Biomedical Engineering Reference
In-Depth Information
5.5.2
Frequency-Domain OCT
Frequency-domain OCT (FD-OCT) [ 20 , 75 ] is a variant interferometric imaging
modality. It has been widely attracted in the biomedical imaging field due to
its higher sensitivity and imaging speed compared to conventional TD-OCT. The
principle of FD-OCT relies on the transformation of the OCT time varying signal
along the optical axis, termed the A-scan, into the frequency domain. The basic
physics behind FD-OCT is based on the inverse scattering theorem. According to
the Wiener-Khintchine theorem, the spectral power amplitude of the backscattered
wave equals the Fourier transform of the axial distribution of the object scattering
potential. FD-OCT has the advantage that the full sample depth information is
obtained in a parallel manner such that no moving parts are necessary. Based
on the implementation, the FD-OCT can be divided into two classes, spectral-
domain OCT (SD-OCT) and swept-source OCT (SS-OCT). In SD-OCT, the optical
frequency components are captured simultaneously with a dispersive element and
linear detector; on the other hand, in SS-OCT, the optical frequency components are
captured by a single detector in a time-encoded sequence by sweeping the frequency
of the optical source. Recent studies have shown that Fourier domain OCT can
provide signal-to-noise ratio that is more than 20 dB better than the conventional
TD-OCT.
However, SD-OCT also has remarkable drawbacks [ 76 - 78 ]. One limitation in
frequency domain is that the optical spectrum at the output of the interferometer
consists of symmetric spectral terms, which cause same image results for positive
and negative OPDs called mirroring effect. Different methods have been developed
to eliminate this problem, such as phase-shifting interferometry or complex signal
processing. The simple detection in the frequency domain usually introduces spatial
frequency ambiguity; autocorrelation artifacts, originating from internal interfer-
ences of the sample and the system; as well as depth-dependent SNR loss, caused
by finite spectral linewidth [ 11 ]. Furthermore, nonuniform FD sampling deteriorates
TD signal quality, and time-consuming post-processing is a necessity for all FD-
OCT techniques. It was found that some of these effects can be largely compensated
by multiple, phase-shifted imaging of the same region (i.e., A-scan), followed
by reconstruction of the complex phase of the FD-signal [ 79 ], resampling, and
transformation into the time domain. Moreover, for getting better lateral resolution,
the technique such as dynamic focusing, which is generally used in time-domain
OCT, is not easily implemented in FD-OCT. In traditional FD-OCT, the imaging
optics are devised with large depth of focus, to accommodate the entire range of the
A-scan, usually 1-2 mm, which limits the possibility of using high-NA objectives to
enhance the transverse resolution. However, recently, by employing a Bessel beam
with axicon lenses, the extended-focal length FD-OCTs were demonstrated with
cellular-level resolution.
Search WWH ::




Custom Search