Prostate cancer is the most common cancer in males and the second most common in humans. Transrectal ultrasound (TRUS) imaging is the gold-standard guide for biopsies to detect prostate cancer and for brachytherapy treatment, where radioactive seeds are implanted in the prostate close to tumors. However, the limited resolution of TRUS imaging restricts the biopsy detection rate to about 25 percent and renders the brachytherapy seeds almost invisible, making it difficult to implant them accurately.
Observing prostate biopsies and brachytherapy under the superior image quality of magnetic-resonance imaging (MRI) would substantially improve diagnostic and therapeutic procedures. MRI lets doctors see potential tumors so they can more accurately determine where to take biopsy samples or deliver treatment. MRI also shows the implanted seeds in its images, enabling more accurate placement and a greater chance of eliminating the cancer with less damage to healthy tissues.
Better Than Metal
But the confined space and powerful magnetic fields (about 100,000 times stronger than the earth’s magnetic field) inside an MRI system create major challenges in accurately placing needles. Steel and other ferrous materials are unsuitable because the MRI’s powerful magnets would attract them. Even nonferrous metals must be kept to a minimum because electromagnetic fields inside an MRI generate eddy currents that could produce excessive heat or distort the imaging. As a result, traditional sensors, actuators and materials that are suitable for a needle-placement robot will not function in this environment.
A team of researchers at Worcester Polytechnic Institute (WPI) addressed this challenge by designing and building an MRI-compatible needle-placement robot made mostly of plastic with a few nonferrous metal components. The robot consists of a 3-degrees-of-freedom (DOF) Cartesian positioning module to align a needle, and a 3-DOF needle-driver module to place the needle and deliver therapy. Ceramic piezoelectric motors and custom shielded, low-noise drive electronics actuate the robot.
“In early versions, we produced the robot by machining various plastic materials,” said Gregory Fischer, assistant professor of mechanical engineering and robotics engineering at WPI. “The cost and time involved in producing these parts was high, which was a problem since it was clear we would need to produce many prototypes in order to perfect the design. Machining also limited our design flexibility because we had to be concerned with how the part would be fixtured during machining, and it restricted the designs to those that were easily machinable.”
So instead of machining, WPI chose additive manufacturing. Parts with complex geometry, which constitute the vast majority of the robot, are made with Fused Deposition Modeling (FDM). FDM technology is an additive manufacturing process that builds plastic parts layer by layer, using data from CAD files. Most of the parts are made with ABS plastic. Parts that could come into contact with the patient are made of ULTEM 9085 because of its biocompatibility. Parts with simpler planar geometries are produced by laser cutting. Plastic bearings and non-ferrous linear guides are off-the-shelf components.
“FDM dramatically reduced both the cost and time involved in making these components while the additive manufacturing process provides nearly unlimited design flexibility. We see this approach less as rapid prototyping and more as a method of flexible manufacturing,” Fischer said. “This new approach has the potential to improve the biopsy detection rate and also improve the performance of brachytherapy.” The needle-placement robot is now gearing up for clinical trials with Johns Hopkins University.