Target audience Interventional radiologists, researchers in medical image analysis and interventional image-guided robotics. Purpose Magnetic Resonance Imaging (MRI) is a superior modality for detection, staging, localization and characterization of Prostate cancer (PCa) [1]. Interventional applications of MRI in PCa management include MRI-guided core needle biopsy that can be recommended for some patient populations, and may lead to improved accuracy of cancer detection. Although MRI enables targeted sampling of the tissue from the suspected cancer regions, precise needle placement is challenging due to several factors that include motion of the patient and prostate gland during the course of the procedure, which may introduce discrepancies with respect to the biopsy plan established in the beginning of the procedure. The objective of this work is to retrospectively quantify the extent of prostate gland motion during transperineal MRI-guided prostate biopsy using image registration technology. Methods Image acquisition MR images were collected in the course of the MRI-guided transperineal prostate biopsies (N=40), all of which were conducted in a wide bore Siemens Magnetom Verio 3T MR scanner. The patients were immobilized on the table top with velcro wrap and sedated. Imaging studies for each patient used a combination of body coil and pelvic array receiver elements and included (1) axial FRFSE T2w MRI (voxel size 0.5×0.5×3 mm, imaging time ~4 min) obtained in the beginning of the biopsy procedure for the purposes of target identification (planning scan) and (2) a series of lower resolution T2w MRI (voxel size 0.75×0.75×3 mm, imaging time ~1 min) collected after needle placement to visually assess targeting accuracy (needle confirmation scan). Prostate gland was contoured manually in the higher resolution T2w scan as part of intra-procedural workflow. Image processing Custom deformable image registration strategy was based on the earlier developed methodology and 3D Slicer software [2,3]. Imaging studies collected for each patient were post-processed to register planning T2w scan to each of the needle confirmation scans. In each case registration was performed separately for the prostate region of interest and for the pelvic bone structures to independently quantify the motion of the prostate and the pelvis. Registration accuracy was confirmed by visual inspection of each image pair. Improvement in the alignment of prostate gland was quantified by comparing the Dice overlap coefficient between the segmentations of the prostate gland in the planning T2w scan and in the final needle confirmation image, before and after registration. Prostate motion characterization Coordinates of the centroid of the prostate gland were automatically calculated for each patient. We summarized patient-specific prostate motion by applying the registration-estimated transformations to the prostate gland centroids and recording centroid translation in antero-posterior, lateral and longitudinal directions. Prostate motion was summarized separately using the pelvis and gland ROI based registration results to assess agreement of the pelvic and prostate gland motion. 3-dimensional motion and axial in-plane 2-dimensional motion were summarized separately, since in-plane motion creates greater potential for targeting errors in the template-guided transperineal biopsy.