Robots have steadily taken up residence in the surgical suite, with surgeons adopting complex tools such as the da Vinci system to perform minimally invasive surgery. But there are some limits, especially for image-guided, semi-autonomous robotics, where medical images guide a procedure. The most precise images are generated in real time through magnetic resonance imaging, but fitting a robot into the tight space of an MRI machine is difficult. Metal parts further complicate matters because of the machines’ strong magnetic fields, and noise interferes with electrical imaging. Now, researchers at Worcester Polytechnic Institute are jumping those hurdles with plastics and ceramics in custom-designed motors that operate an MRI-compatible system being tested on men’s prostate procedures.
Prof. Greg Fischer (right) and his students working on their surgical robot. Image: Worcester Polytechnic Institute
“The main goal is to develop a modular control system for a compact device, says Gregory Fischer, a mechanical engineering professor who leads WPI’s Automation and Interventional Medicine Robotics Laboratory. “The term ‘robot’ often brings to mind an image of a large unit. This has to physically fit into a tube that is 50-60 centimeters in diameter. We are developing enabling technologies for image-guided surgery inside of an image scanner.”
So far, the tool has been used on 14 patients undergoing prostate biopsies at Brigham and Women’s Hospital in Boston. “It’s a clinical application to improve the diagnosis of prostate cancer, to take as few samples as possible, with fewer needle sticks,” says Fischer. “We want to go to the right spot.”
Typically, a prostate biopsy procedure lacks precision. A doctor targets the gland, about the size of a walnut or golf ball, using a grid guide placed between a patient’s legs. The doctor then inserts needles through the skin and into the gland to obtain tissue samples. But there is no image. Some procedures use an ultrasound device to provide an image, but ultrasound does not produce one clear enough to precisely target the affected portion of the gland. As a result, in both cases doctors take more tissue samples than may be needed.
The WPI device will allow surgeons to operate with the guidance of real-time MRI. Image: Worcester Polytechnic Institute
The WPI device allows a doctor to work with a clear image in real time with a high degree of accuracy, says Fischer. The MRI can identify suspicious marks on the prostate, or areas of concern, allowing the doctor to direct the robot to the affected area where it places the needle precisely. In the trials, the doctor pushes the needle through the skin to collect the sample but eventually the robot will complete the task. The precision allows the doctor to take as few as four samples, compared to up to 50 in a “blind” procedure.
“Building a robot to work inside an MRI scanner presented some very interesting design problems,” says Fischer, whose lab has been working on the project for about seven years. Fischer has been interested in the topic for longer. His dissertation work for a doctorate in mechanical engineering at Johns Hopkins University was on MRI-compatible mechatronics, he notes, and one of the earliest devices to be used with MRIs dates to 1995.
To succeed, Fischer’s team had to eliminate a robot’s radio frequency emissions, which interfere with an MRI’s magnetic and electric fields. The machine’s strong magnetic field prevents the use of conventional mechatronics, so the team first experimented with pneumatic actuators to move the robot. Although electrical signals were eliminated, they found using air pressure too difficult to control things precisely.
They then moved to piezoelectric motors that convert electrical signals into oscillations for maneuvering the robot and are commercially available. While they are relatively inexpensive and low-power, they are too noisy. The team overcame the noise issue by designing a piezoelectric drive with a direct digital synthesizer. “It’s entirely our own electronics that drive the actuators,” says Fischer. “It is a very precise electronic custom drive.”
The machine’s electronic controls are also fitted into a suitcase-sized case positioned outside of the MRI scanner. A fiber-optic cable connects the control case to the robot inside the scanner bore. Fischer says lab tests have shown the system successfully operating autonomously, reading the MRI image to steer and insert the needles. For now, the doctor specifies the desired location, working a foot pedal to align it into position, and inserts the needles. Fischer says the doctor chooses the desired target trajectory, and the robot calculates the trajectory of the insertion angle.
In tests, funded through grants from the National Institute of Health, the robot has reduced the time needed to perform the procedure, Fischer says. “The slowest part is patient prep,” he notes, which can take over an hour. “This is faster than manual positioning and requires fewer reinsertion attempts. The actual robotic motion is only about fifteen minutes.”
Fischer’s lab also is adapting the robot for neurosurgery. “The controlling electronics are identical and there are similar types of motors,” he says. “It is just configured differently. I like to think of it as a modular platform. A large portion of the work is identical between the systems.”
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The main goal is to develop a modular control system for a compact device. We are developing enabling technologies for image-guided surgery inside of an image scanner.
Gregory Fischer, Worcester Polytechnic Institute