**2. Strengths and limitations [8]**

scanning laser polarimetry (SLP) and confocal scanning laser ophthalmoscope (SCLO) [4,5]. Stereophotographs of the optic discs was proven to be as efficacious in detecting glaucoma as

The new imaging modalities on the optic disc and RNFL include the confocal scanning laser ophthalmoscopy (CSLO), optical coherence tomography (OCT) and scanning laser polarime‐ try (SLP). The first two technologies can analyze both the optic nerve head and RNFL while

The CSLO technology is used by the Heidelberg Retinal Tomograph (HRT, Heidelberg Engineering, Heidelberg, Germany). It is based on the principle of two conjugated pinholes. Laser light (670nm) enters through one pinhole and focuses on a plane of the retina or the optic disc. The reflected light passes through the confocal pupil and allows reflected light only from that specific plane to enter the photodetector. The focused laser light scans across the optic nerve head (ONH) and RNFL along the x and y axes at planes of different depth acquiring a series of images. This series is reconstructed to produce a three dimensional image. Each series consists of 16 images per mm and for a 4 mm depth scan 64 images are captured. A fundamental part of the SCLO technology is the reference plane. It is defined as a plane parallel to the retina and lies 50 µm below the temporal part of the scleral ring of Elsching. In ONH analysis structures above the reference plane are read as neuroretinal ring and structures below are read as disc cup. SCLO has a transverse resolution of 10 µm and an axial resolution of 300 µm.

**Figure 1.** Light from the laser device passes through a pinhole sitting in front of it and focuses on a certain plane in the retina. The reflected light from the retina enters a confocal pinhole sitting in front of the photodetector. Only light

the objective analysis the optic nerve head with the new modalities [6,7].

SLP analyzes the thickness of the RNFL only.

296 Glaucoma - Basic and Clinical Aspects

The field of view of the image is 15°×15°.

*1.1.2. Confocal Scanning Laser Ophthalmoscopy (CSLO, fig 1)*

The advantages of the new version of CSLO (HRT 3) is the large normative database which includes subjects European, African and Indian ancestry and can analyze both optic nerve head and RNF. Its limitation is that some optic nerve head measurements rely on a reference plane based on a hand drown contour line around the disc margins. The Glaucoma Probability Score does not need a reference plane. HRT measurements can be influenced by intraocular pressure fluctuations [9].

### **2.1. Optical Coherence Tomography (OCT, fig 2)**

Optical coherence tomography uses the principle of interferometry to construct high resolution cross-sectional images of the retina. An 800 nm laser light is split into two beams before entering the eye. The imaging beam consists of short pulses of light (the duration of each pulse is defined as the coherence length). One beam enters the eye and is reflected from the retina and the second beam is reflected from a reference mirror that moves back and forth along the Z axis. When the two reflected light beams constructively interfere they create a signal read by the interferometer. The time delay of the back scattered light from each layer of the retina differentiates the depth location of each layer (time-domain OCT). As a consequence in time domain OCT the instrument needs to perform two scans: a transverse scan across the eye (x axis) and a depth scan (z axis).The upgrade of time-domain OCT is the spectral-domain or Fourier-domain OCT (SD OCT/FD OCT). The SD OCT instead of the mechanical movement of the reference mirror analyzes with the aid of a mathematical equation (Fourier transform: FS(z) ∝ FT{AS(K} ) multiple wavelengths reflected from the retina. SD OCT obtains retina scans much faster (as the movement of the reference mirror along the z axis is omitted and only the scanning of the beam along the x axis is used) and with a better resolution (5-6 µm axial resolution, 10-15 µm transverse resolution) than the time-domain OCT. For the analysis of the optic nerve head the OCT runs six scans across the optic disc in a spoke-like pattern (fig 3). The measurements of the area between the scans are interpolated from the values across the scans. The edge of the optic nerve head is automatically defined as the end of the retinal pigment epithelium (RPE)/choriocapillaris layer. A straight line is taken from one edge of the RPE to the other and a reference plane is set 150 µm above this line. Neuroretinal rim is defined as the area above the reference plane and cup the area below it.

**Figure 3.** Optic nerve head analysis of a normal optic disc. The disc margins are identified by the OCT but the examiner can accurately identify the true disc border by manually moving the blue squares. The parameters measured are

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**Figure 4.** RNFL analysis with OCT. The numbers refer to RNFL thickness in μm. The green shaded area represents the normal RNFL thickness in the normative database of the OPKO spectral-domain OCT/SLO (Opko/OTI, Ophthalmic Technologies Inc, Toronto, Canada). Ninety five percent of the age-matched subjects with normal RNFL thickness will be included in the green area. On the other hand <5% of the subjects with normal RNFL thickness will fall in the yel‐ low shaded area and <1% of the normal subjects will be in the red shaded area. In this patient the blue contour line of their RNFL thickness has the characteristic double hump appearance and falls in the green area. The RNFL thickness is normal for the age of this patient. The double hump pattern of the RNFL is due to the increased thickness of the fiber layer in the superotemporal and inferotemporal sector. The RNFL thickness is measured around a 3.46 mm diameter

shown in the figure

circle centered on the optic disc.

**Figure 2.** The beam from super-lumiscent diode laser source is split as it travels through the beam splitter (BS). One beam goes to the reference mirror (mirror) and the second beam in the tissue to be examined. The two beams are reflected back and they interfere as they enter the interfereometer (spectrometer in the figure). The mirror moves back and forth in order to create constructive interference at different depths (represented by different colors in the sample) of the examined tissue (z axis). The beam also travels across x axis in order to capture a slice of the sample.
