Adaptive Optics for Vision Science

Chapter 10.4 - OCT Opthalmoscope

10.4   OCT OPHTHALMOSCOPE

Optical coherence tomography is a noninvasive, interferometric imaging
modality that provides significantly higher sensitivity and axial resolution
than the conventional flood illumination and SLO architectures discussed
previously. The term OCT was coined in 1991 in reference to the first optical
B-scan (x-z plane) images collected of the in vitro human retina using interferometry
[34]. This was preceded by substantial work in the 1980s on a
technique called low-coherence interferometry that provided depth-resolved
A-scan (z) images of the retina using essentially the same interferometric
principles [35, 36]. Since the early 1990s, OCT technology and knowledge
have grown rapidly. This has led to an increasingly large and diverse array of
OCT designs, all of which can, in principle, be combined with AO to increase
their transverse resolution and sensitivity. In an AO-OCT combination,
OCT provides the high axial resolution, and AO the complementary high
transverse resolution. Together, the AO-OCT combination musters a
potentially powerful imaging tool whose 3D resolution and sensitivity in the
eye can substantially surpass those of any current retinal imaging modality.
The benefits of adding AO to an OCT system, however, are not without
costs, with the primary ones being increased system complexity and
expense.

The numerous OCT designs that have been used to image the living human
retina can effectively be categorized into five main embodiments, three in the
time domain and two in the spectral domain. Time and spectral domains refer
to the temporal and spectral detection of the OCT signal. Figure 10.5 shows
the five embodiments along with leading performance specifications for image
acquisition rate, sensitivity, and resolution. Further details of these example
systems can be found in the citations listed in the rightmost column. Note
that many other research groups not listed have made significant contributions
to the broad field of OCT, and their work is readily available in the vast
OCT literature.

As indicated in the figure, AO has been combined with time-domain en
face
(x-y) flood illumination OCT using a CCD [37] and tomographic scanning
(x-z) OCT [38]. The two have been reported to achieve 3D resolutions
of 2.3 μm × 2.3 μm × 14 μm and 5 to 10 μm × 5 to 10 μm × 3 μm in the eye,
respectively, though results have been limited, demonstrating more the operation
of the techniques rather than clinical or scientific benefits. More recently,
AO has been successfully combined with spectral-domain OCT to achieve
the highest reported 3D resolution in the living human retina at 3.0 μm ×
3.0 μm × 5.7 μm [39]. System performance was sufficient to observe the interface
between the inner and outer segments of individual photoreceptor cells,
resolved in both the lateral and axial dimensions. Such observations have not
been reported with conventional flood illumination and SLO systems endowed
with AO. Finally, en face scanning spectral-domain OCT instruments
with AO are under development at several sites including the University of

FIGURE 10.5  Leading performance specifications for three time-domain (1–3) and two spectral-domain (4–5) OCT embodiments as cited in the representative literature for the restrictive case of imaging the living human retina. Three of the five methods have been integrated with AO and reported in the literature. A fourth is in the process of being integrated. A-scan refers to a one-dimensional axial scan (z) through the retinal tissue. C-scan refers to an en face or traversal two-dimensional slice (x-y).

California at Davis, the University of Vienna, and Indiana University with
the first two already presenting early results.

Due to the variety of OCT embodiments that can be coupled with AO, the
rapid and somewhat unpredictable developments in the OCT field, and the
fact that AO-OCT instruments are less established than AO conventional
flood illumination and SLO systems, this section will emphasize a more global
picture of AO-OCT rather than focus on the details of a single OCT
embodiment.

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