1 January 2008;published 12 February 2008Prostate brachytherapy is an effective treatment option for early-stage prostate cancer. During aprostate brachytherapy procedure, transrectal ultrasound TRUS and fluoroscopy imaging modali-ties complement each other by providing good visualization of soft tissue and implanted seeds,respectively. Therefore, the registration of these two imaging modalities, which are readily availablein the operating room, could facilitate intraoperative dosimetry, thus enabling physicians to implantadditional seeds into the underdosed portions of the prostate while the patient is still on the oper-ating table. It is desirable to register TRUS and fluoroscopy images by using the seeds as fiducialmarkers. Although the locations of all the implanted seeds can be reconstructed from three fluo-roscopy images, only a fraction of these seeds can be located in TRUS images. It is challenging toregister the TRUS and fluoroscopy images by using the identified seeds, since the correspondencebetween them is unknown. Furthermore, misdetection of nonseed structures as seeds can lead to theinclusion of spurious points in the data set. We developed a new method called iterative optimalassignment IOA to overcome these challenges in TRUS-fluoroscopy registration. By using theHungarian method in an optimization framework, IOAcomputes a set of transformation parametersthat yield the one-to-one correspondence with minimum cost. We have evaluated our registrationmethod at varying noise levels, seed detection rates, and number of spurious points using datacollected from 25 patients. We have found that IOA can perform registration with an average rootmean square error of about 0.2 cm even when the seed detection rate is only 10%. We believe thatIOA can offer a robust solution to seed-based TRUS-fluoroscopy registration, thus making intraop-erative dosimetry possible. ©