**4. TD-OCT**

ror in the reference arm of the interferometer. A B-scan depicts a cross-section image

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‐

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

**Property TD-OCT SD-OCT**

An interferometer measures **sequentially** the echo delay time of light that is reflected by the retinal microstructures

**Image acquisition time** 1 – 2 seconds 60 times faster

6 radial scans are performed, 20 μ wide and 6 mm long (the area between the 6 scans is not imaged)

Cross-section images of the retina are obtained every 1.6 seconds: 400 scans/ second

**Image axial resolution** 10 - 15 μm 3-7 μm

**Presentation of the results** Two-dimensional images of the sample

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

**Principle** Low coherence interferometry Fourier transformation

**Modality of sampling** It samples one point at the time It samples all the points simultaneously

A spectrometer evaluates **simultaneously** the light reflection by the retinal microstructures

In a 6-mm diameter area, 65.000 scans are performed, without excluding areas; 128-200 scans over the same area

25.000 – 52.000 scans/second

internal structure 3D reconstruction possible

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

meaning a lateral x depth map which is generated by collecting many A-scans [4].

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

ology not evident with conventional 2D images [4].

evaluation of tissues [11,12].

**Modality of acquisition**

**Sampled area**

**Rate of acquisition**

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

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

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 and blue correspond to thinned retinal areas [8].

#### **4.3. Image resolution**

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

#### **4.4. Image processing and correction for eye motions**

Image acquisition takes about 1-2 seconds. Taking into account that the image resolu‐ tion is extremely high, the correction for eye motions is very important to avoid the ob‐ taining of blurred images. As consequence, image processing techniques had to be developed [8].

**5.3. Commercially available SD-OCT devices**

available SD-OCT devices are presented in table 2 [15].

3D-OCT 1000

3D-OCT 2000 (Topcon)

**6.1. Overview**

**Table 2.** Commercially available SD-OCT devices

tients is summarized in table 3 [8].

**6. Application area of OCT in AMD**

SD-OCT has superior depth resolution as compared to TD-OCT. Currently, the axial resolu‐ tion varies from 3 – 7 μm, depending on the SD-OCT model [18]. Several SD-OCT instru‐ ments are available at the current moment: Cirrus HD-OCT (Carl Zeiss Meditec), RTVue-Fourier DomainOCT (Optovue), Copernicus OCT (Reichert/-Optopol Technology), Spectral OCT/SLO (Opko/Oti), Spectralis HRA+OCT (Heidelberg Engineering), Topcon 3D OCT-1000 (Topcon) and RS-3000 Retiscan (Nidek). All the above mentioned instruments provide high quality images and offer the possibility of tridimensional reconstruction of the macula [13]. The main technical characteristics and differences between the commercially

New Insights into the Optical Coherence Tomography – Assessement and…

http://dx.doi.org/10.5772/53357

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**Device (Company) Axial resolution (μm) A-scans/second** Cirrus HD-OCT (Carl Zeiss Meditec) 5 27 000 Spectralis (Heidelberg Engineering) 7 40 000 RTVue-100 (Optovue) 5 26 000

Spectral OCT/SLO (OPKO/OTI) 5 27 000 SOCT Copernicus (Optopol) 6 25 000 SOCT Copernicus HR (Canon/Optopol, Inc) 3 50 000 SDOCT (Bioptigen) 4 20 000 Retinascan RS-3000 (Nidek) 7 53 000

Most cases of neovascular AMD are complicated by intraretinal fluid accumulation and RPE detachments. For many years, the therapeutic decision in neovascular AMD was based on the results of fundus biomicroscopy, fluorescein and indocyanine green angiography. In this context, the posttherapeutical evolution in many CNV membranes remained unsatisfactory. OCT technology offers subtle imaging of CNV membranes that appear as hyperreflective bands on OCT. Frequently, the identification of CNV depends on the reflectivity of the adja‐ cent structures and on the CNV localization related to it. OCT is more sensitive than biomi‐ croscopy in identifying retinal edema and small neurosensory and RPE detachments. The OCT relationship with fundus biomicroscopy and fluorescein angiography (FA) in AMD pa‐

6 18 000

### **5. SD-OCT**

#### **5.1. Principle of SD-OCT**

The development of the SD-OCT technique is originating in the Fourier mathematical equa‐ tion (1807). The french mathematician Joseph Fourier described the decomposition of a peri‐ odic function into a sum of simple sinusoidal-based oscillating functions. The practical effect of this abstract statement is the possibility to measure simultaneously all echoes of light, in contrast to TD-OCT where the echoes of light are measured sequentially by moving a mirror in front of the reference beam. The SD-OCT devices use a central wavelength of 800-850 nm, a stationary reference arm, a high speed spectrometer that analyses simultaneously all the frequencies and a charged-coupled device (CCD) line-scan camera. The mechanical scan‐ ning is not needed in order to detect light echoes simultaneously. As consequence, the aqui‐ sition speed increases to 25,000-52,000 A-scan/second and the amount of data that can be obtained during one session was improved significantly [15]. The axial resolution is of 3-7 μm (as compared to 10 – 15 μm with TD-OCT devices), significantly improving the signalto-noise ratio. Therefore, the detection of individual retinal layers and lesions components became possible [16].

#### **5.2. Clinical impact of SD-OCT**

The practical impact of the improvement in axial image resolution is the early detection of small cystic changes associated with the wet form of AMD. The early diagnosis is very im‐ portant for the early treatment and the better preservation of the visual function. Given the possibility to get images simultaneously in various planes, the 3D reconstruction is possible with SD-OCT, allowing the obtaining of hundreds of high-resolution images per second and the accurate measurement of the macula (total volume) in various conditions: edema, fluid, drusen, CNV. The reduction of the examination time considerably decreases the artifacts re‐ lated to eye movements and poor fixation of the low vision patients [17]. Another significant advantage of SD-OCT is represented by the increased retinal scan coverage. The SD-OCT images have proven to be clearer and with higher quality as compared to the ones obtained by the successive TD-OCT systems (OCT1, OCT3, stratus). The SD-OCT systems are contin‐ uously improving, by adding complementary functions: fundus photography, angiography, microperimetry. The ultra-high resolution images obtained by SD-OCT allow a better differ‐ entiation between the retinal and subretinal layers [4].

### **5.3. Commercially available SD-OCT devices**

**4.4. Image processing and correction for eye motions**

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

developed [8].

**5. SD-OCT**

**5.1. Principle of SD-OCT**

became possible [16].

**5.2. Clinical impact of SD-OCT**

entiation between the retinal and subretinal layers [4].

Image acquisition takes about 1-2 seconds. Taking into account that the image resolu‐ tion is extremely high, the correction for eye motions is very important to avoid the ob‐ taining of blurred images. As consequence, image processing techniques had to be

The development of the SD-OCT technique is originating in the Fourier mathematical equa‐ tion (1807). The french mathematician Joseph Fourier described the decomposition of a peri‐ odic function into a sum of simple sinusoidal-based oscillating functions. The practical effect of this abstract statement is the possibility to measure simultaneously all echoes of light, in contrast to TD-OCT where the echoes of light are measured sequentially by moving a mirror in front of the reference beam. The SD-OCT devices use a central wavelength of 800-850 nm, a stationary reference arm, a high speed spectrometer that analyses simultaneously all the frequencies and a charged-coupled device (CCD) line-scan camera. The mechanical scan‐ ning is not needed in order to detect light echoes simultaneously. As consequence, the aqui‐ sition speed increases to 25,000-52,000 A-scan/second and the amount of data that can be obtained during one session was improved significantly [15]. The axial resolution is of 3-7 μm (as compared to 10 – 15 μm with TD-OCT devices), significantly improving the signalto-noise ratio. Therefore, the detection of individual retinal layers and lesions components

The practical impact of the improvement in axial image resolution is the early detection of small cystic changes associated with the wet form of AMD. The early diagnosis is very im‐ portant for the early treatment and the better preservation of the visual function. Given the possibility to get images simultaneously in various planes, the 3D reconstruction is possible with SD-OCT, allowing the obtaining of hundreds of high-resolution images per second and the accurate measurement of the macula (total volume) in various conditions: edema, fluid, drusen, CNV. The reduction of the examination time considerably decreases the artifacts re‐ lated to eye movements and poor fixation of the low vision patients [17]. Another significant advantage of SD-OCT is represented by the increased retinal scan coverage. The SD-OCT images have proven to be clearer and with higher quality as compared to the ones obtained by the successive TD-OCT systems (OCT1, OCT3, stratus). The SD-OCT systems are contin‐ uously improving, by adding complementary functions: fundus photography, angiography, microperimetry. The ultra-high resolution images obtained by SD-OCT allow a better differ‐

SD-OCT has superior depth resolution as compared to TD-OCT. Currently, the axial resolu‐ tion varies from 3 – 7 μm, depending on the SD-OCT model [18]. Several SD-OCT instru‐ ments are available at the current moment: Cirrus HD-OCT (Carl Zeiss Meditec), RTVue-Fourier DomainOCT (Optovue), Copernicus OCT (Reichert/-Optopol Technology), Spectral OCT/SLO (Opko/Oti), Spectralis HRA+OCT (Heidelberg Engineering), Topcon 3D OCT-1000 (Topcon) and RS-3000 Retiscan (Nidek). All the above mentioned instruments provide high quality images and offer the possibility of tridimensional reconstruction of the macula [13]. The main technical characteristics and differences between the commercially available SD-OCT devices are presented in table 2 [15].


**Table 2.** Commercially available SD-OCT devices
