Biomedical Engineering Reference
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often called the “carrier frequency,” which is related to the scanning speed of the
reference mirror. This procedure helps eliminate extraneous signals arising from
background light. Then, for the next scan of the reference mirror, the probing beam
is shifted to an adjacent position and so on, to yield a set of consecutive A-scans.
These A-scans are then combined into a single picture to form a cross-sectional
image of the object, the B-scan. Three-dimensional, volumetric data sets can be
generated by acquiring sequential cross-sectional images by scanning the incident
optical beam in a raster pattern.
5.5.1.2
Line-Field OCT (Linear OCT) and Full-Field OCT
Conventional point-scan optical coherence tomography (OCT) [
55
-
57
] can enable
noninvasive, noncontact 3D cross-sectional imaging of structures in highly scatter-
ing specimens with very high resolution. However, for reconstructing the 3D image,
the conventional OCT system needs a single pointer raster-scanning scheme, in
which the focus of the probing beam scans across the sample surface and detects the
backscattered light with a single-element photodetector. Thus, a fiber optics-based
point-scan time-domain OCT scheme requires at least three scans (one depth and
two lateral scans) for acquiring three-dimensional images. The lateral scans address
laterally adjacent sample positions, whereas the depth scan detects longitudinal
depth positions of light-reflected sites in the sample. The major limitation in point-
scan OCT approach is the mechanical scanning system giving rise to motion
artifacts due to mechanical jitter and limited repeatability.
Line-field [
58
-
60
] and full-field OCT (FF-OCT or wide-field (WF-OCT)) [
61
,
62
] are alternative OCT concepts which aim to improve the image acquisition speed
and to simplify the optical setup of conventional point-scan OCT by realizing direct
line-field or full-field sample imaging onto an array or line detector such as CCD
[
61
]orCMOS[
63
] camera. FF-OCT is based on bulk optics Linnik-type Michelson
interferometer with relatively high-NA microscopic objectives, but identical, on
both arms of the interferometer. Whereas WF-OCT uses imaging optics with a
single lens to image a large sample area, compromising the lateral resolution, in the
case of line-field OCT, the line illumination was achieved with a spherical focusing
lens and a plano-concave cylindrical lens (CL). Recently, FF-OCT has been of
increased interest as a nonscanning, high-resolution, en face imaging method in
several medical and nonmedical applications. Moreover, en face image can provide
new information, which may complement that provided by the longitudinal cross-
sectional image.
The basic configuration of full-field/line-field OCT systems is based on a bulk
optics low-coherence interferometer with a CCD camera placed at the output as
detector. The CCD can enable the acquisition of 2D en face=1D image in a single
exposure without any lateral scanning. However, the direct use of CCD cameras
for heterodyne detection in time domain is limited by the relatively slow frame
rate of the commercially available CCD camera (typically
100 Hz for 512
512
pixels). Therefore, the depth-scan acquisition speed has to be drastically reduced
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