Optical Coherence Tomography (OCT)
James Strong, CRA, OCT-C
Penn State Hershey Eye Center
Optical Coherence Tomography (OCT) is the most valuable advance in retinal diagnostic imaging since the introduction of fluorescein angiography in 1959. OCT is a non-invasive imaging technique relying on low coherence interferometry to generate in vivo, cross-sectional imagery of ocular tissues. Originally developed in 1991 as a tool for imaging the retina, OCT technology has continually evolved and expanded within ophthalmology as well as other medical specialties. Specialized anterior segment OCT machines became available in 2005 and the introduction of Spectral (Fourier) Domain OCT (SD-OCT, FD-OCT) technology now provides greater tissue resolving power, significantly higher scan density, and faster data acquisition than original Time Domain OCT.
Cross-sectional visualization is an extremely powerful tool in the identification and assessment of retina abnormalities. The high resolving power (10um – Time Domain, 5um – Spectral Domain) provides excellent detail for evaluating the vitreo-retinal interface, neurosensory retinal morphology, and the RPE-choroid complex. The ability to perform volumetric and retinal thickness analysis also provides a quantitative and repeatable method to evaluate surgical and pharmacological interventions.
Individual high resolution line scans are a simple way to identify overt as well as very subtle retinal interface pathologies, such as a persistently adherent posterior hyaloid, fine epiretinal membranes, and vitreomacular traction. In a procedure that is easily tolerated by most patients, well-placed line scans can differentiate between pseudo holes, lamellar holes and full thickness macular holes with a high degree of confidence. Line scans can also confirm the presence of retinal edema from various causes. When combined with serial thickness map or volume analysis, these different data sets provide a detailed picture of disease progression or therapeutic response.
OCT is also quite useful in the assessment of subretinal fluid, neurosensory detachments, pigment-epithelial detachments, and choroidal neovascular membranes. OCT confirmation of persistent subretinal fluid can influence the treatment plan when considering intravitreal injection therapy. RPE irregularities associated with both wet and dry AMD can be monitored using line scans. With experience, OCT imaging may allow differentiation between wet and dry AMD eliminating the need for more invasive testing such as fundus photography and fluorescein angiography; while in other cases OCT is a valuable adjunct to these modalities.
Utilization of OCT imaging as a pre and post surgical assessment tool can provide invaluable information in the surgical management of macular holes and retinal detachments. OCT can provide visualization of surgical outcomes, confirming reattachment and normal contour. Immediate post-surgical imaging can sometimes be challenging due to ocular turbidity that can result in significantly reduced OCT signal strength; but images adequate for subjective, if not quantitative, interpretation can usually be obtained. Pre-surgical scanning, especially in cases of poor ocular media, can often reveal pathologies that could complicate surgery, such as the presence of undetected macular hole, CNV, edema or VMT. The OCT’s scanning beam technology allows successful imaging even through a small pupil or tiny peripheral opening in a dense cataract that would otherwise confound thorough ophthalmoscopic examination.
Optical Coherence Tomography generates cross sectional images by analyzing the time delay and magnitude change of low coherence light as it is backscattered by ocular tissues. An infrared scanning beam is split into a sample arm (directed toward the subject) and a reference arm (directed toward a mirror). As the sample beam returns to the instrument it is correlated with the reference arm in order to determine distance and signal change via photodetector measurement. The resulting change in signal amplitude allows tissue differentiation by analysis of the reflective properties, which are matched to a false color scale. As the scanning beam moves across tissue, the sequential longitudinal signals, or A-scans, can be reassembled into a transverse scan yielding cross-sectional images, or B-scans, of the subject. The scans can then be analyzed in a variety of ways providing both empirical measurements (e.g. RNFL or retinal thickness/volume) and qualitative morphological information.