Objectives

This research has couple of specific objectives: First, we will design and develop finer resolution based on common-path OCT system to simultaneously monitor depth properties of the biological tissues. Second, we will develop and implement a scanning (2-D) and non scanning (1-D) micro probes that can be fully integrated with the targeted surgical tools (endoscope and biopsy/therapeutic injection needles). Finally, we will evaluate and validate the integrated system performance.

Introduction

Optical coherence tomography (OCT) is an emerging non-invasive optical imaging technique that can be used to perform high-resolution cross-sectional in vivo and in situ imaging of microstructure in biological tissues. Particularly, common-path optical coherence tomography (CPOCT) has gained much interest in recent years due to its robust and stable configuration resulting from the fact that sample and reference arms share the same fiber optic path. The common path optical coherence tomography configuration has a couple of unique advanced characteristics such as the freedom to use any arbitrary sample probe length and insensitivity to temperature and strain variations. In addition, it eliminates the need for dispersion and polarization matching between the reference and sample arms. Thus, CPOCT systems are inherently simpler and more robust which results in easier to obtain high resolution OCT images. 

For ophthalmic applications, there have been many successful efforts to obtain high-resolution retinal images using non-intruding probes for two-dimensional or three-dimensional optical cross-sectional imaging scanning tools. However these OCT imaging probes are not designed to be inserted into eyes and can obstruct the both surgeon’s view and the use of surgical tools. Therefore, there is a need for minimally invasive OCT probes that can be inserted directly into various organs and be compatible with commercial biomedical devices. The diameters of most current catheters or surgical needles are around from 0.3 mm to 1.6 mm. In addition to the compatibility issue, the imaging should not exhibit performance degradation while it is in situ or in vivo conditions. In the case of eye, for instance, which is filled with vitreous humor, the imaging probe should perform equally well both inside and outside the eye. For a typical CPOCT probe which uses probe tip reflection as the reference, when the fiber is submerged in the water, the reflected signal is reduced due to the reduced index difference at the interface of fiber end.

As a preliminary result, gold-coated micro-fiber probes (Au-µFP) were fabricated and their imaging capability was demonstrated using a frog (Rana catesbeiana, or North American Bullfrog) eye as an imaging sample. Au-µFP allows a strong reference reflection from the probe tip even when the probe is submerged in the liquid or in contact with the tissue. No focusing lens was implemented with Au-µFP in order to limit the probe size to the current fiber diameter of 125-μm.

The goal of the research is to design CPOCT system and fabricate micro-fiber probes that satisfy the entire requirement described above so that they can be fully integrated with micro-retinal surgical instruments working in close proximity to the tissue, which will provide a tool for measuring tissue distances, and for obtaining cross-sectional images of the internal retinal tissue planes.
 

Preliminary Results

Fig. 1.  Schematic of in situ frog retina imaging using common path optical coherence tomography with gold-coated micro-fiber optic probe: (a) CPOCT configuration; (b)experimental setup.

Fig. 2.  Optical characteristics of the probe: (a) measured spectrum for longitudinal resolution; (b) estimated transverse resolution

Fig. 3.  A-mode (depth) scan images of reference signal: (a) gold coated probe in air; (b) gold coated probe in water.

Fig. 4.  Scanned false color OCT images of retina obtained from frog eye: (a) micro-fiber probe; (b) bulk lens optic probe

References

[1] Xiaolu Li, Jae-Ho Han, Xuan Liu, and Jin U. Kang, “SNR Analysis of All-Fiber Common-Path Optical Coherence Tomography,” Applied Optics, vol. 47, no. 27, pp. 4833-4840, Sep. 2008.

[2] Jae-Ho Han, Scott Hendrickson, and Jin U. Kang, “In Situ Frog Retina Imaging Using Common-Path OCT with a Gold-Coated Bare Fiber Probe,” OSA Conference on Lasers and Electro-Optics (CLEO), CFM6, San Jose, CA, May 2008.