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Transcript
1 A Distributed Pressure Sensing System to Prevent Bowel Perforations Design Team Casey Birmingham, Scott Cooper, Bob Hertig, Iljoo Kim Design Advisor Dr. Saeed Sokhanvar Abstract Gastrointestinal perforations are a deadly risk associated with colonoscopy and occur in about 0.1% of the approximately 14.2 million colonoscopies performed each year in the United States. A distributed pressure sensing system on the insertion tube of the colonoscope could alert an endoscopist when they are on the verge of causing a perforation. The scope of this project is to design and test a distrubted pressure sensor suitable for application on a colonoscope. Micro-bend optical sensors were chosen for the project because, unlike most pressure sensing systems, they are largely immune to axial forces such as those produced when a colonoscope bends. They are also extremely small and will not disrupt other medical equipment. Bending analysis of a colonoscope and optical fiber was conducted, as well as in-depth calculations to determine the light lost from the system due to micro-bending. By selecting the correct wavelength of light, type of fiber, bend radii of the fiber, and fiber placement, the system will detect when a pressure in the range of 8-14 KPa is being applied, which is the range leading up to critical force for bowel perforation. The sensitivity of the system is also compliant with the range of pressures present. A bench top test using the selected system will be conducted to investigate the performance of the system. Future work on the project will include fitting the pressure sensing system to a colonoscope and running the output to a visual interface to alert the doctor. 2 The Need for Project Bowel perforation is a Bowel perforation occurs in about 14,200 colonoscopies potentially deadly risk associated performed in the US each year. Victims of gastrointestinal perforation with colonoscopy. Reducing that face potential medical expenses, emergency surgery, pain and risk would positively affect both suffering, and possible death. Doctors are faced with the reality of patients and doctors. unintentionally injuring their patients, as well as resulting expensive lawsuits. The Design Project Objectives and Requirements The goal of this project is to Design Objectives design a device to reduce the The goal of this project is to design a device to reduce the number number of perforations during of perforations during colonoscopy, specifically due to scope colonoscopy. Due to complexity “looping”. Due to the complexity of the engineering required to and time constraints, the scope of develop said system, the scope of this phase of the project is to develop this phase of the project is to proof of concept for a distributed pressure sensing system to be develop a distributed pressure implemented on a colonoscope. The system must not be sensitive to sensing system to be used on a axial forces produced by scope bending, it must work in the proper colonoscope. range of pressures, and it must account for the additive nature of pressures induced along the entire length of the scope vs. one large, potentially critical pressure. Design Requirements The critical perforation pressure for a human colon is about 2 psi, or 14 KPa. The system must be sensitive to pressures in the range leading up to and including this pressure. A typical scope is 14mm in diameter and 160 cm in length. The system must be effective on multiple axes and at multiple points along the length of the insertion tube. Design Concepts considered The group considered an Several design concepts were considered and evaluated throughout ergonomically shaped scope tip the course of this project. The initial concepts were an ergonomic tip design, as well as multiple and two pressure-sensing designs. Initial sensor design concepts that pressure sensing ideas. The group chose a sensing design, and considered resistive, FiberBragg Grating, and Micro-bend Optical Fiber sensors. were considered after the shift in project scope will also be discussed. Ergonomic Distal Tip This concept was a redesign of the distal end of the insertion tube to make it less likely to become stuck or lodged in the bowel wall, making “looping” less likely to occur. The tip would be streamlined by 3 making it more of a conical shape. This option was not chosen because the current design of the scope tip is vital to the functionality of the scope’s instrument channels and optic systems. Force Sensor at the Distal Tip A force sensor placed at the distal tip would alert the doctor when the tip of the scope is lodged against an obstruction and no longer moving through the colon, which can cause loops. This design would not be effective when another section of the insertion tube was lodged. Tactile Sensor Array The tactile sensor array design was chosen as the design to be completed. An array of sensors placed at strategic points along the length of the insertion tube measure the pressure applied by a scope loop on the bowel wall. This serves as the basis for a system to alert the doctor before a loop is actually occurring. Sensing Options While designing the system, multiple sensing technologies were considered. Resistive sensors were considered for their accuracy, but were found to be unsuitable for this application due to their sensitivity to axial forces and high cost. Fiber-Bragg Grating (FBG) sensors were researched as another potential solution, and were found to be too expensive and complex for the time and resources available to devote to this project. FBG sensors are a type of optical fiber sensor that can measure location and magnitude of an applied pressure. Micro-bend Optical Force (MBOF) sensors are another type of optical fiber sensor that work on the principle of light lost due to pressure applied. MBOF sensors were chosen to be included in the system design because of their small size, low cost, accuracy, and immunity to axial forces due to scope bending. Recommended Design Concept The recommended design concept Design Description uses optical fibers to implement a The tactile sensing array is designed to measure the pressure tactile pressure-sensing array. An applied to the colon wall through the bending of optical fibers. A optical power meter measures conceptual prototype of this pressure-sensing array is going to be light loss through the array. constructed on a flexible tube with properties similar to that of a Applied pressure and optical light colonoscope. The pressure-sensing sheath consists of one hundred loss were correlated through 14mm O-rings and two optical fibers embedded in silicone rubber. 4 extensive mathematical modeling. The O-rings are spaced 6mm apart along the length of the insertion A prototype of this design will be tube. The optical fibers are laid over the O-rings. Each fiber runs to constructed to verify and calibrate the distal tip of the colonoscope and back, covering the insertion tube the sensing array. on two axes separated by 90º. In total, fiber will cover the flexible tube along 4 axes, each separated by 90º. The sensing sheath will be embedded in silicone rubber to provide stability and protection to the equipment. Bending of the scope will be addressed with a slack-feed mechanism to avoid false readings. The light in the fiber is provided and measured by an optical light loss test kit made by FIS. A laser provides light, which is run through a splitter to provide light to both fibers. After running the length of the insertion tube, the fibers are joined by another splitter and connected to the measurement terminal of the kit. The kit outputs light loss measured in dB based on the difference between the intensity of input and output light. Analytical Investigations This measurement technique works on the principle of light propagation through a fiber. When a fiber bends to a small radius of curvature, a significant amount of light is allowed to propagate out of the fiber. This reduced light level can be measured to determine the radius of curvature in the fiber. The curvature radius indicates the applied pressure in a direction normal to the fiber. When the sensor encounters a force perpendicular to the colonoscope, the fibers will bend over the O-rings, producing a small bend radius in the fiber. The resulting bend radius depends on the applied pressure. A small pressure will create a large bend radius, resulting in little to no light loss through the fibers. The maximum critical pressure will do the opposite, resulting in a measurable light loss. The maximum allowable pressure was determined to be 14kPa through extensive research. Knowing this, a fiber bending analysis was created. Based on the sensor geometry described above, a fiber radius of curvature of 2.7mm was calculated at the critical pressure. The resulting light loss over a 2cm contact length is 1.2dB. The light loss under these conditions will determine the maximum light loss threshold. This maximum threshold will be used in future design iterations to alert the doctor of a potentially dangerous situation. 5 Key Advantages of Recommended Concept This pressure measurement technique has several advantages over conventional methods. Optic systems are not affected by nor interfere with electromagnetic fields, which is advantageous in certain medical environments. Unlike conventional pressure sensors, optical fibers are not affected by lateral stresses along the insertion tube. Optical fibers are slender and have the same form factor as colonoscopes, which make them suitable for distributed pressure sensing. This design can be easily retrofitted to any colonoscope and avoids changing the design of the instrument. Physical properties of light provide another important advantage: increased measurement sensitivity. Light loss grows exponentially with applied pressure. This property provides two key benefits to the design. First, it helps eliminate the effect of cumulative light loss throughout the optical fiber. If the colonoscope experiences a pressure of 7kPa, the resulting light loss will be negligible. This means that this safe level of pressure can exist in several spots along the colonoscope without skewing the overall light loss measurement. The second benefit is increased measurement sensitivity as the pressure approaches the critical threshold. Financial Issues The cost of the prototype The initial prototype presented in this project cost $845 to make, presented in this project is $845, $600 of which was a laboratory quality optical fiber test kit with light $600 of which is a laboratory source and power meter. Future iterations of this project will be more quality optical fiber test kit. inexpensive to prototype because the group has this test kit in its Future prototypes will be cheaper possession. because the group has this test kit in their possession already. Recommended Improvements Improvements to the strength of Several group members plan on continuing this project next the product include integrating the semester as an elective course. Integrating the sensing system with a sensing system with a colonoscope and designing a visual feedback system to alert the colonoscope and designing a doctor when critical force is being applied will improve the prototype visual feedback system. presented here. Making the system disposable will also be considered.