Adaptive Optics and Optical Coherence Tomography

Tomographic techniques are used to produce two-dimensional images of the internal structure of the sample under study.

Optical Coherence Tomography (OCT) is based in Michelson interferometry. The interference pattern between light scattered back from the object and reflected in the reference arm is used to create a profile of the reflectance of the internal structure of the object. A broad band source is used, so interference only occur between light scattered back from the object that has covered the same optical path as the light reflected from the reference arm, so the amount of light scattered from a point at a certain depth in the sample can be determined. This provides the system with depth sectioning capability, and its longitudinal or depth resolution depends on the bandwidth of the light source used.

In traditional OCT the sectional images obtained are parallel to the optical axis, i.e., the images obtained are longitudinal, as shown in figure 1.b.

Figure 1. Different directions of scanning used in OCT. b) B-scan, or longitudinal scan, used in longitudinal OCT. c) C-scan, or en-face scan, where the fast scanning axis is perpendicular to the optical axis.

OCT has been widely used in in-vivo retinal imaging, because it is a non-invasive technique, and since it shows a very high signal to noise ratio (SNR), the light levels needed to create these images are very low (some results have been presented showing up to 40 dB SNR [1]).

As already stated the depth resolution of OCT images depends only on the bandwidth of the light source used. Its value ranges from 1-3mm for light sources such as white lamps or femto second lasers, to a 15-20mm for super luminescent diodes. On the other hand, lateral resolution is limited by the aberrations of the eye. Typical values for lateral resolution of OCT images is around 10mm.

In en-face OCT, the scanning direction is such that the images obtained are perpendicular to the optical axis, as shown in figure 1.c. This is achieved by using a couple of scanning mirrors that sweep the probing beam over the layer of the retina that is being imaged. This technique that has been developed in the Applied Optics group, in the University of Kent at Canterbury, uses a set-up that is very similar to that used in scanning laser ophthalmoscopy (SLO), and a simple modification of the system provides it with the capability to obtain simultaneous SLO and en-face OCT images.

Adaptive optics is a technique that was originally developed to improve astronomycal images by correcting for the effect of the atmospheric turbulence in ground based telescopes. The technique has also been used to improve the quality of in-vivo retinal images by correcting for the aberrations of the human eye. It has been implemented with fundus cameras, SLOs and OCT systems showing improvement in the quality of the images obtained with each of those systems.

In our lab, we have implemented an AO - simultaneous en-face OCT/SLO system, and some of the results obtained with it are shown here.