ATLAS Optical Coherence Tomography of Macular Diseases and Glaucoma Vishali Gupta, Amod Gupta, Mangat Ram Dogra
INDEX
×
Chapter Notes

Save Clear


1Introduction to OCT2

Introduction to Optical Coherence Tomography1

 
Time Domain OCT
 
BASICS OF TIME DOMAIN (TD) OPTICAL COHERENCE TOMOGRAPHY
 
What is OCT?
Optical Coherence Tomography (OCT) is a new diagnostic tool that can perform tomography/cross-sectional imaging of biologic tissues with ≤ 10 microns axial resolution using light waves. Since retina is easily accessible to the external light, it is especially suited for retinal disorders. This imaging technique provides information regarding the retinal tomography and is akin to in vivo histopathology of the retina. The conventional imaging techniques including fundus photography and fluorescein angiography yield diagnostic information about retinal topography. OCT yields information about retinal tomography that is complementary to the conventional topographic techniques.
 
PRINCIPLE
We are all familiar with principle on which ultrasound works where the high frequency sound wave is launched into the eye with the help of a probe. The sound wave is reflected from different boundaries between microstructures. The working principle of OCT is similar to the ultrasound with two major differences:
  1. It uses light instead of sound. The speed of light being almost a million times faster than sound allows the measurement of structures with a resolution of ≤ 10 microns as compared to 100-micron scale for ultrasound.
  2. Ultrasound needs contact with the tissue under study whereas OCT does not require any contact.
 
HOW DOES TIME DOMAIN OCT WORK?
It is a non-contact, non-invasive device where a broad bandwidth near-infrared light beam (820 nm) is projected onto the retina. The light gets reflected from the boundaries between the microstructures and also gets scattered differently from tissues with different optical properties. It then compares the time delay of the light reflecting from the various layers of the retina with the time delay of the light reflected from a reference mirror at a known distance. The interferometer then combines the reflected pulses from the retina as well as reflecting mirrors, resulting in a phenomenon known as interference. This interference is then measured by a photodetector, which determines the distance traveled by various light pulses by varying the distance to the reference mirror. This finally produces a range of time delays for comparison.
The interferometer integrates several data points over 2 mm of depth to construct a tomogram of retinal structures. It is a real time tomogram using false color scale. Different colors represent the degree of light back scattering from different depths of retina. The image, thus, produced has axial resolution of ≤ 10 microns and 4transverse resolution of 20 microns. The only commercially available equipment based on time domain technology is Stratus OCT (Carl Zeiss Meditech, Dublin CA).
 
LIMITATIONS OF TIME DOMAIN TECHNOLOGY
The time domain based technology had significant limits in signal acquisition time and thus comprehensive three-dimensional imaging of the retina was not possible. The signal acquisition time was prolonged because of the fact that this technology relied on the mechanical movements of the internal components for thickness measurements, thus limiting the speed of acquisition. In addition, there was no proper recording of the eye movement during acquisition and movement of the device or patient during scanning would reduce the quality and also markedly decrease the spatial registration (the ability to determine the precise location of the B-scan OCT image). Also, there was no comprehensive coverage of retina that could result in missing a pathology if the scan line would not pass through the area of pathology. Recently, these shortcomings of the time-domain technology have been overcome by the introduction of spectral-domain OCT that allows cross-sectional and three-dimensional tomograms of the posterior segment of the eye.
 
Spectral/Fourier Domain Technology
Spectral/Fourier domain detection techniques measure the echo time delay of light by measuring the spectrum of the interference between light from the tissue and light from a stationary unscanned reference arm. Light returning from the sample and reference paths is combined at the detector, which is a spectrometer in Spectral domain (SD)-OCT. The spectrometer resolves the interference signals throughout the depth of each A-scan immediately by means of a Fourier transformation. This is possible because the spectrometer resolves the relative amplitudes and phases of the spectral components scattered back from all depths of each A-scan tissue sample, without varying the length of the reference path. The depth is analyzed by inverse fourier transformation of the back-scattered light spectrum. Eliminating the necessity of moving a mechanical reference arm makes it possible to acquire OCT image data about 70 times faster than conventional (time domain) OCT (Figures 1.1A and B).
zoom view
Figure 1.1A: Line diagram showing the difference in the techniques measuring the echo time delay of light(Courtesy: Rick Torney, Carl Zeiss Meditec).
The vast increase in scan speed makes it possible for these new generation OCTs to acquire three-dimensional data sets, or entire cubes of data in about the same time (depending on the selected scan type) than conventional OCT. Data over a 6 × 6 mm cube (upto 128 lines and 512k resolution) can be acquired in about 2 seconds with an axial resolution of 5-7 µm and transverse resolution of about 10 µm. The comprehensive coverage of the retina by a 6 × 6 mm cube allows better delineation of pathology with less sampling errors.
5
zoom view
Figure 1.1B: Principle of Spectral domain OCT(Courtesy: Rick Torney, Carl Zeiss Meditec).
 
WHAT ADDITIONAL INFORMATION DOES SPECTRAL OCT GIVE?
SD OCT gives cross-sectional images that can be useful in studying several pathologies including choroidal neovascularization, vitreomacular traction, macular hole, epiretinal membranes, retinal detachment, etc. In addition, the 3D surface maps can be obtained that help in mapping the surface topography. The quantification of retinal thickness data can be done as in time domain OCT; however, the image registration is superior in SD OCT due to the incorporation of eye tracker. The retinal thickness measured by SD OCT tends to be higher for any given situation compared to time-domain because time domain OCT measures from inner retina to inner segment/outer segment (IS/OS) junction while SD OCT measures from inner retina to inner or outer retinal pigment epithelium (RPE) band. Table 1.1 shows the comparison between the two OCT systems.
 
Switching from Time to Spectral Domain
It is important to note that one cannot switch from TD OCT to SD OCT in a given patient during follow-up. The foveal thickness measured by spectral domain OCT has been found to be higher than that measured with the time domain OCT both in normal eyes as well as eyes with retinal pathologies. There could be several reasons for this variability. Firstly, the spectral domain OCT segmentation measures the retinal thickness from RPE to the ILM, whereas time domain Stratus OCT measures the retinal thickness from IS/OS to the ILM. It has been demonstrated that the outer boundary of the neural retina was inaccurately delineated in time domain OCT using the automated measurement tool that considers the inner hyper-reflective line (HRL), whenever present, as the outer boundary of the neural retina, thus excluding a significant part of the structure for ultimate retinal thickness measurements at the specific regions of the normal as well as abnormal maculas. The distance between IS/OS and RPE is approximately 45 microns, thus spectral domain OCT would be inherently thicker compared to time domain OCT. Secondly, the retinal thickness measurement in Stratus OCT may be biased by the retinal thickness in the central subfield alone as the scan is obtained by ‘Fast Retinal Thickness scan’ wherein only six linear line-scans pass through the foveal center. The spectral domain OCT, on the other hand, measures the retinal thickness by an evenly distributed square centered on the fovea and is likely to indicate true retinal thickness.
So far there is no conversion factor from time to spectral domain OCT and thus it is suggested that any particular patient maybe followed on one machine only.
6
Table 1.1   Comparison between time and spectral domain OCT (Courtesy: Rick Torney, Carl Zeiss Meditec)
Spectral Domain OCT
Time Domain Stratus OCT
Benefit of Spectral Domain
Light source
840 nm Broader bandwidth
820 nm
Provides higher resolution
Detector
Spectrometer
Single detector
No moving parts—faster scan acquisition
Axial resolution
6-7 µm
10 µm
Better visualization of retinal
Transverse resolution
10 µm
20 µm
layers and pathology
Maximum A-scans per B-scan
8,000
512
Better visualization of tissue/pathology
Scan depth
2 mm
2 mm
Slightly better penetration of light
Scanning speed
About 28,000 A-scans per second
400 A-scans/sec
Better registration, 3D scanning and analysis
Suggested Reading
  1. Costa RA, Calucci D, Skaf M, et al. Optical coherence tomography 3: Automatic delineation of the outer neural retinal boundary and its influence on retinal thickness measurements. Invest Ophthalmol Vis Sci 2004; 45:2399–2406.
  1. Forooghian F, Cukras C, Meyerle CB, Chew EY, Wong WT. Evaluation of time domain and spectral domain optical coherence tomography in the measurement of diabetic macular edema. Invest Ophthalmol Vis Sci. 2008 May 30. [Epub ahead of print].
  1. Gupta V, Gupta P, Singh R, Dogra MR, Gupta A. Spectral-domain cirrus high-definition optical coherence tomography is better than time-domain stratus optical coherence tomography for evaluation of macular pathologic features in uveitis. Am J Ophthalmol 2008;145: 1018–22.
  1. Hee MR, Izatt JA, Swanson EA, et al. Optical coherence tomography of the human retina. Arch Ophthalmol 1995; 113:325–32.
  1. Huang D, Swanson EA, Lin CP, et al. Optical coherence tomography. Science 1999;254:1178–81.
  1. Leung CK, Cheung CY, Weinreb RN, et al. Comparison of macular thickness measurements between time domain and spectral domain optical coherence tomography. Invest Ophthalmol Vis Sci 2008 Apr 30 [Epub ahead of print].
  1. Schuman JS, Puliafito CA, Fujimoto JG. Optical Coherence Tomography of Ocular Diseases (2nd edn). Slack Incorporated 2004.