Page 58 - Computational Retinal Image Analysis

48 CHAPTER 3 The physics, instruments and modalities of retinal imaging

shorter axial range B-scan images can be produced in real-time [58]. As illustrated
in Table 1, the MS is the only spectral domain technique that truly allows for real-
time operation.
In Fig. 22A a B-scan image of the anterior chamber of the eye, produced in real-
time (50 Hz) using a MS SS-OCT instrument is demonstrated. The real-time opera-
tion is achievable as the MS technique allows to limit the mathematical calculations
to a specific region of interest. This would not be possible with the same CPU if FFT
based technique is employed.
The sensitivity advantage of SD-OCT over conventional TD-OCT was not only
demonstrated theoretically [55] but also proven in clinical studies. It has been for ex-
ample shown that when used to evaluate macular morphology alterations, SD-OCT
performs better as these alterations are typically related to choroidal neovasculariza-
tion [59], of which SD-OCT is able to provide better images. This is obvious if we
compare the TD base B-scan image from Fig. 1 to the SS based B-scan from Fig. 23.
Fig. 23 was produced in real-time, at 50 Hz using a swept-source of 100 nm spectral
bandwidth and central wavelength 1060 nm.
TD-OCT still has the advantage of producing en-face views in real-time, how-
ever of lower sensitivity than the SD-OCT methods. The MS approach can never-
theless deliver en-face images in real-time whilst taking advantage of the greater
sensitivity of the SD methods. Such an example is presented in Fig. 24B, where
real-time en-face images from 36 axial positions are displayed simultaneously, to-
gether with one (or more) B-scan views (Fig. 24A) and a confocal image allowing
for guidance.
3.8.6 Modern topics in optical coherence tomography for eye imaging
In the recent years, several functional extensions of optical coherence tomography
have emerged to broaden the potential clinical applications of OCT by providing
novel ways to understand tissue activity.
Optical Coherence Tomography—Angiography (OCTA). OCTA can be used to
measure or monitor the motion and flows of biologic fluids. This method provides a
depth resolved profile of the flow velocity in the blood vessel, with the resolution de-
termined by the coherence length of the source employed. OCT- microangiography
can provide enhanced visualization of retinal and choroidal vasculature. The beauty
of this technology is that it does not require any dye injection as initially demon-
strated in combined en-face fluorescence/TD-OCT instruments [60].
Spectroscopic OCT (SOCT) allows simultaneous OCT measurements in multiple
spectral windows. SOCT can provide information on the oxygenation or on the con-
centration of specific constituents of the tissue by exploiting their spectral absorption
behavior. The larger the number of interrogating wavelengths the better the quan-
titative analysis. The availability now of large bandwidth sources, such as super-
continuum ones allows novel implementation of SOCT to provide depth resolved
distribution of chromophores in tissue, not technologically possible using combina-
tions of discrete sources as well as the development of ultra-high resolution OCT
(UHR-OCT) instruments.