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MRI-Compatible Robots

A new motor made of nonmagnetic and dielectric materials can power a robotic medical device inside an MRI scanner, significantly improving cancer diagnoses and treatments.

Engineers at Johns Hopkins University have designed a new motor to be compatible with magnetic-resonance imaging (MRI) so that it can drive an image-guided robot for medical applications, such as cancer biopsies and therapies. The MRI-compatible motor makes it possible for doctors to remotely perform procedures within the scanner, using the MRI to guide the robot.

MRI-safe robot This robotic medical device, designed to give surgeons remote access to the prostate gland, is made of ceramic, rubber, and plastics, so it’s safe to use inside an MRI scanner. It’s powered by a new type of motor and guided by the magnetic-resonance images. Testing has shown that the needle may be placed within one millimeter of a target selected in the image, resulting in more-precise cancer biopsies.

“The real novelty of what they have done is the compatibility with the high-magnetic-field environments like the MRI,” says David Trumper, a professor of engineering at MIT.

When physicians perform cancer biopsies on organs such as the prostate gland, they are commonly guided by ultrasound scanners. But such imaging methods are only able to image, for instance, the outer shape of the prostate. Therefore, doctors are blindly picking samples to test, running the risk that they will miss an actual tumor. Surgery guided by an MRI and a remote-controlled robot would be far more accurate because the robotic needle could be aligned directly with the tumor as seen by the MRI. But until now, there has been no device capable of operating inside the scanner’s small tunnel without distorting the images, which depend on a strong magnet and electric currents.

The Johns Hopkins robot, built using conventional techniques, was designed to enable transperineal needle access to the prostate under direct MRI guidance. Essential to coming within a millimeter–a necessary level of precision–of the targeted tumor is a new motor specifically developed for this application. The motor provides controllable pneumatic actuation so that the robotic device is able to steadily and slowly move alongside the patient in the MRI scanner, says Dan Stoianovici, an associate professor of urology and mechanical engineering at Johns Hopkins and the director of the Urology Robotics program, at which the robot was developed.

The engineers created a new type of pneumatic step motor. It’s based on the idea that end-to-end motion of a piston within its cylinder is always exact and is very easy to achieve by pressurizing the cylinder. A pneumatic step motor is not an entirely new concept; previous versions were based on hydraulic and thermal power, but they lacked mechanical efficiency.

The motor designed by the Johns Hopkins engineers has three cylinders connected to a series of gears that achieve rotary motion by being pressurized equally by air flow. The team of engineers used six step motors to power the MRI-compatible robot, MrBot, that they built to give surgeons remote access to the prostate gland. The robotic device is networked with the magnetic-resonance imager so that when an image is taken, it can be mapped through the network connection by way of the robotic controller. The robotic controller is then in the position to start sending air to the robot to get it in motion.

“The motor is low power and low speed, but for the application, it seems pretty well conceived,” says Trumper. “It is a cool design.”

“The robot is a remarkable achievement,” says Ron Rodriguez, associate professor of urology, medical oncology, and cellular and molecular medicine at Johns Hopkins Medical Institutions. It will allow for more-rapid and -accurate cancer-therapy methods, such as radiation placement, explains Rodriguez.

The next step for the engineers is to test the robotic device in clinical trials. If those tests go well, the researchers will look into the development of additional MRI-compatible robots for other medical procedures.

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