**4. First clinical applications and the early diagnostic efficacy**

The time-domain (TD)-OCT imaging was firstly developed in 1990s. It included 3.4 mm scan around the optic nerve head and six radial scans (6 mm) at the macula for assessment of glaucoma, optic neuropathies and retinal diseases [4]. Then, the introduction of the 3D approaches allowed the imaging of ganglion cell complex GCC, RNFL thickness map, comparison to a normal population and comparison of different scans over time for detection of any subtle structural changes reflecting the disease progression. Also, the progression in OCT development allowed better thickness measurements, analysis of the vitreoretinal interface and detection of biomarkers as predictors of the surgical outcome and visual prognosis.

Anterior segment AS-OCT is a non-contact imaging device that provides the detailed structure of the anterior part of the eye including the cornea and anterior chamber angle. It is beneficial in presurgical and postsurgical assessment of corneal and lenticular refractive surgeries. Unlike ultrasound biomicroscopy UBM, it does not allow visualization of the ciliary body.

### **5. Time domain vs. Fourier domain**

#### **5.1 Advantages and disadvantage**

TD-OCT uses low coherence interferometry with the light being split to be sent to both a scanning reference mirror and the sample. The interference occurs when the reflected beams recombine. The intensity information can be extracted from the

#### **Figure 2.**

*Principles of different approaches of OCT, time domain versus Fourier domain (figure is taken from Wolfgang et al. [10]).*

interference profile. Different depths in the tissue sample can be scanned by changing the location of the reference mirror [4]. The TD-OCT approach encodes the location reflections and relates them to the position of the moving reference mirror, and the time-encoded signals are recorded sequentially. It is limited by the slow-scan acquisition time and 2D imaging with an axial resolution of ∼10 μm2 [2] (**Figure 2**).

The A-scans in the other approach are obtained using a Fourier transform of the detected frequencies. It uses a single axial scan by evaluating spectrum of interference between stationary reference mirror and reflected light. The light echoes come at the same time from all axial depths with simultaneous capture of all the spectral components, hence the advantages of improving 3D scan resolution with an axial resolution of ∼2-5 μm2 and reducing the acquisition time up to 27,000–100,000 A-scans/second9. The Fourier domain includes both the spectral domain (SD) and the swept-source (SS)- OCT. In the SD-OCT approach, a broad-bandwidth light source, charge-coupled device (CCD) camera and a spectrometer are used to acquire frequency information, while the SS-OCT is sweeping a narrow band through broad range with a photodetector [2] (**Figure 3**). The limits of pixels of CCD camera and the dispersed interference pattern immediately before detection are drawbacks of SD over SS-OCT, while the SS-OCT has the advantages of point detection, less movement artifacts, better signal-to-noise ratio with better visualization of different retinal and choroidal pathologies [4, 8] (**Figure 3**).

#### **5.2 The swept-source technology**

It is a unique combination of high speed, deep tissue penetration and high-resolution OCT technology. It allows simultaneous visualization of the vitreous, retina and choroid. Visualization of choroidal structure may play a role in understating the pathogenesis of retinal diseases. Presenting the optic nerve and macula on the same scan is possible with its wide scans. Longer wavelengths are beneficial in eyes with hazy fundus view due to media opacity as in cataractous eyes [12].

#### **Figure 3.**

*Normal OCT-macula as captured by A: Time-domain OCT, B: Spectral domain OCT and C: Swept source OCT [11].*

The advantage in SS-OCT performance is due to the laser source performance (a wavelength-swept laser). The advances in development of several types of wavelength-swept lasers improve the speed and depth of imaging and eliminating the mechanical movement (active mode-locking (AML) laser) [13].
