**3. Theoretical considerations on OCT examination techniques**

#### **3.1. History**

The concept of optical coherence tomography (OCT) was developed at Massachusetts Insti‐ tute of Technology in the early 1990s and the first commercial version of OCT was made available by Carl Zeiss (Jena, Germany) in 1996 [4]. The first applications of OCT were to provide quantitative and qualitative information about the peripapillary area of the retina and the coronary artery [5]. The first publication on time-domain OCT belongs to Huang and coworkers in 1991 [5]. Hee et al. published the first data on the quantitative evaluation of the macular edema in 1995 [6]. Since its introduction, OCT evolved into a powerful ex‐ amination tool for patients with retinal diseases [7].

#### **3.2. Optical tomography versus ultrasound**

For many years, the cross-sectional imaging of the eye has been possible only with the help of ultrasounds which allowed spatial resoultions of 150 μm. The development of new tech‐ niques was based on the use of higher frequency waves that created the possibility to obtain image resolutions of 20 μm. However, the strong atenuation in the biological tissues limits its use to the anterior structures of the eye [8]. The principles of the two imaging methods (OCT and ultrasounds) are similar, but OCT uses light instead of sound. The primary differ‐ ence between ultrasonic and optical imaging derivates from the different speed of sound and light. The light propagates nearly a million of times faster than sound, which allows the obtaining of measurements resolutions in the range of 10 μm or less at the posterior pole of the eye. Also, the use of light makes the examination more comfortable for the patient, as there is no need for physical contact with the examined eye [8].

### **3.3. OCT Instrumentation for retinal imaging**

The device operates as a fundus camera: a condensing lens of +78 dioptres is used so that the retina could be imaged in the same plane with the instrument. The magnification of the reti‐ nal image is determined by two factors: the refractive power of the condensing lens and the magnification of the ocular. At the lowest magnification, the typical field of view is 30º. If the visual acuity of the eye to be examined is very low and therefore the patient has fixation problems, a guiding light can be placed in front of the fellow eye, in order to stabilize the eyes during image acquisition. With the first generation OCT devices, pupil dilation was recommended with the aim of obtaining high quality images, but the latest generation OCT machines deliver good quality pictures with a 3-mm diameter pupil. The OCT technology can be limited by various conditions of the eye: lens or vitreal opacities, subretinal haemor‐ rhages, lack of foveal fixation, nystagmus [8].

#### **3.4. Principle of OCT**

**2. Problem statement**

**3.1. History**

A variety of retinal diseases such as AMD, central serous chorio-retinopathy, macular hole, vitreo-macular interface syndrome and diabetic maculopathy have taken advantage from the introduction of OCT in the clinical practice. Among these, AMD is the ocular condition that benefited the most from the enormous advantages offered by OCT imaging techniques, in terms of diagnosis, response to treatment and monitoring. Future progress in OCT techni‐ ques has already brought and is expected to bring new insights in understanding the patho‐ physiology of this potentially blinding disease. With the aim of defining the role of OCT in the assessment and follow up of AMD, the theoretical principles at the foundation of this retinal imaging technique are outlined, making a clear distinction between TD-OCT and SD-OCT methods. The advantages of SD-OCT over TD-OCT methods are revealed. The role of OCT in AMD management is emphasized by the description of typical OCT aspects in vari‐ ous AMD lesions: macular edema, CNV membranes, occult and classic choroidal neovascu‐ larization, pigment epithelial detachments (PED), external retinal cysts, vitreo-macular adhesions. OCT has a crucial role in monitoring AMD and establishing the indication of treatment with anti-VEGF intravitreal injections in the wet form of the disease. The place of OCT in the evaluation of AMD patients is completed by its comparative presentation with retinal biomicroscopy, fluorescein and indocyanine green angiography. The impact of OCT in AMD diagnosis and monitoring is illustrated with examples of various aspects that AMD can display, both with a TD-OCT device (Stratus OCT) and a SD-OCT one (Cirrus OCT). Fu‐

134 Age-Related Macular Degeneration - Etiology, Diagnosis and Management - A Glance at the Future

ture directions in OCT techniques development close the current presentation.

**3. Theoretical considerations on OCT examination techniques**

amination tool for patients with retinal diseases [7].

**3.2. Optical tomography versus ultrasound**

The concept of optical coherence tomography (OCT) was developed at Massachusetts Insti‐ tute of Technology in the early 1990s and the first commercial version of OCT was made available by Carl Zeiss (Jena, Germany) in 1996 [4]. The first applications of OCT were to provide quantitative and qualitative information about the peripapillary area of the retina and the coronary artery [5]. The first publication on time-domain OCT belongs to Huang and coworkers in 1991 [5]. Hee et al. published the first data on the quantitative evaluation of the macular edema in 1995 [6]. Since its introduction, OCT evolved into a powerful ex‐

For many years, the cross-sectional imaging of the eye has been possible only with the help of ultrasounds which allowed spatial resoultions of 150 μm. The development of new tech‐ niques was based on the use of higher frequency waves that created the possibility to obtain image resolutions of 20 μm. However, the strong atenuation in the biological tissues limits its use to the anterior structures of the eye [8]. The principles of the two imaging methods

OCT is based on the interferometry that uses a low-coherence light in order to measure the difference between the reflected light waves from the examined tissue and that from a refer‐ ence path [9]. The light reflected from the retinal structures interferes with the light of the reference beam. The detection of echoes resulting from this interference is measuring the light echoes versus depth [9,10].

*Axial resolution.* Axial resolution is given by the bandwdith of the light source and the level of coherence that depends upon the central wavelength [10]. The light from a superluminis‐ cent diode has long coherence that generates images with poor axial resolution. With shortcoherence light, interference is possible over short distances and therefore high axial resolution is possible [10].

*Transverse resolution.* The transverse resolution is determined by the size of the light spot that can be focused on the retina. The best structural resolution is obtained when the light is focused on the tissue to be examined. The absorbtion of central light by the tissue of interest must also be considered [10].

### **3.5. Time Domain (TD-OCT) versus Spectral Domain (SD-OCT)**

OCT is applied by two main methods: TD-OCT and SD-OCT.

TD-OCT produces two-dimensional images of the sample internal structure. An A-scan rep‐ resents a reflectivity profile in depth which is gradually built up over time by moving a mir‐ ror in the reference arm of the interferometer. A B-scan depicts a cross-section image meaning a lateral x depth map which is generated by collecting many A-scans [4].

**4. TD-OCT**

**4.1. Principle of TD-OCT**

The origin of TD-OCT imaging technique is found in the processes of absorbtion and disper‐ sion of light traversing tissues [13]. The creation of an image with the TD-OCT technique is based on the principle of *low coherence interferometry*. The source of light is represented by a su‐ perluminiscent diode that emits a radiation with the wavelength of 830-840 nm. This emission is split in two arms by an optical beam splitter functioning as interferometer: half the beam is reflected from the reference mirror and is named the *reference beam* and half of it is directed to the target tissue and is named the *detection beam*. The comparison of the tissue-reflected beam with the beam coming from the reference mirror measures the time delay between these two beams [13]. In order to understand the system operating, the corpuscular theory of light must be applied: the beam is made up of short pulses of light. The pulse of light reflected from the ref‐ erence mirror and the pulse of light coming from the analyzed tissue within the eye will coin‐ cide only if they both arrive at the same time, producing the phenomenon called *light interference*. For the light interference to occur, the distance traveled by the two above men‐ tioned beams must be equal. The interference is measured by a light-sensitive detector and it is translated into OCT image on the screen [8]. This method allowed to obtain cross-section im‐ ages of the retina every 1.6 seconds (400 A-scans per second) [14]. TD-OCT has limits represent‐ ed by: long acquisition times, limited image sampling (with the risk of overlooking small

New Insights into the Optical Coherence Tomography – Assessement and…

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macular lesions), limited resolution by motion artifacts and patient blinking [14].

**4.2. Tomographic imaging and volumetry – Interpretation of TD-OCT images**

and blue correspond to thinned retinal areas [8].

cellent, but through a thick haemorrhage is less than 100 μm [8].

**4.3. Image resolution**

The light source moves across the retina and the optical reflection and backscatter from the retinal structures are detected. Successive longitudinal measurements at transversal sequen‐ tial points are performed. This technique generates a two-dimensional image and a crosssectional map displayed in false colours. Each colour is given a certain degree of reflectivity. White and red colours corespnd to highly reflective tissues, whereas black and blue repre‐ sent low reflectivity structures. Green is given an intermediate reflectivity. Examples of hy‐ perreflective tissues are: fibrosis, haemorrhages, infiltrates [8]. The retinal layers are displayed on the linear scans and the retinal thickness can be measured taking as references the vitreo-retinal interface and the retinal pigmented epithelium, given their different reflec‐ tivity. By using 6 radial scans 30 degrees apart, a surface map can be obtained, in which white and red represent high volume structures (for example, macular edema) and black

The most important parameter that determines OCT image resolution is the coherence length of the light source. For the commercially available TD-OCT system, image axial reso‐ lution is in the range of 10-15 μm. The penetration through transparent optical media is ex‐

SD-OCT can be implemented in two formats, Fourier domain (FD-OCT) and swept source (SS-OCT). With SD-OCT units, all the A-scans in the reflected light are acquired at a given point in tissue. The moving mirror is not needed in order to obtain complete A-scans which allows the acquisition of the images about 60 times faster than with TD-OCT. The detection and monitoring of retinal diseases is improved with SD-OCT units because they have ultra high-speed scan rate, superior axial and lateral resolution, cross-sectional (2D) scan, 3D ras‐ ter scanning and a higher imaging sensitivity than the traditional TD-OCT units. The SD-OCT software is much improved compared with traditional TD-OCT and the great number of scans done per unit of time creates the conditions for the SD-OCT systems to generate 3D reconstructions which can be further manipulated with the aim to demonstrate subtle path‐ ology not evident with conventional 2D images [4].

Briefly, the advantages of SD-OCT over TD-OCT are: significant improvement of the image axial resolution, decreased acquisition times, reduction of motion artifacts, increased area of retinal sampling and the possibility to create topographic maps by the three-dimensional evaluation of tissues [11,12].


Table 1 summarizes the main differences between TD-OCT and SD-OCT.

**Table 1.** The main differences between the properties of TD-OCT and SD-OCT
