Research Highlights

Optical coherence tomography (OCT) is a noninvasive technology used to perform in vivo high-resolution, cross-sectional imaging of microstructure in biological tissues¹. It was first used clinically in ophthalmology for the imaging and diagnosis of retinal disease¹. Recently, it has been applied to imaging subsurface structure in skin, vessels, oral cavities, as well as respiratory, urogenital, and GI tracts². Technology that increases spatial resolution will have significant impact on clinical applications of OCT. The axial resolution of OCT is determined by the coherence length of the light source, which can be improved by the development of broadband light source³.

Fig. 1 A: High-resolution OCT image of anterior chamber of eye with image size of 3.0 x 2.0 mm.

Fig. 1 B: the envelope of the interference peaks at line A-A in the above image.

With improvements in OCT resolution made possible by an increase in source bandwidths, dispersion has become a critical limiting factor. For ultrahigh resolution OCT, any unbalanced dispersion in the interferometer will degrade axial resolution. Depth dependent dispersion is especially a critical limiting factor in high-resolution OCT imaging of the eye, for which a long depth of view is required.

We have investigated the influence of depth dependent dispersion by water, the main component of biological tissues such as eye. Our studies showed that by choosing a light source with a center wavelength near 1.0 µm it is possible to eliminate the influence of depth dependent dispersion by water⁴. Using broadband continuum generation from a photonic crystal fiber we developed an ultrabroad band light source near this wavelength with bandwidth of 370 nm⁵. We obtained -to the best of knowledge- the first dispersion-free ultrahigh resolution image of eye with axial resolution of 1.3 µm (Fig. 1). Figure 1A shows an OCT image of the anterior chamber at the limbus of the eye. To investigate the evolution of the OCT resolution with increasing imaging depths inside the eye, the envelope of the interference peaks at line A-A in Fig. 1A was derived and is shown in Fig 1B. The width of the interference fringe at the interface in the front surface is very similar to the width at the interface of aqueous humor and lens, indicating that the resolution of the OCT is not degraded after light passes through 1.64 mm thickness of cornea, aqueous humor, and iris inside the eye. This demonstrates that high resolution is kept in a great depth range. In summary, a dispersion free ultrabroadband source with a center wavelength near 1.0 µm has been developed, and such a source is very promising for ultrahigh resolution OCT in biomedical imaging, especially for ophthalmic imaging.

Acknowledgments

This work was supported by research grants awarded from the National Science Foundation (BES-86924) and National Institutes of Health (EB-00293, NCI-91717, RR-01192). Institute support from the Air Force Office of Scientific Research (F49620-00-1-0371) and the Beckman Laser Institute Endowment is also gratefully acknowledged.

References

1. D. Huang et al., Science 254, 1178-81 (1991).
2. B. E. Bouma and G. J. Tearney, eds., Handbook of Optical Coherence Tomography (Marcel Dekker, New York, 2002).
3. W. Drexler et al., Opt. Lett. 24, 1221-3 (1999).
4. Y. Wang et al., Opt. Express 11, 1411-7 (2003), www.opticsexpress.org.
5. Wang et al., Opt. Lett. 28, 182-4 (2003).