Spectral/Fourier Domain Optical Coherence Tomography

Abstract

Optical coherence tomography (OCT) is a powerful diagnostic imaging tool with which multiple quantitative and qualitative studies on diseases of the eye have been performed. 1–7 Optical coherence tomography imaging is analogous to ultrasonic imaging, except that it measures echo time delays of light instead of sound. 1 ,8 Because light travels extremely quickly, echo time delays cannot be directly measured, and correlation techniques are required. Using a technique called low-coherence interferometry, a beam of light from a low-coherence light source, such as a superluminescent diode (SLD), is directed through a beam splitter and is divided into a sample and a reference beam. Light from the sample beam is reflected back from retinal structures with different echo time delays, depending on internal properties of ocular structures. Light from the reference beam is reflected from a reference mirror whose distance is known. The echoes from the two arms are combined by an interferometer and detected. Standard OCT systems use time-domain detection, where the position of the reference mirror is mechanically scanned, or adjusted, to produce different time delays for the reference beam echoes. In this way, axial scans (measurements of sample beam light echoes versus depth) may be acquired (Fig. 26.1 ). By scanning the beam of light in the transverse direction, a cross-sectional OCT image, or OCT B-scan, may be obtained. With standard time-domain detection, OCT systems typically acquire 400 axial scans per second. In OCT, image resolution can be considered both axially (along the incident light beam) and transversally (perpendicular to the incident light beam). The axial resolution of the OCT system is governed by the coherence length of the light source, which is inversely proportional to the bandwidth. Commercial OCT systems use low-coherence SLD light sources at near-infrared wavelengths of ~820 nm. The Stratus OCT (Carl Zeiss Meditec, Dublin, CA) uses a bandwidth of ~25 nm and achieves 8to 10-μm axial resolution in tissue. Research prototype ultrahigh resolution OCT systems using broadband light sources can achieve axial resolution of 2 to 3 μm. 9–11 Recent commercially available OCT systems using spectral/Fourier domain detection can achieve 5to 8-μm axial resolution. Transverse resolution is determined by the size of the focused light beam incident on the retina and is independent of bandwidth. Because there is a trade-off between depth of focus and spot size, most commercial OCT systems use a 20μm transverse resolution in order to have adequate depth of focus. Aberrations in the human eye have limited transverse resolution to 10 to 15 μm; recently, however, adaptive optics that compensate for ocular aberrations can achieve transverse resolutions of ~4 μm. At this resolution, individual rods and cones may be visualized. 12–14