Optical Coherence Tomography: Technical Aspects - Biomedical Optical Imaging Technologies: Design and Applications - page 193

function is Hermitian, so the reconstructed image is symmetric with respect to
the zero-phase delay of the interferometer, leading to ambiguity in interpretation
of the resulted OCT images. In order to overcome this problem, one has to place
the sample entirely within the positive or negative space. Consequently, only half
of the imaging depth is available for imaging in practice. By resolving the complex
conjugate artifact, the imaging depth range can be doubled, which would in turn
provide additional flexibility to explore the most sensible measurement range near
the zero delay line [ 75 ].
To achieve the full-range complex imaging for FD-OCT, several methods have
been proposed. Phase-shifting methods were among the first attempt to achieve
the full-range imaging [ 79 ]; however this limits the imaging speed and complex
conjugate rejection ratio due to the requirements of several phase steps at the same
sampling location and its accuracy Another approach that uses a 3 3 optical coupler
as phase-shifting element was proposed, which allows simultaneous detection of
two quadraturely phase-shifted interferograms to obtain the complex ambiguity-free
images [ 86 ]. This technique requires two separate detectors for image acquisition,
which adds additional cost and complexity, thus limiting its practical application.
For high-speed full-range complex imaging, an elegant, practical scheme is to
introduce a constant modulation frequency into the spatial interferogram during
transverse scanning of the sample [ 87 ]. In this way, the complex data can be obtained
either by Fourier transform or Hilbert transform of the B-scan interferogram in
transverse direction. By using this method, it is possible to achieve a full-range
complex imaging with high speed. However, the original implementation requires
a piezo-stage to provide modulation in the interferograms, which limits the full
imaging speed due to the mechanical movement of the piezo-stage used.
Recently, the modulation of the spectral interferograms is achieved by the galvo-
scanner [ 88 ], which is used for transversely (x-direction) scanning the sample. In
doing so, the sample beam is offsetted from the pivot axis of the x-scanner, which
causes a path length modulation during x-scanning, thus introducing a modulation
frequency into the B-scan interferograms. The main advantage of this method is
that it does not need any additional phase-shifting elements to realize frequency
modulation and the modulation frequency is inherently given by the system itself.
Another important advantage of using frequency modulation in the interferogram
to achieve full-range complex FD-OCT image is that it is relatively insensitive to
sample movement that is critical for in vivo application. These aspects render high-
speed full-range complex SD-OCT imaging without any restriction on additional
hardware component and without limitation on the imaging speed, which is critical
for in vivo imaging applications.

5.6 Functional OCT

In conventional OCT, the contrast mechanism relies upon the spatial variation of the
coherent backscatters within a cross-sectional plane or volume of tissue or materials.
However, a number of other intrinsic properties of the tissue have been demonstrated