The role of OCT in glaucoma management

Introduction

With glaucoma, there is progressive and irreversible loss of both the retinal nerve fiber layer (RNFL) and ganglion cell layer, with corresponding optic disc and visual field changes.

The structural changes that are most clinically recognized include generalized or localized thinning of the neuroretinal rim and deepening of the optic cup. Several techniques are currently available to detect, document, and quantify optic disc changes, such as clinical examination, photography, and modern imaging devices. The wider availability of the latter, with automated image acquisition and

Corresponding author. Tel./Fax: +390630154853; E-mail: dlepore@rm.unicatt.it

objective analysis, represents an important step in the clinical management of glaucoma by enhancing the detection of structural changes.

Devices allowing early detection and prevention of glaucomatous damage are therefore of vital importance in patient management. The detection of glaucomatous degeneration of the RNFL is likely to be valuable in the early diagnosis of glaucoma because it may precede the clinical findings of optic disc cupping and visual field loss.

The optical properties of the RNFL have allowed recent advances in ocular imaging technology to obtain thickness measurements (Hougaard et al., 2007). Optical coherence tomography (OCT) is a noncontact, noninvasive, diagnostic tool to measure RNFL thickness. It is able to precisely distinguish (8 to 10 mm resolution) the interface between the vitreous cavity and the retinal nerve fiber surface

140

anteriorly and the retinal nerve fibers and retinal ganglion cells posteriorly. OCT obtains measurements of RNFL thickness with a fast image acquisition rate (400 A-scans per second) and good resolution.

How OCT works

OCT imaging is similar to ultrasound B-mode imaging; however, it uses light waves instead of acoustic waves. OCT performs cross-sectional imaging by measuring the echo time delay and intensity of light that is scattered or reflected back from tissue source in question. OCT images are twoor three-dimensional datasets that represent variations in optical backscattering of reflection in a cross-sectional plane or volume of tissue (Fujimoto et al., 2004).

As opposed to sound waves, light is highly scattered or absorbed in most biological tissue, and therefore optical imaging is restricted to tissues that are optically accessible. OCT uses near-infrared interferometry, and is therefore not affected by refractive status, axial length, or by changes in nuclear sclerotic cataract density. However, posterior subcapsular and cortical cataracts do impose limitations to the performance of OCT. Another advantage to OCT is that it can be performed without physically coming into contact with the patient’s eye, thus minimizing discomfort.

Furthermore, light provides a significantly higher spatial resolution than ultrasound waves. Standard OCT images have an axial resolution of 10 mm, which is 10 to 20 times finer than that of standard ultrasound B-mode imaging. Research OCT systems for ultra-high resolution imaging can achieve an even finer resolution of 3 mm.

To perform cross-sectional optical imaging, OCT first measures the distance between tissues, similar to ultrasound A mode. This is accomplished by directing the light into the eye and measuring the echo time delay and intensity of backscattering or back reflection from different eye structures. Because light travels much faster than sound, a direct measurement of optical echoes is not possible. OCT uses a correlation technique known as lowcoherence interferometry. Due to extremely weak

backscattering or back reflection, OCT requires a high-sensitivity detection known as ‘‘optical heterodyne detection,’’ which was originally developed for optical communications systems. The quality of OCT imaging is largely influenced by image resolution, pixel density, and speed of image acquisition.

Longitudinal (axial) image resolutions and transverse image resolutions are determined by completely different mechanisms when using OCT. The axial resolution depends upon the coherence length of the light source. This determines the accuracy with which distance can be measured and is characterized by its frequency or wavelength bandwidth. In contrast to conventional microscopy, OCT has a good axial resolution and is independent of focusing conditions and depth of field. This is achieved with a small focusing angle (or small numerical aperture focusing) and a large depth of focus using low-coherence interferometry. With the use of short pulse laser light sources that

The transverse resolution is determined by the same principles as the transverse resolution in conventional optical microscopy, particularly the spot size of the focused OCT imaging beam, the diffraction properties of light, and the focusing parameters used. To achieve an extremely small spot, it is necessary to have a large beam diameter and a lens with a short focal length. Similar to optical microscopy, higher magnifications have a very short depth of field, and while smaller spot sizes improve the transverse resolution, they have a very shallow depth of focus (the depth of field decreases with the square of the focused spot size). The smallest spot size that can be achieved on the retina is limited by optical aberrations in the eye. Typical ophthalmic OCT systems have a 20 mm spot size on the retina.

Pixel density influences the quality of OCT imaging. It is analogous to pixel density in digital photography. The image must have sufficient pixel density in order to be able to visualize small features with a given resolution. Because OCT images are generated by acquiring successive axial measurements (axial scans) of back reflection or backscattering versus depth at different transverse