Minimally Invasive Surgery (MIS) is a new class of surgical procedures in which the operation is performed with surgical instruments inserted through small incisions in the body. In contrast to open surgery, in which the organ or tissue is exposed through large incisions in the body, MIS procedures generally allow for faster recovery time, less pain and trauma, reduced risk of infection, and shorter hospital stays.
Cutting-edge endoscopes, catheters, and even surgeon-guided robots have been developed to perform these complex but beneficial procedures. While the field of MIS continues to expand into more complex procedures, surgical tools that enable these operations are becoming smaller and more flexible. One of the growing challenges when using these devices is providing knowledge to the surgeon as to where the tool is inside the body. Whether it’s a manually or robotically driven surgical instrument, knowing the precise location of the instrument tip, or even the shape and position of the entire instrument, is critical to a successful operation.
Using technology inspired by NASA, Luna Innovations (Roanoke, VA) developed a shape and position sensor using optical fiber. This fiber optic cable is minimally intrusive, virtually weightless, and can provide real-time feedback of its own dynamic shape and position. When embedded or surface-attached to surgical tools or other devices, the fiber will monitor the dynamic 3D shape, independent of the temperature or load environment.
The roots of shape-sensing optical fiber technology started in 1996 at NASA’s Langley Research Center in Hampton, VA. Researchers were asked to provide 10,000 strain sensors for the X-33 Launch Vehicle with a weight budget of virtually zero. Using optical fiber with Fiber Bragg Grating (FBG) strain sensors was an obvious choice because of their light weight, but in order to get 10,000 FBG sensors on a single fiber with 1 cm or less spacing between each sensor, a new demodulation technique had to be used. NASA researchers developed Optical Frequency Domain Reflectometry (OFDR), a technique that permits tens of thousands of sensors with the same nominal reflected wavelength to be read with very high spatial resolution, giving the most complete picture of any of the viable fiber-optic technologies.
In the OFDR technique, a continuously tunable laser is used to spectrally interrogate a multitude of FBG sensors along a fiber. The reflected light from these elements is then detected, demodulated, and analyzed. This interferometric technique enables detection from hundreds to thousands of FBG sensors along a single fiber. Unlike other reflectometry techniques, the gratings can and do have overlapping spectra. This enables mass production of the sensing fiber during the fiber draw process, narrows the necessary laser spectrum, and increases the number of gratings that can be multiplexed on a fiber and measured with a single demodulation system. The OFDR technique makes practical the collection of data from a dense array of spatially distributed sensors that is unrealistic with other techniques currently available.
For shape measurements, the optical fiber consists of high-density linear arrays of FBG strain sensors in multiple fiber cores aligned in the axial dimension and packaged as a monolithic structure in a particular geometry. Advanced algorithms use the strain differential as seen by the fiber optic sensors to calculate the bends at every discrete element along the length. Because of the sensor density, each individual sensing element can be integrated to reconstruct the total shape of the fiber.
In this example, the fiber is designed with three cores that are arranged axially in an equilateral triangle within the fiber cladding, and the strain is measured by Fiber Bragg Gratings (FBGs) that are written into the cores of the fiber using a high-powered pulsed excimer laser at a constant axial spacing. The FBGs consist of a periodic change in refractive index, which reflects a very narrow band of light, the exact wavelength of which is dependent on the period of the refractive index variation. When the fiber is under strain, this period is slightly perturbed, thus changing the wavelength of the light that is reflected back to the interrogator. It is through this mechanism that distributed strain measurements are procured.
The grouping of three FBGs at any point in the multi-core fiber's cross-section is referred to as a sensor triplet. The portion of fiber between one sensor triplet and the next is referred to as a tether segment. To determine the optical fiber's shape, we begin at the first sensor triplet and use the strain measurements from each core's FBG to compute the characteristics of the bend at that point. Using this data, the next sensor triplet's location within the optical fiber is extrapolated. By repeating this process for each sensor triplet in the fiber, the overall shape of the fiber can be determined.1
The global market for minimally invasive surgery (MIS) devices and instruments is expected to reach $18.5 billion by 2011, with average annual growth rate (AAGR) of 7.5% between 2006 and 20112. Incorporating location and position information into the next-generation of devices will enhance the navigation of surgical instruments and improve patient care.
The challenge of determining the dynamic shape and position is of interest outside the medical field such as in aerospace and ocean surveillance. Shape modification of aircraft wings during flight can provide significant performance improvements by matching the aerodynamic shape to flight conditions and by maintaining smooth lifting surfaces without control surface discontinuities. Ocean surveillance ships have a mission to gather underwater acoustical data, including submarine hunting patrols, counter drug missions, and deep water search and rescue. Ships on hunting duty often use towed sonar arrays, which consist of a string of long, flexible modules containing hydrophones that are used to locate underwater noise sources. Shape technology can enhance performance by providing information on the disposition of the flexible cable.
This article was written by Trevor Rice, Business Development Manager for Luna Innovations, Roanoke, VA. For more information, Click Here .
- Characterization of a fiber-optic shape and position sensor (Proceedings Paper) Proceedings Vol. 6167; Smart Structures and Materials 2006: Smart Sensor Monitoring Systems and Applications, Daniele Inaudi; Wolfgang Ecke; Brian Culshaw; Kara J. Peters; Eric Udd, Editors, 616704 Date: 30 March 2006. 2. “Trends in the Noninvasive and Minimally Invasive Medical Device Market”, June 2006, www.piribo.com, Business Communications Company.