Adaptive Optics for Vision Science

Chapter 10.4.1 - OCT Principle of Operation

10.4.1   OCT Principle of Operation

Optical coherence tomography is commonly referred to as the optical analog
of ultrasonography, a common clinical method for measuring distances
between ocular surfaces. Ultrasonography directly measures the time of flight
and intensity of sound pulses that are launched into the eye (using a contact
transducer) and are reflected or backscattered from internal tissue interfaces
that exhibit an acoustic impedance mismatch. Resolution is typically on the
scale of 100 μm. A strict optical analog of ultrasonography does not exist,
however, as optical detectors are too slow to directly measure the speed of
light (which is 5 orders of magnitude faster than that of sound in tissue) over
distances relevant for ocular imaging. To circumvent this technical bottle-



FIGURE 10.6  (Top) Conceptual layout of OCT for imaging the eye. Principal components
of the interferometer include a low-coherence light source, an optical path
length modulator (e.g., translating reference mirror), and a detector. (Bottom) The
detected intensity (when the mirror is at position zref), I(zref) ≡ ½Ψ½2½Ψretina + Ψref½2,
is a sum of two DC components, Iref½Ψref½2 and Iretina½Ψretina½2, plus an interference
term whose amplitude is proportional to the product of , a
reflection from a slice of retina. The latter is first convolved with the coherence function
of the light source, in this case a Gaussian with coherence length, lc. The
coherence function acts as the axial point spread function of the instrument.
The translational change of the reference mirror is given by Δz. Extraction of
I(zref)retina can be realized in a variety of ways and is used to construct an image of
the retina.


neck, OCT cleverly employs a Michelson interferometer in conjunction
with a low-coherence light source to coherently filter light reflecting from
the sample (Fig. 10.6). Indirect measurements of the time of flight, and
therefore distance, can be made from this interference pattern. Axial
movement of the coherent filter through the thick sample, that is, the retina,
in conjunction with temporally resolved detection of the interference
signature permits the reconstruction of reflectivity profiles over a range of
depths in the sample from which one-, two-, and three-dimensional images
of the tissue are generated. This detection scheme is termed time-domain
OCT.

As a complement to time-domain OCT, spectral-domain OCT records the
reflected sample signature in the spectral (optical wavelength) domain rather
than the time domain and requires no modulation or scanning of the reference
channel. At the most basic level, conversion of a time-domain OCT
system to a spectral-domain OCT system is realized by replacing the point
detector in the detection channel with a spectrometer and (linear or areal)
detector array. Processing the recorded spectral image consists of several
steps, the most important of which are reference subtraction, interpolation
from wavelength to wave number space, Fourier transformation, and dispersion
balancing. The final processed result is an intensity reflectivity profile
through depth in the sample.

A good resource for further information on the technology and theory of
OCT is the Handbook of Optical Coherence Tomography [45]. A very nice
theoretical and historical development of the early years of OCT can be found
in Fercher [46]. Many of the recent advances in spectral-domain OCT are
still largely only in reviewed journal publications, such as those listed in
Figure 10.5.

Lastly, OCT has been extended well beyond intensity imaging to other
detection schemes, including polarization-sensitive, phase-sensitive, Doppler,
and spectroscopic OCT. While each extracts different optical information
from the sample, all will benefit from AO in terms of increased transverse
resolution and sensitivity. As such, we confine our attention to intensity
imaging assuming that AO integration will follow a similar path for these
other OCT modalities.

 

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