**9. Future directions**

A significant progress for neovascular AMD imaging was the development of FD-OCT tech‐ nologies. They use a central wavelength of 800-850 nm, a stationary reference arm, a high speed spectrometer and a charged-coupled device (CCD) line-scan camera. The mechanical scanning is not needed in order to detect light echoes simultaneously. As consequence, the aquisition speed increases to 25,000-52,000 A-scan/second. The axial resolution is of 3-7 μm, significantly improving the signal-to-noise ratio and the detection of individual retinal lay‐ ers and lesions components is possible [16].

#### **9.1. Limits of the current OCT examination techniques**

Despite the significant advantages previously mentioned, there are some limitations of SD-OCT: motion and segmentation artifacts, interinstrument comparability [11,12]. Despite the significant progress in retinal imaging offered by the current OCT techniques, TD-OCT and SD-OCT have shortcomings originating in the limitation of resolution, both axial and lateral. The absorbtion of infrared radiation by the anterior segment structures and ocular media limits the image resolution. The axial resolution is limited by the image scattering by the oc‐ ular structures (so-called speckle noise) and the lateral resolution limitation is determined by the restricted numerical aperture of the optical system [16].

#### **9.2. Swept source OCT**

**Figure 14.** Comparative aspects of the two eyes of the same patient: in the Right Eye the RPE band appears irregular

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

**Figure 15.** Macular cube 200x200 shows increased central macular thickness, elevations of the RPE band, associated

with increased vitreo-macular adhesion revealed by the 3D presentation of the macular cube

and there is some fluid in the retina.

SS-OCT is another form of FD-OCT that uses a light source with the wavelength of ap‐ proximately 1,050 nm. A short cavity-swept laser replaces the superluminiscent diode la‐ ser. The emission has different frequencies that can be rapidly tuned over a broad bandwidth [11, 28]. A high speed complementary metal oxide semiconductor camera (CMOS) and two parallel photodetectors are used in order to obtain scan rates of 100,000-400,000 A-scan/second, with the axial resolution of 5.3 μm over a 4-mm imaging range.(trebuie modificat) [29]. Advantages: images to the level of individual photorecep‐ tors are obtained, particularly when coupled with adaptive optics. With SS-OCT the socalled fringe washout (signals at the edges of the B-scan) is reduced as compared to SD-OCT; the sensitivity is better with imaging depth; the image range is longer: approximately 7.5 mm, which allows the evaluaiton of the anterior segment without the use of complex imaging techniques that might generate errors [9]; the efficiency of detec‐ tion is higher; the dual balanced detection can be performed. The above mentioned ad‐ vantages considerably decrease the patient-induced errors by movements and breathing and permit the better penetration in case of ocular media opacities [30]. Limits: even with best patient cooperation, images are still subject to artifacts. Therefore, various algo‐ rithms have been imagined in order to improve the resolution by eliminating these arti‐ facts [30]. The potential application of OCT in the sub-RPE space and choroid is limited by its shallow penetration: approximately 1-3 mm. The degree of choroidal penetration is determined by several factors: the proportion of scattered photons, the absorbtion spec‐ trum of water, the scatter by the ocular media, the absorbtion by melanin [10]. Photon scattering is a phenomenon that influences the image formation in OCT: photons that are singly scattered add to the OCT signal, whereas photons that are scattered multiple times contribute to the background noise [31]. The large amount of water within the eye limits the light wavelengths that can be used [32]. The absorbtion spectrum of water has two regions where the light absorbtion is low: at approximately 950 nm and between 1,000-1,100 nm [30]. The devices with wavelenghts in the range of 1,000-1,100 nm can be used for the enhanced sub-RPE imaging, with ultrahigh-speed image aquisition and axial resolution in the range of 8 μm [10]. This is useful in the managements of sub-RPE space diseases, particularly in AMD.

**Author details**

Simona-Delia Ţălu

**References**

cian 2011;8(1) 16-17.

379: 1728–38.

1178-1181.

berg, 2005

S74.

Address all correspondence to: simonatalu@yahoo.com

University of Medicine and Pharmacy, Cluj-Napoca, Romania

mol. 2010 ;33 : 605-609 doi:10.1016/j.jfo.2010.09.015.

mography. Prog Retin Eye Res. 2006;25 325-353.

Department of Surgical Sciences and Medical Imaging, Ophthalmology, "Iuliu Haţieganu"

New Insights into the Optical Coherence Tomography – Assessement and…

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

157

[1] Lujan B J. Revealing Henle's Fiber Layer Using Spectral-domain OCT. Retin Physi‐

[2] Lim LS, Mitchell P, Seddon JM et al. Age-related macular degeneration. Lancet 2012;

[3] El Maftouhi Q, Wolff B, Faysse MM. Kystes rétiniens externes dans la dégénéres‐ cence maculaire liée à l'âge exsudative : un nouvel aspect de l'OCT. J Fr d'Ophtal‐

[4] Talu SD, Talu S, Use of OCT Imaging in the Diagnosis and Monitoring of Age Relat‐ ed Macular Degeneration, in Age Related Macular Degeneration.The recent advances

[5] Huang D, Swanson EA, Lin CP et al. Optical coherence tomography. Science 1991;22

[6] Hee MR, Puliafito CA, Wong C et al. Quantitative assessment of macular edema with

[7] Ghazi N, Kirk T, Allam S et al. Quantification of Error in Optical CoherenceTomogra‐ phy Central Macular Thickness Measurement in Wet Age-related Macular Degenera‐

[8] F. G.Holz R. F. Spaide. Essentials in ophthalmology. Medical Retina, Berlin Heidel‐

[9] Costa RA, Skaf M, Melo LA Jr et al. Retinal assessment using optical coherence to‐

[10] Keane PA, Ruiz-Garcia H, Sadda SR. Clinical applications of long-wavelength (1,000 nm) optical coherence tomography. Ophthalmic Surg Lasers Imaging 2011;42 S67-

optical coherence tomography. Arch Ophthalmol. 1995;113 1019 – 1029.

tion. Am J Ophthalmol 2009;148:90–96 doi:10.1016/j.ajo.2009.02.017

in basic research and clinical care, ed. Gui-Shuang Ying, Rijeka, Intech, 2012.

#### **9.3. Adaptive optics**

Adaptive optics correct ocular aberrations during image aquisition, making possible the ob‐ taining of image resolution at the cellular level.

#### **9.4. Enhanced Depth Imaging OCT**

The principle of Enhanced depth imaging OCT (EDI-OCT) consists in placing the objec‐ tive lens of the Spectralis SD-OCT device (Heidelberg Engineering) closer to the eye, with the obtaining of an inverted image, which allows the deeper structures to be placed closer to the zero delay with the subsequent better visualization of the choroid. This principle is combined with the high speed scanning, eye-tracking system, image-averag‐ ing technology, reduced noise and greater coverage of the macula. All these improve‐ ments lead to the possibility to create high resolution, repeatable and reliable images of the choroid [33].

### **10. Conclusion**

At the present moment, OCT offers the most valuable data on the retinal structure. AMD is the retinal disease that benefited the most from the development of OCT techniques, espe‐ cially the wet form of this disease. SD-OCT has superior depth resolution as compared to TD-OCT: currently, the axial resolution varies from 3 – 7 μm, depending on the SD-OCT model. OCT is more reliable than biomicroscopy in assessing the macular thickness and small neurosensory and RPE detachments. OCT is more reliable than FA in identifying the intraretinal and subretinal fluid, as on FA the fluid in the inner retina can mask the fluid in the outer retina. OCT is less precise in evaluating the geographic extent of fluid within the macula, as compared to FA. Due to the cross-sectional imaging, OCT allows the localization of a CNV in relationship with the RPE and the neurosensory retina. OCT is superior to FA in identifying CNV membranes that are obscured by pooling of dye or by tiny retinal haemor‐ rhages. Cystoid macular edema is strongly associated with the classic CNV. OCT is very useful in AMD monitoring and decision-making after treatment: if fluid persists, re-treat‐ ment is indicated.
