2 Fundamentals of Retinal Optical Coherence Tomography
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Fig. 2.1 Scheme of a typical fiber optics TD-OCT system
the reference and the sample is recombined at the fiber-coupler and half is collected
by a photodetector in the detection arm of the interferometer. Also, the reference arm
light is typically attenuated by orders of magnitude to improve the signal-to-noise
ratio (SNR).
By rapidly varying the reference arm mirror and synchronously recording the
magnitude of the resulting interference signal, a single axial profile or A-scan is
obtained which is a graph of the optical reflectivity versus distance in the eye. A
sequence of such A-scans is obtained by scanning the probe beam across the entire
retina which forms a B-scan tomogram. As a result, a cross-sectional view of the
structure, similar to a histology section is attained.
The axial resolution of an OCT image depends on the coherence length which is
a fundamental property of the light source, whereas transverse resolution for OCT
imaging is determined by focused spot size, as in microscopy. TD-OCT can achieve
a translational resolution of about 8–10 µm, but the A-scan rate is only a few kHz
which limits real-time 3D imaging application. For example, the motion of human
eye often blurs the OCT images in the long imaging time duration.
2.2.2 Fourier Domain OCT
Fourier domain optical coherence tomography (FD-OCT) obtains the longitude information by measuring the reflective spectrum of the tissue. FD-OCT systems do not
need mechanical time delay, and thus the A-scan rates are greatly improved. There
are two mechanisms of FD-OCT systems: spectral-domain systems (SD-OCT) and
swept-source systems (SS-OCT) [9, 10]. In the case of SD-OCT (Fig. 2.2), a broadband and continuous-wave low-coherence source is employed to illuminate the interferometer. The reference arm length is fixed, and the length of the reference arm and
sample arm is approximately equal. The sample arm also contains a 2D lateral scanner and an objective lens. The detection arm includes a spectrometer, which consists
29
Fig. 2.1 Scheme of a typical fiber optics TD-OCT system
the reference and the sample is recombined at the fiber-coupler and half is collected
by a photodetector in the detection arm of the interferometer. Also, the reference arm
light is typically attenuated by orders of magnitude to improve the signal-to-noise
ratio (SNR).
By rapidly varying the reference arm mirror and synchronously recording the
magnitude of the resulting interference signal, a single axial profile or A-scan is
obtained which is a graph of the optical reflectivity versus distance in the eye. A
sequence of such A-scans is obtained by scanning the probe beam across the entire
retina which forms a B-scan tomogram. As a result, a cross-sectional view of the
structure, similar to a histology section is attained.
The axial resolution of an OCT image depends on the coherence length which is
a fundamental property of the light source, whereas transverse resolution for OCT
imaging is determined by focused spot size, as in microscopy. TD-OCT can achieve
a translational resolution of about 8–10 µm, but the A-scan rate is only a few kHz
which limits real-time 3D imaging application. For example, the motion of human
eye often blurs the OCT images in the long imaging time duration.
2.2.2 Fourier Domain OCT
Fourier domain optical coherence tomography (FD-OCT) obtains the longitude information by measuring the reflective spectrum of the tissue. FD-OCT systems do not
need mechanical time delay, and thus the A-scan rates are greatly improved. There
are two mechanisms of FD-OCT systems: spectral-domain systems (SD-OCT) and
swept-source systems (SS-OCT) [9, 10]. In the case of SD-OCT (Fig. 2.2), a broadband and continuous-wave low-coherence source is employed to illuminate the interferometer. The reference arm length is fixed, and the length of the reference arm and
sample arm is approximately equal. The sample arm also contains a 2D lateral scanner and an objective lens. The detection arm includes a spectrometer, which consists