Image registration accuracy of a 3-dimensional transrectal ultrasound-guided prostate biopsy system.

OBJECTIVE For a follow-up prostate biopsy procedure, it is useful to know the previous biopsy locations in anatomic relation to the current transrectal ultrasound (TRUS) scan. The goal of this study was to validate the performance of a 3-dimensional TRUS-guided prostate biopsy system that can accurately relocate previous biopsy sites. METHODS To correlate biopsy locations from a sequence of visits by a patient, the prostate surface data obtained from a previous visit needs to be registered to the follow-up visits. Two interpolation methods, thin-plate spline (TPS) and elastic warping (EW), were tested for registration of the TRUS prostate image to follow-up scans. We validated our biopsy system using a custom-built phantom. Beads were embedded inside the phantom and were located in each TRUS scan. We recorded the locations of the beads before and after pressures were applied to the phantom and then compared them with computer-estimated positions to measure performance. RESULTS In our experiments, before system processing, the mean target registration error (TRE) +/- SD was 6.4 +/- 4.5 mm (range, 3-13 mm). After registration and TPS interpolation, the TRE was 5.0 +/- 1.03 mm (range, 2-8 mm). After registration and EW interpolation, the TRE was 2.7 +/- 0.99 mm (range, 1-4 mm). Elastic warping was significantly better than the TPS in most cases (P < .0011). For clinical applications, EW can be implemented on a graphics processing unit with an execution time of less than 2.5 seconds. CONCLUSIONS Elastic warping interpolation yields more accurate results than the TPS for registration of TRUS prostate images. Experimental results indicate potential for clinical application of this method.