A MRI Compatible Device for MRI Guided Transrectal Prostate Biopsy

This paper reports a novel remotely actuated device for transrectal prostate biopsy, designed for use in a conventional high-field MRI scanner. The device provides three-dimensional MRI guided biopsy needle placement with sub-millimeter accuracy under physician control. Its compact size enables use in both closed and open configuration MRI scanners. Preliminary in-vivo canine experiments demonstrating the functionality of the device are reported. Introduction Prostate cancer is the most common type of cancer found in American men, after melanoma. The American Cancer Society predicts that in the year 2001 there will be about 198,100 new cases of prostate cancer in the United States and that about 31,500 men will die of t h s disease. The current standard of care to verify the existence of prostate cancer is transrectal ultrasound (TRUS) biopsy. Transrectal Ultrasound is not accurate for tumor localization [ l ] and therefore precludes the precise identification and sampling of individual cancerous tumor sites. Consequently, the sensitivity of TRUS biopsy is only between 60% and 85% [2-31. MRI with an endorectal coil provides images with higher spatial resolution and contrast than can be obtained using TRUS [l]. MRI guided transperineal prostate biopsy has been demonstrated inside an open MRI scanner [4]. Currently, a surgical assist robot for the procedure is under development [ 5 ] . In contrast to ths approach, we have developed and prototyped a prostate biopsy device that (a) operates inside a conventional high-field MRI scanner that has hgher SNR than most open configuration scanners, (b) employs a transrectal approach, (c) provides precise mechanical delivery of the biopsy needle, and (d) is remotely actuated from outside the gantry. Our goal with this device is to enable direct identification and biopsy of candidate prostate tumor sites, and also provide precise delivery of needles for therapeutic procedures. Methods The patient is positioned prone in the scanner, with pelvis elevated. The biopsy device houses a MRI compatible biopsy needle, and is equipped to allow precise operator control of translation, rotation, and insertion of the needle. All components are MRI compatible. Fieure 1: Design of the biopsy device (1 Rectal probe, 2 Positioning stage, 3 Needle insertion mechanism). The rectal probe (1) houses the biopsy needle and three MRI registration micro-coils. The coils serve as fiducials to provide the spatial position of the probe in the MRI coordinate system [6]. Two coils are positioned along the main axis of the probe. The third coil is positioned at a certain offset of the axis to register the rotation of the probe. The probe has a curved channel to guide the needle. A stationary endorectal sheath is designed to house an imaging coil. The positioning stage (2) provides the rotation and the translation of the probe. These motions are manually actuated from outside the MRI gantry via a mechanical linkage. A housing provides support for the shafts and connects to the mount, which is secured to the table of the scanner. The needle insertion mechanism (3) inserts the biopsy needle to a pre-determined target depth. The insertion actuation is similar to actuation in the positioning stage. Results and Discussion The feasibility of the rectal probe design (Figure 2A) was tested in-vivo on a canine subject, using a prototype of our device that accommodates a MRI compatible 18g single action biopsy device from US Biopsy. The probe was manually positioned in the rectum with the guidance of three registration coils. Figures 2B and 2C show a MRI image of the probe in the rectum and the biopsy needle placed in the prostate. In addition a fully functional prototype of the positioning stage was built and tested ex-vivo. Further experiments with the fully integrated biopsy device are in progress.