Prostate cancer is the most common cancer diagnosed in men in the United States. Many techniques have been developed to diagnose and treat prostate cancer. 3D Trans-Urethral Ultrasound (TUUS) is a new technique for the diagnosis and treatment of prostate disease. This research focuses on the potential of TUUS for therapy-guidance during and after transperineal interstitial permanent prostate brachytherapy (TIPPB). Computed tomography (CT) is currently used to determine the source locations and the effective radiation dose distribution throughout the tissue after the completion of the procedure. TUUS may be a viable alternative to CT for determining source locations within the prostate. Placement of the TUUS catheter into the urethra provides excellent 2D images of the prostate. In addition, the catheter can be used to acquire 3D volumetric data for 3D analysis of the prostate, associated tissue, and radioactive sources. Initial work on source localization was conducted on an ultrasound-equivalent prostate phantom. Cylindrical dummy radiation sources with diameter of.8 mm and length of 4.5 mm were placed into the prostate phantom for assessment with TUUS. The TUUS imaging device is a lOfr catheter with a linear array of ultrasonic crystals at one end. The ultrasound catheter operates at 10MHz and is controlled by the Acuson Sequoia ultrasound workstation. The catheter was placed in the phantom urethra and 3D scans were acquired. A corresponding CT was acquired for comparative purposes. Segmentation of the prostate capsule was done semi- automatically. Dummy radiation seed segmentation was conducted manually. Additional processing was necessary to account for image artifact and correctly reconstruct the seeds. The prostate shell and radioactive source reconstructions provide an excellent 3D representation. Comparison to the CT data suggests that the TUUS data provided: 1) greater resolution and 2) better soft tissue differentiation. The reconstructed sources were measured and corresponded to the physical dimensions of those placed in the phantoms. The method is now being evaluated on cadavers and patients.