Three-dimensional (3D) optical coherence tomography (OCT) is an advanced method of noninvasive infrared imaging of tissues in depth. Heretofore, commercial OCT systems for 3D imaging have been designed principally for external ophthalmological examination. As explained below, such systems have been based on a one- dimensional OCT principle, and in the operation of such a system, 3D imaging is accomplished partly by means of a combination of electronic scanning along the optical (Z) axis and mechanical scanning along the two axes (X and Y) orthogonal to the optical axis.
In 3D OCT, 3D imaging involves a form of electronic scanning (without mechanical scanning) along all three axes. Consequently, the need for mechanical adjustment is minimal and the mechanism used to position the OCT probe can be correspondingly more compact. A 3D OCT system also includes a probe of improved design and utilizes advanced signal-processing techniques. Improvements in performance over prior OCT systems include finer resolution, greater speed, and greater depth of field.
Figure 1 includes a simplified schematic representation of the optical subsystem of a typical prior OCT system. In this system, near-infrared light from an incandescent lamp or other low-coherence source is sent through optical fibers and a fiber-optic coupler to a reference mirror. Some of the light is also sent through the fiber optics to a lens that, in turn, focuses the light to a point that lies at or near the depth of interest in a specimen. In the fiber-optic coupler, light reflected from the reference mirror is combined with light scattered from a focal point in the specimen and is then sent along another optical fiber to a photodetector. When the length of the optical path from the light source to the mirror equals or nearly equals the corresponding length to the focal point in the specimen, the photodetector puts out a signal representing a pixel at the focal point in the specimen. Scanning along the depth (Z) axis is accomplished by using the piezoelectric transducer to move the reference mirror closer to, or farther from, the light source. Scanning along the X and Y axes is accomplished by mechanical motion of the probe along X and Y.
The lower part of Figure 1 depicts a typical instrumental response (point-spread function) in the photodetector output obtained in scanning along the Z axis. The response includes oscillations attributable to interference between the light scattered from a point in the specimen and light scattered from the mirror. As the Z displacement increases, the contrast of the interference pattern is reduced by the loss of coherence. Usually, the envelope of the oscillations (in contradistinction to the oscillations themselves) is what is measured. In such a case, the width of the envelope and, thus, the depth resolution, is comparable to the coherence length of the light source.
Figure 2 includes a simplified schematic representation of the optical subsystem of a 3D OCT system. This system is based partly on the same principles as those of the prior system. However, there are several important differences:
- Light from the source is fed through a more-complex fiber-optic subsystem, not only to a photodiode but to three single-mode optical fibers on a probe. Light emerging from the tips of these three fibers illuminates the specimen and creates a 3D interference pattern in the specimen.
- Light scattered from the specimen is collected and sent to the photodetector by a wider, multimode optical fiber. The probe containing the illuminating single-mode fibers and the light-collecting multimode optical fibers is significantly smaller and more rugged, relative to a lens-containing probe in a prior OCT system.
- Instead of utilizing lenses and a piezoelectric actuation of a reference mirror to effect scanning in Z and focusing in conjunction with mechanical scanning in X and Y, the system utilizes a combination of (1) amplitude modulation of the light in the three illuminating optical fibers and of a portion of the source light sent directly to the photodetector, (2) nonlinear detection, and (3) an advanced signal-processing technique that, among other things, exploits the 3D nature of the interference pattern in order to obtain (4) a 3D point-spread function that affords localization in X, Y, and Z. In principle, because mechanical scanning is no longer necessary, it is possible to achieve scanning at a video frame rate.
This work was done by Mikhail Gutin, Xu- Ming Wang, and Olga Gutin of Applied Science Innovations, Inc. for Glenn Research Center.
Inquiries concerning rights for the commercial use of this invention should be addressed to
NASA Glenn Research Center
Innovative Partnerships Office
Attn: Steve Fedor
Mail Stop 4–8
21000 Brookpark Road
Refer to LEW-18352-1.