**3. OCT use in glaucoma and some basic protocols**

inner plexiform layer (IPL) where ganglion cell complex (GCC) consists of RNFL, GCL and IPL thickness [1]. The ganglion cell damage occurs at the lamina cribrosa level in the optic disc; first, the axons get damaged, then the ganglion cells which these axons are connected to disappear through a programmed cell death called apoptosis. This loss of axons and ganglion cells cannot be identified with any clinical diagnosis methods before it exceeds a certain critical threshold [2–5]. Research in this area shows that the earliest symptom that can be detected clinically in glaucoma is the loss or thinning of the retinal nerve fiber layer (RNFL). The loss or thinning of the neuroretinal tissue is usually detected later on [6–8].

Circumpapillary RNFL (cpRNFL) and GCC thickness measurements are the parameters that have high performance in detecting glaucoma-detecting ability and are comparable to

OCT is an imaging method that obtains high-resolution sections of biological tissues, and it is possible to simply define this mechanism as the conversion of the light that is reflected from

Commonly used in the area of ophthalmology especially in the past 20 years, the optical coherence tomography (OCT) has provided significant contributions for the early diagnosis of the glaucoma disease and monitoring and analysis of the glaucoma patients [14–18].

OCT is first developed by Huang et al. in 1991, at the Massachusetts Institute of Technology in Boston [19]. Studies of Dr. Fujimoto who was an important member of the team who developed OCT, on femtosecond lasers and interferometers that can release energy in very short periods of time have been defining in terms of the development of the device [20]. The first OCT device known as the OCT-1 has been introduced by a company called Humprey, which was acquired by Carl Zeiss in 1991. In the following years, OCT-2, which had an increased resolution, and finally OCT-3 (Stratus OCT) were developed. All three devices are referred to

This term is used more commonly, especially after the spectral domain (Fournier domain) OCT technology became available in 2002, in order to clarify the difference between the two technologies. OCT-3 is the last manufactured product with the time domain technology, which provides a significant increase in resolution compared to OCT-1 and OCT-2

Today, all OCT devices that are manufactured have the spectral domain technology. Simply put, while the operating principle of time domain OCT is associated with the delay in the reflection time of light, the actual variable in spectral domain OCT is the change in the optic frequency. The important differences between these devices that demonstrate themselves on clinical basis are the high axial resolution, being affected by eye movements at a minimum level and low artifacts. To date, axial resolution obtained through spectral domain OCT devices has reached up to a value of 3 microns, and these devices are rightfully referred to as OCTs with very high speeds and very high resolutions

cpRNFL thickness [9–11].

156 OCT - Applications in Ophthalmology

the tissue to an image [12–14].

**2. OCT from past to present**

as time domains [21–24].

[25, 26].

[27, 28].

There are several OCT devices working with similar principles but vary in diagnostic ability, acquisition speed and resolution. In this section, three commonly used SD-OCTs that are the Spectralis (Heidelberg Engineering, Dossenheim, Germany), the Cirrus (Carl Zeiss Meditec, Dublin, CA) and the RTVue (Optovue Inc., Fremont, CA) and their features are discussed. Several studies have addressed the diagnostic accuracy of the SD-OCTs one by one or compared them to time-domain technology [1, 14, 32–42].

Since RNFL is, without a doubt one of the most important factors which is also discussed extensively in the literature, a comparison between the protocols through this value will be made and the diagnostic accuracy of this value will be examined [27].

However, in recent studies comparing protocols on the capability of OCT in the diagnosis of glaucoma, no significant differences were observed between the protocols [14, 31, 32].
