Until the end of the year 2006, all Food and Drug Administration (FDA)-approved commercially available OCT machines were based on TDOCT technology, including the Stratus OCT3.35
So far, SDOCT (also called Fourier domain OCT) is the only technology that affords unprecedented simultaneous UHR with ultrahigh acquisition speeds.36
This technology is possible, because it uses a spectrometer that is a fundamentally more efficient way to process information coming back from the reference arm and the eye ().
Figure 6 Spectral domain optical coherence tomography (SDOCT) set-up. ASL indicates air-spaced focusing lens; CCD, charge-coupled device camera; Col, collimator; E, eye; HP-SLD, high-power superluminescent diode; LSC, linescan camera; NDF, neutral density filter; (more ...)
The first prototype video-rate SDOCT system was built by Johannes de Boer, PhD, at the Massachusetts General Hospital.36–42
Although the light source is the same as in TDOCT (ie, either an SLD light source or a titanium:sapphire laser source), the main difference between TDOCT and SDOCT is the way the information is processed as light comes back from the reference arm and the eye (). In contrast to TDOCT, which uses a point detector or photodetector in the detector arm (), SDOCT uses a spectrometer () that is composed of the transmission grating and the air-spaced focusing lens. In SDOCT, depth information is acquired by analyzing the interference patterns in a spectrum of mixed reflected lights.25,35,43,44
This information from the spectrometer then undergoes Fourier transformation to create an image; therefore, this technology has also been referred to as Fourier domain OCT.25,36
Because of the greater efficiency of the spectrometer, ultrahigh speed data acquisition of weaker signals is possible. SDOCT’s higher acquisition speeds allow for the transition from 2-dimensional to 3-dimensional and video ophthalmic imaging (). With SDOCT, A-lines can be acquired at a rate of 29,000/s (at 6-μm axial resolution), which is 73 times faster than the Stratus OCT3. At a 14,500 Alines/s rate, a 3.5-μm axial resolution can be achieved, which is 3 times better than Stratus OCT3.36,41
Since SDOCT can acquire 29 frames/s (1000 A-lines per frame), a 2-dimensional image can be obtained in 1/29 of a second. Because faster acquisition speeds allow for scanning of larger areas of the retina, SDOCT can create RNFL thickness maps of large regions of the posterior pole ().
Figure 7 Spectral domain optical coherence tomography (SDOCT) 3-dimensional image of the normal left eye of a 38-year-old Korean woman. The image is elongated 4 times. The image was scanned with a prototype SDOCT system with a superluminescent diode laser (SLD, (more ...)
Figure 8 Spectral domain optical coherence tomography (SDOCT) integrated reflectance image (top) and retinal nerve fiber layer thickness map (bottom) of the normal right eye of a 43-year-old white man. The image was scanned with a prototype SDOCT system with a (more ...)
Unlike TDOCT, which achieves UHR images by increasing acquisition times, SDOCT can achieve UHR imaging near 2-μm axial resolution without a significant increase in acquisition times and with a titanium:sapphire laser source (, ).42,45
The FDA-approved commercially available SDOCT machines to date only give us axial resolutions of about 5 to 7 μm (compared with Stratus TDOCT at 10 μm), because these machines use the cheaper, easier-to-maintain SLD light source.
Figure 9 Spectral domain optical coherence tomography (SDOCT) image of the normal right optic nerve of a 43-year-old white man. The image was scanned with a prototype SDOCT system with a titanium:sapphire laser (Femtolaser, Austria) with a central wavelength of (more ...)
Figure 10 Spectral domain optical coherence tomography (SDOCT) image of the normal right optic nerve and fovea of a 43-year-old white man. The image was scanned with a prototype SDOCT system with a titanium:sapphire laser (Femtolaser, Austria) with a central wavelength (more ...)
The image quality, expressed as the SNR (signal-to-noise ratio), of the SDOCT system is better than that of TDOCT.37,38
For example, in the experimental prototype SDOCT system that was at the Massachusetts Eye and Ear Infirmary, the SLD light source was centered at 840 nm with a bandwidth range of 50 nm. The axial resolution in a TDOCT system would increase with optical bandwidth, but its SNR is inversely proportional to an increase in optical bandwidth.41,42
As the light source bandwidth does not affect the SNR performance of an SDOCT configuration, any source can be used in SDOCT without compromising image quality. Unlike titanium:sapphire laser sources, ultrabroad bandwidth superluminescent diode light sources have been developed that are compact, relatively inexpensive, and require low maintenance. The combination of new light sources and SDOCT technology has greatly improved ophthalmic imaging. In addition, the SNR can be reduced further with pulsed illumination instead of continuous wave illumination.46–48
In analogy to stroboscopic illumination, sample motion can be frozen by using a pulsed light source.47
This pulsed illumination subsequently reduces the detrimental effects of sample motion during scanning, thereby providing a 4.4 relative SNR advantage compared with that of continuous wave illumination.48
As a result, less artifacts and clearer images can be acquired. SDOCT has, therefore, evolved into a preferred technology for UHR ophthalmic imaging.
By the end of the year 2006, many imaging companies began to market SDOCT machines in the United States, and some of these machines and companies include the following: RTVue (Optovue, Fremont, CA), Spectralis (Heidelberg Engineering Inc, Heidelberg, Germany), SOCT Copernicus (Optopol Technology, Zawiercie, Poland), Cirrus HD-OCT (Carl Zeiss Meditec Inc, Dublin, CA), 3D OCT-1000 (Topcon, Paramus, NJ), etc. At the time of this writing, not all of these machines have glaucoma software or an age-matched normal database.
Because the new SDOCT machines give 3-dimensional data instead of TDOCT’s 2-dimensional data, new algorithms for SDOCT machines need to be developed to better evaluate the ONH in glaucoma patients. For ONH imaging and analysis, SDOCT can produce a 2-dimensional integrated reflectance image, which is similar to a CSLO image (, top). Although CSLO and TDOCT have been largely limited to ONH topography and 2-dimensional imaging, SDOCT may now allow us to better measure neuroretinal rim tissue volume. A parameter called the “minimum distance band” (MDB) may better reflect the actual amount of nerve tissue coursing through the ONH ().49
In this parameter, the “minimum distance” refers to the band of tissue or the shortest distance between the retinal pigment epithelium edge and the ONH surface. All nerve axons coming from the photoreceptors that course through the ONH on the way to the brain must necessarily pass through this narrow MDB, which encircles the cup. This band area could provide an objective quantification of the nerve tissue in the neuroretinal rim, and any focal regions of thinning in this band may indicate glaucomatous neuroretinal rim thinning. As determination of the MDB or neuroretinal rim area only requires determination of the highly reflective ONH surface and the also highly reflective retinal pigment epithelium edge, this value should be easily determined in all glaucoma patients. This ability to consistently determine an objective 3-dimensional parameter of nerve tissue becomes more significant when we realize that not all glaucoma patients can generate a reliable RNFL thickness map.
Figure 11 Method for “minimum distance band” (MDB) determination in spectral domain optical coherence tomography (SDOCT) images of the optic nerve head. The inner ring of the MDB is shown by the black crosses and correspond to the optic nerve head (more ...)
Because ultrahigh acquisition speeds now allow for 3-dimensional scanning of large areas of the retina, SDOCT can create RNFL thickness maps of large regions of the posterior pole. This is in contrast to TDOCT Stratus OCT that only determines the RNFL thickness values along a circular scan around the ONH, but does not give any information of RNFL thickness either inside or outside this circular scan (). The importance of RNFL thickness determination in the early diagnosis of glaucoma can not be overstated because RNFL thinning may in theory be the earliest structural change clinically detectable as RNFL thinning has been shown to occur as early as 6 years before VF loss.11
RNFL thickness maps can also be potentially used for a thorough evaluation of the RNFL in the longitudinal monitoring of glaucoma patients, which is extremely important in the clinical management of a life-long disease.50
The problem with relying on RNFL thickness measurements in evaluating glaucoma patients is that this parameter is more difficult to reliably measure compared with ONH topography. When determining RNFL thickness for SDOCT RNFL thickness maps, the anterior and posterior boundaries of the RNFL can be determined owing to the reflectivity changes between the RNFL-vitreous and RNFL-inner plexiform layer.50
When the RNFL undergoes glaucomatous changes and thinning, the reflectivity of this tissue decreases, which makes the posterior RNFL border often difficult to determine. However, ONH topography is always very easy to determine in OCT because of the consistently sharp change in reflectivity between the vitreous and the ONH surface, even if the ONH undergoes glaucomatous cupping. Therefore, ONH topography and new SDOCT ONH parameters (ie, MDB) may be more consistently reliable parameters in the evaluation of glaucoma patients.
Figure 12 The right eye of a 77-year-old white woman was found to have mixed mechanism glaucoma. Her fundus photograph (A) showed superior neuroretinal rim thinning that corresponds to an inferior nasal step (B) on Humphrey visual field testing (24-2). From the (more ...)
SDOCT imaging also has the capability of creating 3-dimensional video movies of the ONH and RNFL. Using Doppler principles, spectral domain optical doppler tomography also provides information on retinal blood flow dynamics.51,52
Better noninvasive methods to assess blood flow abnormalities in vivo may help us to better understand the vascular theories of glaucomatous damage. SDOCT also has the capability of directly measuring in vivo RNFL birefringence, which may change in glaucoma and that may help in more accurate RNFL thickness determinations.53
Anterior segment imaging is also possible with SDOCT.54