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

3. 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).

4. 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.

5. 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.

6. 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

7. Optics Base Reflected Light Mirror Gold Plated Mirror Compliant Mechanism Optics 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

8. 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

9. Optics Base Compliant Mechanism Torsion Hinges Gold Plated Mirror 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.

10. 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

11. 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

12. 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

13. 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.

14. 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.

15. 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.

16. Mirror Pseudo Rigid Hinge Current Result

17. 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.

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

19. References • P. Patterson, P.M. Mills, J. Zara. “Amplified Bimorph Scanning Mirror For Optical Coherence Tomography.” • S. Kota. “Design of Compliant Mechanisms: Applications to MEMs.” SPIE Vol. 3673, March 1999 • 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. • 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) • Howell, Larry. Compliant Mechanisms. Canada: John Wiley & Sons, Inc., 2001

20. 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.