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How optical coherence tomography changed our lives
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Optical coherence tomography is one of the most extraordinary advancements introduced in ophthalmology.
In 1991, Huang et al published about a technique called OCT, which was aimed at obtaining “noninvasive cross-sectional imaging in biological systems.” In 1996, time-domain OCT 1 (Carl Zeiss Meditec) debuted with a velocity of 100 axial scans per second. Although looking back at tomographic images originally obtained with that “primitive” machine may evoke much skepticism on the validity of this intuition due to the rudimentary quality of the imaging provided, many physicians passionately started scanning normal and pathologic retinas with subsequent clinical and experimental implications and new insights on many ocular diseases. Since then, more sophisticated time-domain OCT models have been released with improved axial resolution and increased scanning speed. In the early 2000s, resolution was brought to 10 µm and scan velocity to 400 axial scans per second.
Paolo Lanzetta
Continuous efforts by researchers and industry interest have led to further relevant developments. Significant limitations of time-domain technology have been overcome through the adoption of spectral-domain OCT with higher resolution (3 µm to 7 µm) and speed (up to more than 50,000 axial scans per second). The rapid development of OCT technology, growing interest and impact in clinical medicine have led to a dramatic increase in publications and citations in peer-reviewed journals — from 200 publications in 2000 to more than 800 in 2006 and more than 14,000 to date. In 2000, OCT publications were cited 1,000 times, which increased to more than 10,000 in 2006. Currently, several spectral-domain OCT platforms are commercially available and have revolutionized the way physicians diagnose and treat pathologic ocular conditions. Also, new insights on the pathogenesis of many retinal diseases have been possible through the use of OCT scanning.
OCT technology has confirmed Gass’ theory originally published in 1988 and revised in 1995 concerning the development and progression of macular hole. At that time, Gass’ observations were only based on his brilliant biomicroscopic and anatomic interpretations. OCT has also been defined as in vivo optical biopsy. Morphology of OCT scans obtained with spectral-domain systems or ultrahigh-resolution prototypes “correlate” with light microscopic analysis of human retinas affected by different conditions, such as macular hole and macular edema, collected by Giarelli and Brancato in the 1980s. Spectral-domain OCT has frequently replaced fluorescein angiography, an invasive imaging method with possible severe adverse events, in the management and follow-up of macular edema secondary to diabetic retinopathy and retinal vein occlusion, choroidal neovascularization secondary to age-related macular degeneration, and other conditions. Changes in intraretinal and subretinal fluid and retinal thickness as evaluated by OCT are commonly used for re-administering anti-VEGF therapy. Surgical approach and postoperative evaluation of diseases of the vitreoretinal interface have been facilitated with the advent of OCT scanning. Today, even intraoperative OCT scanning is available through a portable platform. Early diagnosis of glaucoma may also benefit from the application of OCT scanning protocols with the evaluation of retinal nerve fiber layer thickness.
Current applications of spectral-domain OCT will be further expanded with the advent of improvements and newer technologies.
The use of a longer wavelength, around 1050 nm, allows deeper penetration into tissues, with visualization of structures underneath the retinal pigment epithelium, such as the choriocapillaris and superficial choroid.
The combination of indocyanine green angiography and ultrahigh-resolution OCT/scanning laser ophthalmoscopy can provide simultaneous en face OCT scans and indocyanine green angiograms.
Excellent results with ultrahigh-resolution OCT have been obtained with inexpensive, compact broadband superluminescent diodes that may guarantee an axial resolution of 3 µm or less with improved visualization of the different retinal and photoreceptor layers. The combination of adaptive optics with OCT technology may also achieve an ultrahigh axial and transverse resolution of less than 3 µm, with a subsequent individual photoreceptor cell resolution retinal imaging.
In polarization-sensitive OCT, tissue birefringence with polarized light is combined with high-resolution OCT scanning. Therefore, three parameters can be analyzed with polarization-sensitive OCT: reflectivity, retardation and optic axis orientation with improved characterization of the photoreceptor and retinal pigment epithelium layers.
Functional OCTs are based on color Doppler technology and electrophysiology. Quantitative imaging of fundus blood flow and pulsatility of retinal and choroidal vasculature can be evaluated with color Doppler OCT.
It has also been shown that retinal reflectivity changes may represent the functional activity of the retina in response to light stimulation. Non-contact, depth-resolved optical probing of retinal response to visual stimulation achieved by using functional ultrahigh-resolution OCT in rabbit retinas shows changes in the retinal reflectivity profile in the inner/outer segments of the photoreceptor and plexiform layers.
Finally, OCT technology may be a helpful tool to detect tissue reflectivity changes during subthreshold retinal laser irradiation, which typically lacks any ophthalmoscopically visible endpoint.
There is no doubt that OCT technology is here to stay and continue facilitating our daily activities.
References:
Gass JD. Am J Ophthalmol. 1995;119(6):752-759.
Haouchine B, et al. Ophthalmology. 2001; 108(1):15-22.
Huang D, et al. Science. 1991;doi:10.1126/science.1957169.
Johnson RN, et al. Ophthalmology. 1988;doi: 10.1016/S0161-6420(88)33075-7.
Lanzetta P, et al. Ophthalmology. 2008;doi: 10.1016/j.ophtha.2007.08.007.
Veritti D, et al. Eur J Ophthalmol. 2012; doi:10.5301/ejo.5000078.