Biomedical Engineering Reference
In-Depth Information
example, moving blood cells, from the scattering signals backscattered by the
static tissue background, that is, by the tissue microstructures, to achieve imaging
of blood perfusion. In essence, OMAG mathematically maps the backscattered
optical signals from the moving particles into one image—that is, the blood flow
image—while it simultaneously maps the backscattered optical signals from the
static particles into a second image, which is the microstructural image, identical
to the conventional OCT image. In OMAG, the spatial interferogram in the lateral
direction (B-scan) is modulated with a Doppler frequency feasible to separate the
moving and static scattering components within sample. This Doppler modulation
frequency can be introduced either by mounting the reference mirror in the
reference arm onto a linear piezo-translation stage, which moves the mirror at
a constant velocity across the B-scan (i.e., x-direction scan), or simply by the
inherent scattering fluid flow within sample. In the earlier version of OMAG system
[
119
,
120
], this is achieved by mounting the reference mirror in the reference
arm onto a linear piezo-translation stage that moved the mirror at a constant
velocity across the B-scan (i.e., x-direction scan). However, the latest version
of OMAG utilizes the spatial modulation frequency provided by the inherent
blood flow modulation rather than reference arm modulation [
121
]. OMAG is
an emerging imaging modality with clear potential applications in many basic
research and medical imaging applications. Because of its exceptionally high spatial
resolution and velocity sensitivity, OMAG can provide useful information regarding
microcirculation in a number of applications, both in clinical [
122
,
123
] and basic
research [
124
].
5.6.4
Spectroscopic OCT
Spectroscopic OCT is an alternative mode of OCT, which provides further access
to the composition and functional state of the specimen. Spectroscopic OCT can
be implemented in a variety of ways. One approach is based on spectral ratio
imaging of OCT images using two or more spectral bands with wavelength division
multiplexers, which combine lights with different wavelengths and then electron-
ically distinguish the resulting signal by their different Doppler shifts resulting
from the reference arm scan [
125
,
126
]. By use of state-of-the-art ultrabroadband
femtosecond Ti
W
Al
2
O
3
lasers, spectroscopic imaging over the wavelength range
from 650 to 1,000 nm has been reported by Morgner et al. [
127
]. Another implemen-
tation is based on estimation of depth-resolved spectral information of the source
spectrum, in which the modification of the source spectrum caused by the sample
can be measured directly from the Fourier domain processing of cross-correlation
interferometric data [
128
,
129
].
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