PART ONE
INTRODUCTION
Optical coherence tomography (OCT) is an essential tool for diagnosing and managing retinal diseases and glaucoma. In this handbook, which features detailed schematic illustrations as well as actual OCT scans, we offer a step-by-step guide for interpreting images and data acquired by the revolutionary and novel spectral domain OCT technology.
We believe this handbook is important because spectral domain technology offers significant advances over time domain technology. For example, spectral domain OCT offers:
- Improvements to the sagittal OCT B-scan, which reveals previously undetected structures
- Technical and clinical improvements in the study of glaucoma
- Three-dimensional images that provide new information. (Although this is a new way to view ocular structures, interpretation can be learned quickly and easily, and these images enable more accurate diagnosis.)
All references to spectral domain OCT in this handbook are based on our experiences with the spectral domain CirrusTM HD-OCT manufactured by Carl Zeiss Meditec Inc., Dublin, Calif. (U.S. headquarters). All images were generated by the CirrusTM HD-OCT.
In these pages, we have ascribed great importance to explaining the meaning of OCT images. In addition, we have tried to set forth a logical method for interpreting ophthalmic images. The first phase of analysis subdivides each image into its smallest components. The second phase combines these fine details to synthesize the data, enabling us to arrive at an accurate diagnosis and to decide on appropriate therapy. We hope this handbook helps eyecare practitioners appreciate the new possibilities offered by spectral domain OCT.
Bruno Lumbroso
Overview of Optical Coherence Tomography: Advances in technology usher in a new era of expanded functionality.Chapter 1
Optical coherence tomography (OCT) is an essential tool for noninvasive in vivo analysis of retinal tissue for diagnosis and management of retinal disease and glaucoma. This technique is based on the degree of absorption or dispersion of light traversing the tissue. The light, which is divided into a detection arm and a reference arm, is emitted by a superluminescent diode at a wavelength of approximately 840 nm.
Time domain OCT, which was commercialized at the end of 1996 and improved with increased resolution in 2002, compares a reflected beam of light to a beam of light from a reference mirror.
Time delays between the two beams can then be measured. Although the low acquisition rate exposes the images to aberrations caused by eye movement, increasing the acquisition speed would degrade image resolution. At good resolution (1024 points in 6 mm of tissue), time domain OCT can produce one B-scan every 1.6 seconds (400 A-scans per second).
SPECTRAL DOMAIN OCT
Recently, we have seen the emergence of a new technology called Fourier domain or spectral domain OCT, which employs a different acquisition technique. This technique uses a spectrometer on the detector arm to measure the difference in wavelength between the light from the fixed reference arm and that returning from the tissue. The instrument uses Fourier analysis to analyze the images according to the light wavelength recorded.
This type of OCT technology avoids moving the reference arm and instead analyzes the reflected light using a spectrometer. The immediate advantage of this technology is the high number of scans acquired per second—approximately 27,000 A-scans per second-making true three-dimensional (3-D) imaging possible and practical in a clinial environment.
This sampling frequency has notable advantages. The possibility of artifacts due to eye movement is minimized because the operator can easily position and center the beam to control the image, especially when working with pseudophakic or highly myopic eyes, or when there is relevant media opacity. Given the increased resolution, fine retinal detail is comparable to what is visible on a histology slide.
The CirrusTM HD-OCT combines image accuracy and ease of use. The acquisition process is fully automated, including:
- Auto focus
- Auto optimization
- Auto polarization.
Cycles follow one another in a few seconds, placing the retinal plane in the center of the screen. The operator need only position the scanner on the area of the retina to be studied. The instrument acquires in high-resolution or high-speed mode, taking into consideration the inverse proportions between these variables.
As mentioned previously, the wavelength of the light source is 840 nm, and the sampling frequency is about 27,000 A-scans per second. The instrument acquires 200 to 512 B-scans in rapid succession, from top to bottom, then constructs a 3-D retinal map by aligning the B-scans. Acquisition speed determines the accuracy of the retinal map, so with spectral domain technology, it is possible to obtain very detailed retinal maps.
The analysis speed for calculation of average thickness and the display of ETDRS grid averages is almost the same. The higher density of data collected with spectral domain technology also allows for 3-D display.