Biomedical Engineering Reference
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resolution and the detector size (number of pixels). For a conventional SD-OCT
system, the maximum imaging depth is defined as
0
S
N;
1
4n
l
max
D
(5.43)
where n is the refractive index of the medium,
0
is the center wavelength of
the detected spectrum,
S
is the spectrometer bandwidth, and N is the number
of pixels of the camera. For an ambiguity-free image, this depth range should be
doubled. The term .
s
=N /
1
can be seen as the inverse of the spectral resolution
power ı
1
of the detecting spectrometer and is proportional to the coherence length
of the detected spectral channel, which is responsible for one of the main drawbacks
of SD-OCT, which is the signal decay observed for structures far away from the
relative zero delay.
Line-Field SD-OCT
Line-field SD-OCT is an alternative implementation to improve imaging speed. To
improve imaging speed, transversally multidimensional techniques have already
been extensively investigated for TD-OCT imaging, as described in the above
section, which profit the continued high-speed camera development with full frame
acquisition in the multi-kframe/s range as well as the high stability due to the
transversally static setup. Their major advantages found in almost isotropic high-
resolution sampling of microscopic specimen (within the depth of field), but suffers
from low imaging speed and high power requirements because of very low signal
to noise ratio, limit its potential for in vivo applications. Line-field SD-OCT based
on the introduction of one transversal imaging dimension and detection by a highly
parallel spectrometer has been investigated by multiple groups [
80
-
82
]. These seem
to have potential for high-speed acquisition of cross-sectional scans but involve a
complicated mixture of geometrical imaging in one sensor axis and a spectrometer
setup in the orthogonal direction, which confines area detectors to rather low pixel
counts due to cross talk in the “spatial” axis. For full volume acquisition, they still
need beam translation. Figure
5.14
shows the basic schematic of a line-field SD-
OCT implementation.
5.5.2.2
Swept-Source OCT (SS-OCT)
The concept of FD-OCT can also be implemented using a tunable laser source
over a broad spectral range in conjunction with a single detector. FD-OCT of this
type has been called swept-source OCT (SS-OCT) [
82
]. Despite the difference in
system configuration, both SD-OCT and SS-OCT have a common net result: the
OCT signal is sampled in spectral domain and an SNR improvement is gained
because of the Fourier reconstruction. In SS-OCT, the time required to tune the
wavelength determines the time to produce an A-scan. Recently, a swept source
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