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Compensating for hand tremor in micro-surgical procedures

28 September 2012

Using a special optical fibre sensor, a new 'smart' surgical tool can compensate for hand tremors by making hundreds of precise position corrections per second.

The CAD model and prototype of the fibre optic sensor based microsurgical tool, SMART. Image courtesy of Cheol Song, Johns Hopkins University

Researchers from the Johns Hopkins University Whiting School of Engineering and Johns Hopkins School of Medicine in Baltimore, have combined the Optical Coherence Tomography (OCT) imaging technique as a distance sensor with computer-controlled piezoelectric motors to actively stabilise the tip of a surgical tool.

“Microsurgery relies on excellent motor control to perform critical tasks,” said Cheol Song, a post-doctoral fellow in the Electrical and Computer Engineering Department at Johns Hopkins. “But certain fine micro-manipulations remain beyond the motor control of even the most skilled surgeon.” At its most steady, the human hand naturally trembles, moving on the order of 50-100 microns (about the thickness of a sheet of paper) several times each second.

Various opto-mechatronics techniques, including robotics, have been developed to help augment stability and minimise the impact of hand tremors. None so far has been able seamlessly to merge simple fibre-optic rapid and fine-grained sensing with handheld automated surgical tools. The major challenge for researchers has been finding a way precisely to measure and compensate for the relative motions of a surgical instrument in relation to the target.

The emerging imaging technique of OCT attracted the attention of the researchers because it has higher resolution (approximately 10 microns) than either MRI or ultrasound. It also uses eye-safe near infra red light to image tissues.
To apply this imaging technique to their work, the research team first had to integrate an OCT-based high-speed high-precision distance sensor directly into  a small, handheld surgical device.

The device could then hold a variety of surgical instruments at the tip, such as a scalpel or forceps. The well-known fibre-optic based common path optical coherence tomography (CP-OCT) technique provided the essential capability. As its name suggests, the optical signal of this sensor uses the same path, or optical fibre, to transmit and receive the near infra red light.

Because this single fibre-optic cable is so small and flexible, the researchers could easily integrate it into the front of a tool used for eye surgery. By continually sending and receiving the near-infra red laser beams, the high-speed fibre-optic sensor precisely measures the motion of the probe. This information then feeds to a computer that sends signals to small piezoelectric motors integrated into the surgical device to control the position of the tool tip. This creates a series of “station keeping” manoeuvres that compensate for the surgeon’s hand tremors.

Combined, the sensor and motors can operate accurately at 500Hz, which is much higher than the typical tremor frequency of 0-15Hz.

During the next few years, the researchers hope to take their instrument from the laboratory to the operating suite, and with additional refinements expand its use to other fine-scale surgeries.

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