Christopher M. Brown

Single-fiber flexible endoscope: general design for small size, high resolution, and wide field of view

Flexible endoscopes currently used in medicine have a fundamental tradeoff. Either resolution or field of view (FOV) is sacrificed when the scope diameter is less than 3 mm, since the minimum pixel size is usually greater than 4 microns in a pixel-array such as a camera or fiber bundle. Thus, the number of pixels within the image plane determines the minimum size of a conventional scope. However, an image plane is not required for image acquisition using a scanning single-fiber scope. Both high resolution and wide FOV are possible in a scanning single-fiber scope of 1 to 2 mm in diameter. The technical challenge is to produce a two- dimensional scanned beam of light at the distal tip of the scope. By manipulating a resonant fiberoptic cantilever as the optical scanner, various 2-D scan patterns can be produced. The general design concepts and analyses of the fiberoptic scanner for scaling to small size and high resolution/FOV are reviewed. In our initial experimental tests, the size of the photon detector in a fiberoptic scanning scope is demonstrated to not affect image resolution, unlike existing endoscopes with pixel-based detector systems.

Optomechanical design and fabrication of resonant microscanners for a scanning fiber endoscope

Scanning fiber optical endoscopy shows promise as a small, inexpensive imaging tool. Using this method of image acquisition, a scanning fiber is actuated at mechanical resonance, projecting a light spot across an imaged surface. Light backscattered from scanned spots is measured to form an image. The acquired image field of view, resolv- able pixels, and frame rate are dependent on the dynamics of the optical fiber used as a resonant scanner. A finite-element analysis FEA model was constructed to predict scanning fiber dynamics, and compared with experimental results. A scanning fiber microfabrication process was de- veloped that allows for the controlled manufacture of fiber scanners. Ex- perimental results confirm that the theoretical model was accurate in predicting the system transfer function with less than 6% error in ampli- tude and less than 10% error in resonant frequency at the first two reso- nant modes of a cylindrical and a microfabricated fiber. The scanning fiber microfabrication process proved to be capable of repeatably etching notches in optical fibers as small as 2.00±0.05 mm in length and 15±2 m in diameter. FEA was used to predict the effect of geometry change on microfabricated fiber scan dynamics, yielding candidate de- signs chosen for enhanced performance of future scanning endoscopes.

Scanning Single Fiber Endoscopy: A New Platform Technology for Integrated Laser Imaging, Diagnosis, and Future Therapies

Remote optical imaging of human tissue in vivo has been the foundation for the growth of minimally invasive medicine. This article describes a new type of endoscopic imaging that has been developed and applied to the human esophagus, pig bile duct, and mouse colon. The technology is based on a single optical fiber that is scanned at the distal tip of an ultrathin and flexible shaft that projects red, green, and blue laser light onto tissue in a spiral pattern. The resulting images are high-quality color video that is expected to produce future endoscopes that are thinner, longer, more flexible, and able to directly integrate the many recent advances of laser diagnostics and therapies.

Microfabricated optical fiber with microlens that produces large field-of-view video-rate optical beam scanning for microendoscopy applications

Our goal is to produce a micro-optical scanner at the tip of an ultrathin flexible endoscope with an overall diameter of 1 mm. Using a small diameter piezoelectric tube actuator, a cantilevered optical fiber can be driven in mechanical resonance to scan a beam of light in a space-filling, spiral scan pattern. By knowing and/or controlling the fiber position and acquiring backscattered intensity with a photodetector, an image is acquired. A microfabrication process of computer-controlled acid etching is used to reduce the mass along the fiber scanner shaft to allow for high scan amplitude and frequency. A microlens (<1 mm diameter) is fabricated on the end of the optical fiber to reduce divergence of the scanned optical beam. This added mass of the microlens at the free end of the fiber causes the location of the second vibratory node to shift to near the focal length of the microlens. The result is a microlens undergoing angular rotation along two axes with minimal lateral microlens displacement. Preliminary experimental results indicate that this method of optical beam scanning can deliver laser energy over wide fields of view (>50 degrees full angle), up to video scan rates (>10 KHz), while maintaining a scanner diameter of 1 mm. A comparison can be made to bi-axial mirror scanners being fabricated as a MEMS device (micro-electro-mechanical system). Based on the opto-mechanical performance of these microlensed fiber scanners, flexible catheter scopes are possible for new microendoscopies that combine imaging with laser diagnoses.

Modeling optical fiber dynamics for increased efficiencies in scanning fiber applications

A cantilevered singlemode optical fiber is base-excited to create 2D amplitude-modulated resonant motion as a basis for a scanning fiber endoscope (SFE). Over the past few years, prototype SFEs have been developed with smaller sizes of the distal rigid tip which houses the fiber scanner. Our current prototype is 2 mm in diameter with 15 mm rigid length at the tip of a highly flexible shaft. A spiral scan pattern at 40 degrees field of view generates 250 rings (500 lines) at greater than 10 frames per second with negligible distortion at 10 micron resolution. Future SFEs will use microfabrication techniques to sculpt the optical fiber cantilever to form tapered and microlensed tips for the purpose of increasing field of view without increasing electrical power. Microfabrication of complex optical fiber geometries is guided by linear and nonlinear dynamic models of the resonant motion of these fiberoptic scanners. Linear finite element analysis (FEA) is used to match low amplitude motions of tapered and notched fiber geometries, indicating that more flexible regions or hinges can be designed into future fiber scanners for increased amplitude of motion without sacrificing frequency. Nonlinear models of the fiber dynamics are developed and the results help predict the more complex behavior of microfabricated fiber scanners at wider fields of view. Thus, sophisticated fiber dynamics models are used to guide the development of more efficient scanning fiber image acquisition sensors and systems, such as ultrathin flexible SFEs and low-cost sensors.

Laser induced fluorescence as a diagnostic tool integrated into a scanning fiber endoscope for mouse imaging

Scanning fiber endoscope (SFE) technology has shown promise as a minimally invasive optical imaging tool. To date, it is capable of capturing full-color 500-line images, at 15 Hz frame rate in vivo, as a 1.6 mm diameter endoscope. The SFE uses a singlemode optical fiber actuated at mechanical resonance to scan a light spot over tissue while backscattered or fluorescent light at each pixel is detected in time series using several multimode optical fibers. We are extending the capability of the SFE from a RGB reflectance imaging device to a diagnostic tool by imaging laser induced fluorescence (LIF) in tissue, allowing for correlation of endogenous fluorescence to tissue state. Design of the SFE for diagnostic imaging is guided by a comparison of single point spectra acquired from an inflammatory bowel disease (IBD) model to tissue histology evaluated by a pathologist. LIF spectra were acquired by illuminating tissue with a 405 nm light source and detecting intrinsic fluorescence with a multimode optical fiber. The IBD model used in this study was mdr1a-/- mice, where IBD was modulated by infection with Helicobacter bilis. IBD lesions in the mouse model ranged from mild to marked hyperplasia and dysplasia, from the distal colon to the cecum. A principle components analysis (PCA) was conducted on single point spectra of control and IBD tissue. PCA allowed for differentiation between healthy and dysplastic tissue, indicating that emission wavelengths from 620 - 650 nm were best able to differentiate diseased tissue and inflammation from normal healthy tissue.

Fiber scanner modeling as the basis for new scanning fiber endoscopes

In order to develop a one-millimeter outside diameter endoscope we have moved away from using a bundle of optical fibers that are an integral part of current endoscopes. To achieve the field of view and the resolution requirements in spite of the small diameter we are using a single optical fiber that acts as a scanning source. Photo detectors positioned along the side of the scanning fiber detect the backscattered light and software then reconstructs the image on a monitor. The single optical fiber moves in a spiral scan pattern that is produced using a four quadrant piezoelectric tube as the actuator. The optical fiber is fed through the center of the piezoelectric tube and glued into place.Copyright