Robot for Magnetic Resonance Imaging Guided Focal Prostate Laser Ablation1

Abstract

Approximately, 240,000 men in the U.S. are diagnosed with prostate cancer annually [1]. The majority of these cases represent low-risk, organ-confined disease for which targeted therapy has emerged as a treatment alternative that spares patients from undesired side effects such as impotence and incontinence [2]. Focal laser ablation (FLA) utilizes a diode laser catheter to generate a well-controlled ablation zone, causing rapid heating of targeted cancerous tissue, and leaving the majority of the surrounding gland intact. While FLA for localized prostate cancer is receiving increased attention due to its minimally invasive nature, the procedure has several technical limitations. Most notable are the difficulty of (1) effectively localizing the prostate tumor according to the treatment planning, (2) safely placing the laser catheter to ablate the entire tumor and achieve adequate margins, and (3) accurately monitoring the ablated area. Larger tumors require multiple catheter placements, which can be difficult to achieve in an accurate and repeatable manner using current free-hand/template-based techniques.A carefully designed robotic system can simplify magnetic resonance imaging (MRI)-guided prostate therapy by:In this paper, a robotic system for MRI-guided prostate FLA is presented. We hypothesized that accurate placement of laser catheters to planned tumor locations could be achieved under MRI guidance and robotic positioning, critically important to avoid unnecessary gland punctures as well as to ensure the ablation treatment covering full volume of the tumor. This would maximize the utility of a minimally invasive system. Preoperative parametric MRI information together with intraoperative magnetic resonance (MR) thermometry imaging could update the plan iteratively after each ablation [3].The goal of developing an MR-conditional robotic system imposes several limitations on the use of materials to be plastics and small amounts of nonferromagnetic metals for patient safety and preserving image quality. Spatial limitations imposed by the small cylindrical bore (55–70 cm) of an MRI scanner and workspace required by prostate targeting dictate the available volume for the robot to occupy and the volume over which the robot must provide targeting capabilities. The prostate targeting volume was determined to be an approximately 50 mm square region in the transverse plane; open space between the needle guide and the bore opening was needed to allow operation.A slim robot design built around a Core-XY belt system was chosen as a solution to the design requirements (Fig. 1(a)). The Core-XY system utilizes a single belt, in which the targeting is controlled by two pneumatic motors to position the end effector of the robot. Each motor integrates a pair of fiber-optic lines, generating quadrature pulses for positional encoding with a resolution of 0.005 deg. The end effector movement (ΔX, ΔY) is related to the rotation of two motors (ΔA, ΔB) mathematically as follows:(1)ΔX=12(ΔA+ΔB)(2)ΔY=12(ΔA−ΔB)(3)ΔX+ΔY=ΔA=npulses,A4(2πrpulley)(4)npulses,A=2ΔAπrpulleyThe robot registration to the MRI coordinate system was achieved using five gadolinium fiducial markers (PinPoint, Beekley, Inc., Bristol, CT) mounted on the robot. This robot was designed for use in conjunction with Visualase® laser ablation catheter kits and its laser generator.A remote insertion guide (Fig. 1(b)) placed proximal to the surgeon allowed the user standing at the bore opening to insert the catheter under real-time MRI guidance and without needing to remove the patient from the bore. After positioning the needle guide, the remote insertion guide was aligned manually with the robot by the surgeon, and the catheter was inserted through the guide toward the robot. The alignment cone (Fig. 1(c)) corrected any misalignment caused by the distance between the robot and the remote insertion guide by channeling the inserted catheter to the end effector needle guide.A LabVIEW™ (Nat