• Comprehensive Ophthalmology, Retina/Vitreous

    Following its original description by Huang and workers in 1991, optical coherence tomography (OCT) has steadily evolved into an integral element in the diagnosis and management of patients with posterior segment disease.1-4 OCT presented 2 significant advances over existing fundus imaging modalities:

    • Cross-sectional imaging of the retina morphology
    • Quantification of the effects of disease in the form of retinal thickness maps5

    This original OCT technology, time-domain OCT (TD-OCT), was hindered by its relatively slow scanning speed, which limited the amount of retina that could be sampled during a scan acquisition and allowed eye movements to interfere with measurements in patients with unstable fixation. A recent advance in OCT technology, spectral-domain OCT (SD-OCT), has significantly increased scanning speeds (more than 100 fold in some cases) and has been touted as a dramatic advance in retinal imaging. Although SD-OCT instruments have been shown to provide spectacular 3-dimensional depictions of the retinal morphology, their clinical value has not been firmly established. This article reviews the advantages and limitations of SD-OCT technology and explores the potential clinical benefits of these instruments.

    Both TD-OCT and SD-OCT use a long-wavelength (near-infrared) broad bandwidth light source to illuminate the retina, and then assess the light reflected from retinal tissue interfaces to derive axial/depth information. TD-OCT instruments use a moving reference mirror to localize the depth of the reflected light. The movement of this mirror ultimately limits the speed by which TD-OCT instruments can scan and image the retina. SD-OCT devices, in contrast, obtain depth information using a spectrometer and advanced mathematics (Fourier transform), with the mirror remaining stationary. Although the latest generation of commercially available TD-OCT devices, the Stratus OCT (Carl Zeiss Meditec, Dublin, CA), can acquire 400 A-scans/sec, SD-OCT devices can achieve speeds upwards of 40,000 A-scans/sec. The high speed of SD-OCT provides a number of clinical advantages, which are summarized below.

    High-Density Sampling

    The relatively slow scanning speed of TD-OCT limits the retinal area that can be scanned during an imaging session, as a patient can keep his or her eye open and fixated for only a short time. The most commonly used Stratus OCT macular scan consists of six 6-mm-long radial line B-scans, oriented 30 degrees apart, and thus samples only approximately 5% of the macular region. As a result, small lesions that fall between the line scans may be completely missed. Similarly, retinal thickness maps generated from TD-OCT data require interpolation of all data points between the scan lines.6 This interpolation allows small errors in thickness measurements to be propagated over large areas. The rapid scanning speed of SD-OCT, in contrast, allows dense sampling of the retinal area being mapped. This detailed mapping is useful in many clinical situations, a few of which include:

    • Identification of focal abnormalities such as retinal angiomatous proliferation
    • Recognition of subtle areas of photoreceptor disruption that may account for otherwise unexplained visual loss7
    • Identification of small pockets of subretinal fluid in patients undergoing therapy for choroidal neovascularization

    Precise Registration

    Another major limitation of the slow scanning speed of TD-OCT is inaccurate identification of the retinal location from which the scan was obtained. Eye movements occur during scan acquisition, particularly in patients with retinal disease, who often have poor fixation. This problem also reduces confidence when comparing TD-OCT scans obtained on the same eye over time. In contrast, the rapid scanning capability of SD-OCT reduces the artifact introduced by eye movement. Some commercially available SD-OCT instruments also include eye tracking capability, which further minimizes movement-induced inaccuracies.

    In addition, the dense scanning afforded by SD-OCT allows identification of the retinal vascular pattern in the OCT data. This information may be used for registration of the OCT B-scans to fundus images. The precision of registration of SD-OCT-acquired data provides several clinically relevant benefits. For example, retinal abnormalities noted on clinical exam or fundus photography may be precisely correlated with OCT features.8Also, retinal features and thickness maps can be monitored for change over time. This capability is particularly valuable when monitoring disease progression or assessing therapeutic efficacy. Precise registration also paves the way for other emerging OCT technologies such as Doppler OCT and functional OCT.9,10

    Physician and Patient Education

    Dense data acquisition by SD-OCT instruments also permits 3-dimensional depiction of the retinal morphology (Figure 1).11 Visualization of the retina in this fashion can be a useful tool for educating patients regarding their disease. In addition, this mode of visualization of the retina can aid the clinician in understanding the disease and in recognizing the anatomic relationships between retinal structures. This may be of particular benefit in eyes with complex vitreomacular interface diseases,12,13 where 3-dimensional visualization can facilitate surgical planning (Figure 1).

    Image courtesy of SriniVas R. Sadda, MD.

    Figure 1. Three-dimensional image of an eye with a focal area of vitreomacular traction to the edge of a full-thickness macular hole obtained by an SD-OCT (Topcon 3D OCT-1000, Paramus, New Jersey).

    Although high speed is the principal advantage of SD-OCT, the new instruments also boast higher axial resolution and greater sensitivity.14 Whereas the stated axial resolution of Stratus OCT is 8 to 10 microns, the axial resolution of commercial SD-OCT devices ranges from 4 to 7 microns. This higher resolution and sensitivity of SD-OCT may facilitate the identification of fine retinal structures. As an example, the authors have found that the external limiting membrane is more consistently detectable with SD-OCT compared with TD-OCT. The higher sensitivity of SD-OCT may also translate into more accurate identification of retinal boundaries (segmentation) and more reliable retinal thickness maps. The latter expected benefit, however, has yet to be established in large studies.

    Limitations

    With the benefits of SD-OCT come some potential drawbacks. The new technology is not simply an upgrade to existing equipment, but requires the purchase of an entirely new, and generally more costly, instrument. In addition, second- or third-generation SD-OCT instruments may become available in a short time frame, presenting the clinician with the prospect of making another large investment to keep up with the latest technology. Also, although the increased market competition has many potential benefits for consumers, it also requires an investment in time on the part of the buyer to study the various devices to select the instrument(s) best suited for his or her practice.

    The dense datasets produced by SD-OCT do produce a burden of larger file sizes, which present problems for both data storage as well as transmission of image data to remote workstations. On the other hand, the file sizes for most SD-OCT volume cubes are still considerably smaller than a set of 7-field color photographs using current high-resolution digital camera backs. In addition, many vendors have developed compressed file formats that can be more easily transmitted. Also, commercial software platforms are already available that allow users to view and manipulate image data from various SD-OCT devices in a single viewer. Future deployment of an open-source OCT file format will further facilitate the free exchange of image data between devices.

    In summary, SD-OCT represents a significant step forward in OCT imaging technology that yields not only stunning visualizations of the retina, but also clinically useful and important information. As such, SD-OCT will likely supplant TD-OCT as the standard of care for retinal diagnosis in the near future.

    References 

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    13. Koizumi H, Spaide RF, Fisher YL, Freund KB, Klancnik JM Jr, Yannuzzi LA. Three-dimensional evaluation of vitreomacular traction and epiretinal membrane using spectral-domain optical coherence tomography. Am J Ophthalmol. 2008;145(3):509-517.
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    Author Disclosure

    Drs. Sadda and Walsh share in royalties from intellectual property licensed by the Doheny Eye Institute to Topcon Medical Systems, Paramus, NJ. Drs. Sadda and Walsh also serve as consultants for Heidelberg Engineering, Vista, CA.