Download A Distributed Pressure Sensing System to Prevent Bowel Perforations

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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.
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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
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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.
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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.
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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.