Plastic Bearings and Linear Guides Put the Moves on MRI-guided Robot

Edited by: Leslie Langnau, Managing Editor

Equipment for use with MRIs faces several unique challenges, not the least of which is dealing with the magnetics. These researchers met those challenges with piezoelectric and plastic components.

MRI compatible piezoelectricactuated robot is made of ABS plastic. PhD candidate Hao Su and Professor Gregory Fischer used a Stratasys Dimension system to prototype and then produce the small robot.

Mechanical systems play crucial, functional roles in any mechatronic design. For PhD candidate Hao Su and Professor Gregory Fischer, from the Worcester Polytechnic Institute in Massachusetts, the choice of DryLin T linear guide systems and iglide plastic plain bearings from igus were important to facilitate the different motion needs of a robot design built to work inside a magnetic resonance imaging (MRI) machine.

Each year approximately 1.5-million core-needle biopsies are performed, leading to 220,000 newly diagnosed cases of prostate cancer. In the quest to improve the way prostate cancer is detected and treated, Su and Fischer developed an MRI compatible piezoelectric-actuated robot that works inside a 3T Tesla closed-bore MRI with real-time image guidance.

Transrectal ultrasound, or “TRUS,” is currently hailed as the best treatment for this type of cancer because it is low in cost and easy to administer quickly. However, TRUS-guided biopsy has a detection rate of only 20 to 30%, and cannot effectively monitor the implantation and placement of radioactive seeds used during treatment.

TRUS is currently considered the gold standard for guiding biopsy and brachytherapy, however, MRI-based medical diagnosis offers some unique benefits. Brachytherapy—defined as a form of radiotherapy where a radiation source is placed inside or next to the area requiring treatment—is often delivered in low doses using rice-grain-sized radioactive seeds placed in an irregular, complex pattern inside the prostate. Although this treatment is effective, a teleoperated robotic system can increase the accuracy of seed delivery while at the same time minimizing tissue damage and avoiding non-tissue structures.

teleoperated-robot--inside-a-3T-Tesla-closed-bore-MRI

The teleoperated robot is used inside a 3T Tesla closed-bore MRI. Surgeons use its real-time image guidance to position and insert a bevel tip needle. Because the needle trajectory is often curved, robot control helps the surgeon precisely position radioactive “seeds” inside the prostate. The patient lies supine on the MRI table bed, and the robot is placed on the bed near him. Because of the size of the MRI, the robot must be small as well as immune to the MRI’s magnetic fields

The WPI team designed the small teleoperated robotic system specifically for prostate percutaneous procedures taking place under MRI guidance. The slave robot features real-time in situ needle steering capability in 3-Tesla MRI with a high-resolution fiber optic force sensor that measures the needle insertion force, which can be inferred to increase the cancer detection rate.

The needle placement robot consists of a 3-degrees of freedom (DOF) Cartesian positioning module and 3-DOF needle-driver module. The robot uses DryLin T linear guide systems and iglide plastic plain bearings to facilitate different types of motion. As part of its Y.E.S. (Young Engineers Support) Program designed to foster the mechanical design ideas for those with a passion for engineering, igus supplied the WPI team with the parts at no cost. The DryLin T linear guides facilitate translational motion of the positioning module, which provides gross positioning for the robot’s needle driver. The needle driver is a vital part of the system, as it enables the rotation and translational movement of the needle cannula: a flexible tube inserted into the patient’s body cavity for MRI-guided diagnosis and therapy.

The needle driver has a needle guide sleeve, a collet locking mechanism and passive optical tracking fiducial frame. Two iglide A200 plastic plain bearings are used in the front and rear of the driver to constrain the needle guide. The bearings enable the robot’s motor to rotate the needle using the collet mechanism by way of a timing belt. This rotating needle reduces tissue damage while enhancing targeting accuracy. Another 10 iglide bearings were used to create a revolute joint, also known as a “pin joint” or “hinge joint,” to provide single-axis rotation.

A perfect picture with plastics
Since the robot operates inside a 3-Tesla, closed-bore MRI machine with only a 500-millimeter cylinder diameter, there were a number of elements that had to be taken into consideration during the design process. Specifically for the robot’s mechanical system, the components had to be non-ferrous for the patient’s safety during the radiation process, but also compact due to the limited installation space. The robot itself is about 8 in. by 3 in. Drylin T linear guide systems and iglide A200 plastic plain bearings suited these needs.

Drylin T linear guide systems and iglide A200 plastic plain bearings met the motion as well as the size and non-ferrous needs of the designers.

DryLin T linear guides are comprised of hard-anodized aluminum rails and carriages and high-performance plastic sliding elements, which do not interfere with the MRI procedure. The linear slides operate without messy lubrication, which is important in a sterile medical environment. They also feature a lower-profile for applications where installation space is an issue.

iglide A200 plastic plain bearings are comprised of FDA-compliant polymers specifically designed for applications with contact to food or drugs. Like DryLin T, the iglide bearings also operate dry running to prevent contamination.

Said Su, “There are very few products on the market that both satisfied our requirements and delivered a good mechanical performance. igus helped us address this issue by donating its products. The linear slides and plastic bearings were our first choice because of their compatibility with MRI. They are also smooth and quiet during operation and have excellent wear resistance. This robot is the first needle placement robot under real-time MRI guidance and it would significantly enhance biopsy and brachytherapy accuracy, and thus dramatically elevate the efficacy of prostate cancer prevention and management.”

Most recently, thanks to its innovative design and potential for changing the face of prostate cancer treatment, the MRI-guided needle placement robot was accepted into the 2011 IEEE International Conference on Robotics and Automation. The conference will focus on technology in robotics and automation that aims to help build a better human society.

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