Biomedical Engineering Reference
In-Depth Information
in order to permit sufficient sampling of the interferometric signal. The temporal
phase-stepping interferometric scheme is commonly used for the reconstruction
of en face OCT images. Conventional FF-OCT uses a three-phase or four-phase
step algorithm for the image reconstruction. Over the past decade, several FF-OCT
schemes were reported based on a wide variety of optical configurations to realize
the phase stepping and the type of PSI (phase-shifting interferometry) algorithm
[
64
] used for the extraction of the interferometric signal. Typically, four images
with the different phase shifts are recorded successively to reconstruct the en face
OCT image. The phase-stepped interference images sequentially recorded by the
CCD detector can be expressed as follows:
I
0
D
E
S0
C
E
R0
C
2E
S0
E
R0
cos.'
S
'
R
/;
(5.36)
I
=2
D
E
S0
C
E
R0
C
2E
S0
E
R0
cos.'
S
'
R
C
90
ı
/;
(5.37)
D
E
S0
C
E
R0
C
2E
S0
E
R0
cos.'
S
'
R
C
180
ı
/;
I
(5.38)
D
E
S0
C
E
R0
C
2E
S0
E
R0
cos.'
S
'
R
C
270
ı
/:
I
3=2
(5.39)
Then, for reconstructing the en face OCT image I
OCT
, the conventional four-
phase shift algorithm can be used as shown below:
I
OCT
D
.I
0
I
/
2
C
.I
3=2
I
=2
/
2
1=2
:
(5.40)
In FF-OCT, different interferometric configurations like Michelson [
61
], Mirau
[
65
], or Linnik [
66
,
67
] configuration can be used. For wide aperture systems, the
spherical aberration in the axial direction is very dominant for Michelson and Mirau
configuration compared to Linnik. In case of Linnik configuration, optical path
length and focusing in both arms of the interferometer can be adjusted independently
[
12
]. Moreover, in Linnik configuration, high-NA objectives can be used to obtain
high spatial resolution. In general, the axial resolution of the tomographic image
in FF-OCT depends on both coherence length of the source (coherence effect) and
the NA of the objective (aperture effect). A variant scheme of FF-OCT that utilizes
objective with high NA (<0:4) is called full-field optical coherence microscopy
(FF-OCM) and is used for obtaining very high resolution [
68
,
69
]. In this scheme,
the aperture effect is more dominant than coherence effect. Figure
5.12
shows the
basic setup of FF-OCT based on Michelson (Fig.
5.12
a) and Linnik configurations
(Fig.
5.12
b).
The FF-OCT scheme can utilize a wide variety of optical sources, including
a source with either spatially coherent illumination or spatially incoherent
illumination. In FF-OCT, depending upon the nature of illumination, the multiple
scattered light from the specimen can result in strong cross-talk-generated noise
between parallel detection channels; thus, each detector should ideally not detect
light originating from outside its conjugate volume. To avoid cross talk, one must
prevent interference occurring between adjacent pixels at the detection plane [
70
].
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