Biomedical Engineering Reference
In-Depth Information
The polarization state of light can be expressed in terms of Stokes vector S as
2
4
3
5
2
4
3
5
D
a
x
C
a
y
a
x
a
y
2a
x
a
y
Cos'
2a
x
a
y
Sin'
I
Q
U
V
D
S
;
(5.44)
where I , Q, U ,andV are Stokes parameters; a
x
and a
y
are amplitudes of two
orthogonal components of the electric vector; and ' represents the phase difference
between the two orthogonal components. By using a 4
4 Muller matrix, the effect
of an optical device on the polarization of light can be expressed, and the matrix
acts on the input state S
1
to give the output state S
2
:
2
3
2
3
2
3
I
2
Q
2
U
2
V
2
M
00
M
01
M
02
M
03
M
10
M
11
M
12
M
13
M
20
M
21
M
22
M
23
M
30
M
31
M
32
M
33
I
1
Q
1
U
1
V
1
4
5
D
MS
1
D
4
5
4
5
S
2
D
:
(5.45)
If we consider the Poincare sphere representation, a polarization state S can be
represented by a point (Q, U , V ). The Poincare sphere representation provides a
convenient method to evaluate changes of Stokes vector. By determining either the
Stokes vector or the Muller matrix, the full quantification of birefringence properties
of the biological sample can be obtained. PS-OCT imaging of the Stokes vector and
Muller matrix has been demonstrated by several groups [
96
-
99
]. It is attractive for
medical applications, providing an extra contrast mechanism that could potentially
lead to optical diagnosis of certain pathologies [
100
-
102
].
5.6.2
Doppler OCT
Doppler OCT (DOCT) or optical Doppler tomography (ODT) is another functional
mode, which can permit quantitative imaging of fluid flow in highly scattering
specimens [
103
-
108
]. The technique is similar to Doppler velocimetry. Doppler
velocimetry suffers from imprecise imaging due to the long coherence length of
the light source used, which results from the interference of the static, and Doppler
shifted components of light occurs over a long optical path. The low-coherence
gating property of OCT overcomes this problem, permitting quantitative imaging of
fluid flow in highly scattering media, such as monitoring in vivo blood flow beneath
the skin. In DOCT, the velocity resolution depends upon detection electronics, scan
angle, and acquisition time. Reported flow velocity resolutions are in the region
10-100 ms
1
; however, recent development in Fourier domain Doppler OCT
(FD-DOCT) has shown that a velocity resolution of just a few micrometers per
second may be achieved [
109
-
116
]. The DOCT can provide 3D tomographic map
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