Chapter 1

■Optical coherence tomography (OCT) is a valuable and indispensable tool in the current management of macular diseases.

■As in other imaging modalities, artifacts occur with both time-domain and spectral/Fourierdomain OCT imaging.

■Recognition of these artifacts will improve the management of macular diseases and prevent treatment decisions based on erroneous data.

■Stratus® OCT retinal thickness measurements are more to error in eyes with subretinal pathol-

ogy when compared with eyes with intraretinal pathology.

■In time-domain OCT, the topographic maps are derived from the individual radial scans; thus, maps should never be interpreted for artifacts without examining the individual scans.

■Despite the inherent advantages of spectral/ Fourier-domain OCT over the time-domain Stratus® OCT, the Stratus® OCT remains quite useful for the day-to-day clinical practice.

1.1  Introduction

Optical coherence tomography (OCT) is a noninvasive medical diagnostic imaging modality that utilizes light to image tissue using low coherence interferometry [1]. It measures the echo time delay and intensity of light that is reflected back from different structures in a tissue. In the OCT machine, a beam of light from a superluminescent diode is divided through a beam splitter into sample and reference beams. Light from the sample beam is directed toward the tissue of interest and depending on the composition of the internal tissular structures, the sample beam will be reflected with different echo time delays. Light from the reference beam is reflected from a reference mirror located at a known distance. Both the reflected reference and sample beams are combined by a modified Michelson interferometer and detected by the OCT [1, 2]. Detection of these beams is based on timedomain or spectral-domain protocols.

1The authors have no financial or propietary interest in any of the products or techniques mentioned in this chapter.

The most widely used OCT machine, Stratus® OCT (Carl Zeiss–Meditec, Dublin, California, USA), is based on time-domain detection. In time-domain OCT, the position of the reference mirror is adjusted, creating different time delays for the echoes of the reference beam. Axial scans (A scan) are obtained by directing the sample beam at different tissue depths and recording the reflected light echoes. The Stratus® OCT can make from 128 to 768 A scans in a single scan pass. Each A scan has 1,024 data points and is 2 mm long. By scanning the sample beam of light in a transverse direction, a cross-sectional OCT (B scan) image is obtained. Once the information is obtained, a tomogram is constructed using a false color scale that represents the amount of light backscattering from microstructures at different depths of the imaged tissue. In the false color scale, bright colors such as red to white represent high reflectivity and dark colors such as blue to black represent minimal or no reflectivity. The ganglion cell layer, the inner and outer nuclear layers have a low reflectivity, and therefore appear blue–black in the false color scale. Conversely, the retinal pigment epithelium (RPE)/choriocapillaris and the nerve fiber layer exhibit a high reflectivity [2, 3]. The hyperreflective band