**2. Theoretical consideration on OCT**

The principle of tomography consists in the reconstruction of cross-sectional images of an object using its projections. The concept of OCT developed in 1990 and the first commercial version of OCT belongs to Carl-Zeiss in 1996. The first clinical application of OCT technology was in the field of ophthalmology. The obtained images reveal the optical properties of the scanned tissues and not the tissues themselves, and they are very similar to the histological sections.

There are two OCT methods: time domain (TD-OCT) and spectral domain (SD-OCT). The properties of the two methods are synthesized in **Table 1** [3, 4].

the evaluation of macular thickness. SD-OCT allows photoreceptors external segments be differentiated from the retinal pigmented epithelium (RPE) and are included in the value of

**Device (company) Axial resolution (μ) Scans/s** Cirrus HD-OCT (Carl Zeiss Meditec) 5 27,000 Spectralis (Heidelberg Engineering) 7 40,000 RE Vue (Optovue) 5 26,000

Spectral Domain Optical Coherence Tomography in the Diagnosis and Monitoring of Diabetic…

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

6 18,000

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Many SD-OCT devices are used in the clinical practice. Their axial resolutions and scanning

OCT and ultrasounds are complementary methods in assessing the vitreo-retinal interface. Ultrasound scans provide a more complete image of the vitreo-retinal interface, but with significantly lower resolution. OCT offers a high resolution image of a limited area. Since OCT

OCT is a noninvasive, well-tolerated method, easy to understand and explain. It offers qualitative information on retinal thickness, and it is reliable and reproducible. OCT reveals the

OCT technology holds the promise for the unprecedented capability to describe and monitor

The normal retinal layers have different reflectivity on OCT scans. Thus, the nerve fibers and the retinal pigmented epithelium (RPE) display high reflectivity, the plexiform and the nuclear layers have intermediate reflectivity and the photoreceptors display low reflectivity [3, 4]. The most commonly scan algorithms used in the clinical practice are the line and the volume (cube). Retinal thickness is automatically measured by the use of device software. The

uses light to acquire images, its use is limited by media opacities [5, 6].

presence and extension of vitreo-macular traction (VMT) [5, 6].

the macular thickness [3, 4].

3D OCT-1000

3D OCT-2000 (Topcon)

speeds are presented in **Table 2**.

**Table 2.** SD-OCT devices used in ophthalmology.

the changes in retina geometry.

**3. Normal OCT aspect of the macula**

The most common application of OCT is the measure of macular thickness. With TD-OCT, macular thickness map is calculated from six radial scans crossing at the fovea from which result the medium macular thickness and the total macular volume. With SD-OCT, image resolution is significantly higher. Different landmarks are used to calculate macular thickness with TD-OCT and SD-OCT. With TD-OCT, photoreceptors external segments are not differentiated from the retinal pigmented epithelium (RPE) and are therefore excluded from


**Table 1.** Properties of TD-OCT and SD-OCT.

Spectral Domain Optical Coherence Tomography in the Diagnosis and Monitoring of Diabetic… http://dx.doi.org/10.5772/intechopen.78681 43


**Table 2.** SD-OCT devices used in ophthalmology.

OCT represented a breakthrough in the management of DME, both for its diagnosis and monitoring of treatment effects. OCT technology offers valuable quantitative data (macular thickness, extension of edema, macular volume) and qualitative ones (macular morphology,

The advantages of OCT over other imaging modalities are as follows: its noninvasiveness,

The principle of tomography consists in the reconstruction of cross-sectional images of an object using its projections. The concept of OCT developed in 1990 and the first commercial version of OCT belongs to Carl-Zeiss in 1996. The first clinical application of OCT technology was in the field of ophthalmology. The obtained images reveal the optical properties of the scanned tissues and not the tissues themselves, and they are very similar to the histological

There are two OCT methods: time domain (TD-OCT) and spectral domain (SD-OCT). The

The most common application of OCT is the measure of macular thickness. With TD-OCT, macular thickness map is calculated from six radial scans crossing at the fovea from which result the medium macular thickness and the total macular volume. With SD-OCT, image resolution is significantly higher. Different landmarks are used to calculate macular thickness with TD-OCT and SD-OCT. With TD-OCT, photoreceptors external segments are not differentiated from the retinal pigmented epithelium (RPE) and are therefore excluded from

> A spectrometer evaluates *simultaneously* the back scattering of the light by the retinal structures

vitreo-macular interface) [3].

42 OCT - Applications in Ophthalmology

sections.

rapidity and safety profile [3, 4].

**2. Theoretical consideration on OCT**

properties of the two methods are synthesized in **Table 1** [3, 4].

**Property TD-OCT SD-OCT** Principle Low-coherence interferometry Fourier equation

back scattering of the light by the retinal

Acquisition time 1–2 s 60 times faster

Display 2D 3D Axial resolution 10–15 μ 3–7 μ

Sampling Point by point All points simultaneously

Target area Six radial scans 20 μm wide and 6 mm long Φ 6 mm = 65,000 scans Acquisition rate 400 scans/s 25,000–52,000 scans/s

Acquisition An interferometer evaluates *sequentially* the

structures

**Table 1.** Properties of TD-OCT and SD-OCT.

the evaluation of macular thickness. SD-OCT allows photoreceptors external segments be differentiated from the retinal pigmented epithelium (RPE) and are included in the value of the macular thickness [3, 4].

Many SD-OCT devices are used in the clinical practice. Their axial resolutions and scanning speeds are presented in **Table 2**.

OCT and ultrasounds are complementary methods in assessing the vitreo-retinal interface. Ultrasound scans provide a more complete image of the vitreo-retinal interface, but with significantly lower resolution. OCT offers a high resolution image of a limited area. Since OCT uses light to acquire images, its use is limited by media opacities [5, 6].

OCT is a noninvasive, well-tolerated method, easy to understand and explain. It offers qualitative information on retinal thickness, and it is reliable and reproducible. OCT reveals the presence and extension of vitreo-macular traction (VMT) [5, 6].

OCT technology holds the promise for the unprecedented capability to describe and monitor the changes in retina geometry.

### **3. Normal OCT aspect of the macula**

The normal retinal layers have different reflectivity on OCT scans. Thus, the nerve fibers and the retinal pigmented epithelium (RPE) display high reflectivity, the plexiform and the nuclear layers have intermediate reflectivity and the photoreceptors display low reflectivity [3, 4]. The most commonly scan algorithms used in the clinical practice are the line and the volume (cube). Retinal thickness is automatically measured by the use of device software. The distance between the vitreo-retinal interface and the anterior surface of the RPE is generally comprised between 200 and 275 μm; the foveal depression ranges from 170 to 190 μm and the thickness of the peripheral retina is between 220 and 280 μm [5].

therapeutic option in these cases: vitrectomy with release of vitreo-macular traction/dissection of ERM, from the early stages, when the chances for a good functional outcome are the highest. Assessment of vitreo-macular interface is an important step in evaluating diabetic patients. Not only does OCT indicate the moment for vitrectomy in these patients, but it also monitors the postoperative morphological outcomes: favorable evolution (decrease of macular thickness),

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The detached posterior vitreous face appears on OCT scans as a thin horizontal or oblique line with low/medium reflectivity in the nonreflective vitreous, above or inserting into the retina. If PVD is incomplete, it may adhere to the foveal or peripapillary region. ERM appears on OCT as a hyperreflective line on the surface of the retina. Its presence leads to retinal modifications: increase of macular thickness, loss of foveal depression, formation of intraretinal cysts and pseudoholes. The difference between PVD and ERM is made according to their reflectivity (low in PVD, high in ERM). OCT provides other details related to ERM (degree of opacity, thickness, and distance from the macula) and to its effects on the underlying retina:

OCT is a very reliable and reproducible method to assess and monitor macular thickness following various treatments for DME: intravitreal injections with anti-VEGF and steroids, laser photocoagulation, vitrectomy. OCT also identifies macular atrophy which explains functional failure despite resolution of edema. Monitoring patients with DME must include two major

Cystoid macular edema (CME) appears like large ovoid spaces of low reflectivity separated by hyperreflective septae that represent intraretinal cystoid-like cavities. Posterior hyaloid traction (PHT) appears like a highly reflective band on the surface of the retina. Serous retinal detachment (SRD) appears as a dark accumulation of subretinal fluid beneath the high reflective and dome-like elevation of detached retina. The highly reflective band which represents the outer surface of the retina helps differentiating subretinal fluid from the intraretinal fluid. Tractional retinal detachment (TRD) is identified as the area of low signal underlying the highly reflective border of detached retina. It often takes the appearance of a pick-shaped configuration [5, 6].

There is a correlation between macular thickness and visual acuity in patients with DME. The

In DME OCT technology has significant impact at various levels: it elucidates the pathogenic mechanisms of DME; it has a major contribution in identifying hyaloid-macular traction; it identifies the subclinical DME allowing early treatment; it makes it possible to correlate macular thickness with visual acuity; it monitors the evolution of DME following treatment [5–7].

Along with the development of OCT imaging, various classifications of DME have been elaborated. The first OCT classification of DME belongs to Otani, and it is based on morphological

development of ERM or of lamellar macular hole [6].

distortion, edema, neurosensory detachment [6].

**5. OCT classification of macular edema**

parameters: functional (visual acuity) and anatomical (OCT) [5, 6].

The most common finding in diabetics is diffuse retinal thickening (DRT) [6].

OCT pattern that was associated with worse visual outcome is CME [6].
