Mechanical Design Improvement in the Optical Coherence Tomography Scanner

1
Mechanical Design Improvement in the Optical
Coherence Tomography Scanner

• Joshua RudawitzMentor Professor Kerr-Jia Lu

2
Overview

• Introduction to Optical Coherence Tomography
• The current limitations of existing technology
• Mechanical Solutions
• Laser vector analysis
• MEMS and Compliant Mechanisms
• Topology I and II
• Pseudo Rigid Body Model
• Summary of current results

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Optical Coherence Tomography

• This technology uses optical imaging to produce
  high-resolution (10µm or less) cross sectional
  images of tissue.
• Optical Coherence Tomography (OCT) is similar to
  ultrasound in that pulses of infrared light are
  sent out and the echo is translated into an image
  based on interferometry.
• These images are able to show differences in
  tissue, such as cancers, and therefore offers an
  alternative to conventional cancer detection
  (biopsy).

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Optical Coherence Tomography

• Current technology uses the natural frequency of
  the structure to provide the amplification of the
  mirror, with the use of a bimorph actuator.
• Due to the scalability of this technology it is
  being used in endoscopes for non-invasive cancer
  detection procedures.

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Optical Coherence Tomography

• Limitations
• Large scan angles have been achieved, however
  they still provide a limited range of sight.
• There needed to focus the scan angle to achieve
  both a front and side view to be able to see more
  of the desired area.
• With regards to the bladder, without the side
  view it is difficult to scan areas close to the
  entrance of the endoscope.

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Mechanical Solution

• Goals
• To design a mechanism that will allow for both
  front and side views.
• Include MEMs technology and compliant mechanisms.
• Manufacturing
• Mechanical Amplification

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Achieving Front and Side Viewing

• Constructing a model to predict the scanning
  angle as a function of the mirror rotation.
• Required additional analysis to ensure close to
  straight line, or a curve with a large radius

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Vector Analysis of Laser

• Results
• It was found that with just 45 degrees of
  actuation a viewing angle of close to 90 degrees
  can be achieved.
• This range would be from the tip of the endoscope
  to the side of the endoscope.
• By rotating the endoscope the entire area could
  be scanned.

Curved Path of Laser
Viewing Angle
Tip of Endoscope
Tip of Endoscope
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Design of Mirror Structure

• Due to a need for a more focused location of the
  scan angle the current mirror configuration was
  not sufficient.
• A two torsion hinge design was created.

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Topology I

• The initial design was first analyzed using basic
  force and geometry equations.
• Using ANSYS for non-linear analysis, it proved
  difficult to converge the model.
• In addition, it showed that due to the lack of
  stiffness in the structure, the hinges did not
  act expectedly.
• The structure was acting like a cantilever beam

Torsion Hinges
Mirror
Torsion Hinges
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Topology II

• This topology is based on compliant mechanisms
  theory of a Pseudo-Rigid-Body Model
• This relates a cantilever beam model to a rigid
  link with a torsional spring

Mirror
Pseudo Rigid Hinge
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Pseudo Rigid Body Model

• This allows for a good design approximation of
  large non-linear deflection.
• The approximation is based on modeling the beam
  (hinge) as a pin joint with a torsion spring.
• The strain energy stored in a bending beam can be
  simulated using a torsion spring.

P
P
Torsion Spring
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Pseudo Rigid Body Model

• Assuming the loading case of Force and Moment in
  the Same Direction
• It can be modeled as an initially curved beam,
  with a moment that correlates to the initial
  radius of curvature.

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Pseudo Rigid Body Model Analysis

• By using the pseudo rigid body model, it is then
  possible to predict the path of the end point of
  the hinge and the tangent angle.
• The tangent angle is important because that will
  define the orientation of the mirror after the
  beam has been bent.

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Pseudo Rigid Body Model Analysis

• Results for two positions of the hinge were found
  by using MATLAB.
• Where hinge geometry was the design variable.
• These two positions correspond to the required
  motion of the mirror to achieve the viewing angle
  of 90.
• Using the included stress equations it was
  determined that the material would hold up to
  this range of motion and further positions can
  easily be modeled through the use of the MATLAB
  files created.

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Current Result
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Summary

• A suitable viewing angle was found to achieve the
  desired goal of front and side viewing, done
  using vector analysis.
• The vector analysis of the optics showed that
  with 45 of rotation a 90 viewing angle is
  achieved, close to a straight line.
• A pseudo rigid body model was created to
  approximate non-linear structural response.
• Pseudo rigid body results show that the hinge can
  go through the required motion without structural
  failure.
• The model developed can be used to design the
  remaining hinges to provide the desired mirror
  motion.
• Continuing research on
• Actuators to provide the needed range of motion.
• Compatible frequencies with current data
  acquisition programs.
• Building scale models for testing.

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Acknowledgments

• Professor Kerr-Jia Lu
• Professor Jason M. Zara
• The George Washington University- Institute for
  Biomedical Engineering

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References

1. P. Patterson, P.M. Mills, J. Zara. Amplified
   Bimorph Scanning Mirror For Optical Coherence
   Tomography.
2. S. Kota. Design of Compliant Mechanisms
   Applications to MEMs. SPIE Vol. 3673, March 1999
3. J. Zara, S. Yazdanfar, K.D. Rao, J.A. Izatt, and
   S.W. Smith. Electrostatic Micromachine Scanning
   Mirror for Optical Coherence Tomography. Optics
   Letters. Vol. 28, No. 8, April, 15, 2003.
4. G. Tearney, M. Brezinski, B. Bouma, S. Boppart,
   C. Pitris, J.F. Southern, and J.G. Fujimoto. In
   vivo Endoscopic Optical Biopsy with Optical
   Coherence Tomography. Science, New Series, Vol.
   276, No. 5321 (Jun. 27, 1997)
5. Howell, Larry. Compliant Mechanisms. Canada John
   Wiley Sons, Inc., 2001

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Vector Analysis of Laser

• The goals of this step
• To ensure that by shifting the laser to the side
  an acceptable viewing angle is achieved.
• To confirm that the path was acceptable for
  medical purposes.