Development and Evaluation of an Actuated MRI-Compatible Robotic System for MRI-Guided Prostate Intervention

Abstract

This paper reports the design, development, and magnetic resonance imaging (MRI) compatibility evaluation of an actuated transrectal prostate robot for MRI-guided needle intervention in the prostate. The robot performs actuated needle MRI-guidance with the goals of providing (i) MRI compatibility, (ii) MRI-guided needle placement with accuracy sufficient for targeting clinically significant prostate cancer foci, (iii) reducing interventional procedure times (thus increasing patient comfort and reducing opportunity for needle targeting error due to patient motion), (iv) enabling real-time MRI monitoring of interventional procedures, and (v) reducing the opportunities for error that arise in manually actuated needle placement. The design of the robot, employing piezo-ceramic-motor actuated needle guide positioning and manual needle insertion, is reported. Results of a MRI compatibility study show no reduction of MRI signal-to-noise-ratio (SNR) with the motors disabled. Enabling the motors reduces the SNR by 80% without RF shielding, but SNR is only reduced by 40% to 60% with RF shielding. The addition of radio-frequency shielding is shown to significantly reduce image SNR degradation caused by the presence of the robotic device. An accuracy study of MRI-guided biopsy needle placements in a prostate phantom is reported. The study shows an average in-plane targeting error of 2.4 mm with a maximum error of 3.7 mm. These data indicate the system's needle targeting accuracy is similar to that obtained with a previously reported manually actuated system, and is sufficient to reliably sample clinically significant prostate cancer foci under MRI-guidance.

Fig. 1

CAD model of the APT-III actuated robot for prostate intervention, showing the actuated robot (motor housing, translation stage, rotation stage, steerable needle guide, and endorectal probe with of steerable needle guide and integral MR antenna.) The robot is carried on a passive positioning arm attached to a linear slide on the MR scanner table. Biopsy gun and outline of prostate are shown, indicating prone positioning in a transrectal prostate biopsy procedure. The APT-III employs the rectal probe, steerable needle guide, and passive arm designed originally for the APT-II; all other parts are newly designed specifically for the APT-III.

Fig. 2

Close-up photograph of the APT-III actuated robot for prostate intervention with an automatic biopsy needle inserted in needle guide.

Fig. 3

CAD model of the rotation stage for the actuated robot. Three pairs of HR-1 Nanomotion motors rotate a ceramic ring placed on the rotation shaft.

Fig. 4

CAD model of the translation stage for the actuated robot. The translation stage controls the tilt angle of the steerable needle guide within the transrectal probe. A pair of HR-4 Nanomotion motors pushes on ceramic drive strips and provides linear motion of a drive shaft.

Fig. 5

Photograph of the controller box designed to be placed inside the MRI scanner room near the control room wall waveguide that allows passage of electrical and fiber-optic cables. The box contains motor amplifiers, motion controller, and Ethernet media converter.

Fig. 6

Targeting images, biopsy needle confirmation images, glass needle confirmation images and in-plane errors for seven biopsies of a prostate phantom using the actuated transrectal prostate robot. First row: A target (cross hairs) is selected on axial TSE T2 weighted images. Second row: The biopsy needle tip void is visualized in an axial TSE proton density image. The desired target approximately matches the actual position of the needle. Third row: The glass needle tip void is visualized in an axial TSE proton density image. The void for the glass needle is much smaller than for the biopsy needle and closer to the selected target. Numbers indicate the in-plane needle targeting error for the needle placement.

Fig. 7

Experimental setup for SNR tests in a 3 T MRI scanner. Left image shows unshielded robot, saline phantom, and imaging coils. Right image shows the robot with the addition of radio-frequency (RF) shielding.

Fig. 8

MRI Signal-to-Noise Ratio (SNR) for T1 scans (top), T2 scans (middle), and TFE scans (bottom). T1, T2, and TFE MRI scan parameters are given in Table I. Eleven different robot test configurations (labeled 1–11) are given in Table II.

Fig. 9

Representative T2 phantom images from the SNR tests showing (from top to bottom) 1. Baseline; 2,3: Off; 4,5: Disabled; 6,7: Roll; 8,9: Pitch; and 10,11: Both. Left images are for the case of the unshielded robot. Right images are for the case of the shielded robot. T2 MRI scan parameters are given in Table I. Eleven different robot test configurations (labeled 1–11) are given in Table II. SNR values are given in parenthesis for each scan.