Ring navigation: an ultrasound-guided technique using real-time motion compensation for prostate biopsies.

Prostate cancer has the second highest noncutaneous cancer incidence in men. Three-dimensional (3D) transrectal ultrasound (TRUS) fused with a magnetic resonance image (MRI) is used to guide prostate biopsy as an alternative technique to conventional 2D TRUS sextant biopsy. The TRUS-MRI fusion technique can provide intraoperative needle guidance to suspicious cancer tissues identified on MRI, increasing the targeting capabilities of a physician. Currently, 3D TRUS-MR guided biopsy suffers from image and target misalignment caused by various forms of prostate motion. Thus, we previously developed a real-time motion compensation algorithm to align 2D and 3D TRUS images with an update rate around an ultrasound system frame rate. During clinical implementation, observations of image misalignment occurred when obtaining tissue samples near the left and right boundaries of the prostate. To minimize transducer translation on the rectal wall and avoid prostate motion and deformation, we are proposing the use of a 3D model-based ring navigation procedure. This navigation keeps the transducer positioned towards the centroid of the prostate when guiding the tracked biopsy gun to targets. Prostate biopsy was performed on three patients while using real-time motion compensation in the background. Our navigation approach was compared to a conventional 2D TRUS-guided procedure using approximately 20 2D and 3D TRUS image pairs and resulted in median [first quartile, third quartile] registration errors of 2.0 [1.3,2.5] mm and 3.4 [1.5, 8.2] mm, respectively. Using our navigation approach, registration error and variability were reduced, potentially suggesting a more robust technique when performing continuous motion compensation.