Optical Coherence Tomography (OCT)

Introduction:

Optical Coherence Tomography (OCT) is an emerging, non-invasive imaging technology for performing high-resolution cross-sectional or 3D images. It has revolutionized the clinical practice of ophthalmology by enhancing the patient care. It has the ability to detect problems in the eye prior to any symptoms being present in the patient. Optical coherence tomography is often described as the optical analog of ultrasound, generating images using the time delay and magnitude of light echoes which uses light waves to take cross-section pictures of the retina’s distinctive layers, layers of retinal nerve fibers and the optic nerve head and detect the early onset of a variety of eye conditions and eye diseases such as macular degeneration, glaucoma and diabetic retinopathy which are known to cause blindness.

OCT 3D Cross Sectional Images of Eye, Cross Section of Eye and OCT Machine; Respectively.

The OCT allows for detection of other diseases such as macular holes, hypertensive retinopathy and even optic nerve damage. Using an OCT allows for early treatment in patients and dramatically improves the success of these treatments, especially in diseases such as wet macular degeneration where the eye disease progresses rapidly.

An HD - OCT Scan of a Healthy Eye.

Apart from that, OCT can provide cross- sectional images of tissue structure on the micron scale in situ and in real time. Using OCT in combination with catheters and endoscopes enables high-resolution intraluminal imaging of organ systems. OCT can function as a type of optical biopsy and is a powerful imaging technology for medical diagnostics because unlike conventional histopathology which requires removal of a tissue specimen and processing for microscopic examination. OCT can be used where standard excisional biopsy is hazardous or impossible, to reduce sampling errors associated with excisional biopsy, and to guide interventional procedures.

History of OCT:

• According to Stefan Talu et al., (2011); OCT  technology  was  conceived  in  the  Department  of  Electrical  Engineering and Computer Science at Massachusetts Institute of Technology.

• In the laboratory of James G. Fujimoto, PhD, the first retinal imaging was done in 1989 by David Huang, MD, PhD, and Joel S. Schuman, MD and the data were reported in Science in 1991 (Huang et al 1991).  

• Eric Swanson,  in 1993, designed the first  clinical  prototype  ophthalmic  OCT  that was built in an engineering laboratory and installed at the New England Eye Center, Tufts -  New  England  Medical  Center,  Tufts  University  School  of  Medicine  in  Boston, Massachusetts,  USA.

• The  ocular  imaging  of  human  subjects  in  vivo  began  in  1994 (Schuman 2008). 

• The technology was ultimately sold on 1993 to Humphrey Instruments, a division of Carl Zeiss America™, which was later acquired by Zeiss.  

• Mr. Swanson, Dr. Puliafito, Dr. Schuman, Dr. Huang and Dr. Fujimoto created a start-up company in 1994 known as Advanced Ophthalmic Diagnostics (AOD) to transfer the technology to industry.  

• In  1994  the  technology  was  patented  and  subsequently transferred  to  industry  to  Carl  Zeiss  Meditec,  Inc  (Dublin,  California).  

• Clinical studies were performed between 1994 and 1995, and the first commercial available OCT, called OCT 1000, from Zeiss, was marketed in 1996 (Cruz 2007; Schuman 2008). 

• The 2nd-generation of the Zeiss instrument, resulting in OCT 2000, was introduced in 2000. 

• The adoption of OCT proceeded slowly, with sales of only a few hundred units during a period of several years. 

• The 3rd-generation, OCT 3 (Stratus OCT™) introduced in 2002, is considered the current standard for retinal imaging. 

• The  first  advanced  OCT,  known  variously  as  Fourier  domain  OCT,  SD-OCT,  or hsHR-OCT, was commercially available in 2006. 

• In contrast to the relatively slow TD-OCT market from 1993, Fourier Domain systems have moved relatively fast.  

• Measurements of  axial  eye  length  were  the  first  biomedical applications  of  low-coherence  interferometry  (Fercher  et  al  1988).  

• The  first  in  vivo  OCT  images  were presented  by  Fercher  and  associates  (Fercher  et  al  1993)  and  later  in  1995  the  first images of retinal disease.  

• In 1994, Izatt and colleagues (Izatt et al 1994) presented the first imaging of the anterior eye.  

• Subsequently, researchers have implemented for OCT the use of light wavelengths instead of time delay to determine the spatial location of reflected light. 

• TD-OCT, the original OCT method, is based on the encoding of the location of each reflection in the time information relating the position of a moving reference mirror to the location of the reflection. 

• Spectral-domain  detection  technology  (also  named  Fourier-domain  detection technology  based  on  the  use  of  Fourier  transformation  of  the  frequencies  of  light reflected), acquires all information in a single axial scan through the tissue and detect all echoes  of  light  from  different  delays  simultaneously  improving  imaging  speed.  

• It is evaluated the frequency spectrum of the interference between the reflected light and a stationary reference mirror. 

• In 2002, Wojtkowski and  colleagues (Wojtkowski et  al  2001)  presented the first SD-OCT  ophthalmic scans  where  in  vivo  scans  of  the  iris, lens,  macula, and  optic  disc were all displayed. 

OCT Techniques:

• OCT imaging is somewhat  analogous to  ultrasound B-mode  imaging except that it  uses light instead of sound, but as an optical echo  technique has a higher resolution and does not require contact with the examined tissue.  OCT can be applied by two main methods:  

1. Time  domain  OCT  (TD-OCT)  
2. Spectral domain OCT (SD-OCT).  

• Each  method  has  its  own  distinct  advantages  and disadvantages (Brezinski 2006). An  imaging  system  has  three  scanning  means, one  to  scan  the  object in  depth and two others to scan it transversally. 

• 1D scans are labeled as: A and T-scans, while 2D scans are labeled as  B-  and  C-scans; A- and T-scans are 1D reflectivity profiles while B and C are 2D reflectivity maps or images (Podoleanu 2008).   

• TD-OCT  produces  two-dimensional  images  of  the  sample  internal  structure;  the tissue-reflectance information in depth is gradually built up over time by moving a mirror in the reference arm of the interferometer. 

• SD-OCT can be implemented in two formats, Fourier Domain (FD-OCT) and Swept Source (SS-OCT) (Podoleanu 2008). SD-OCT  units acquire  entire A-scans in  reflected light  at a  given  point in tissue. Information  on  depth  is  transformed  from  the  frequency  domain  to  the  time  domain, without  using  a  moving  reference  mirror  obtain  complete  A-scans.  

• The  absence  of moving parts allows the  image to be acquired very rapidly about 65 times faster than the older TD-OCT. 

• The SD-OCT units has significantly better ability to detect and monitor of retinal  diseases,  because  these  ones have  ultra  high-speed scan  rate,  superior axial and lateral resolution, cross-sectional (2D) scan, 3D raster scanning and a high imaging sensitivity  compared  with  the  TD-OCT  units.  Also,  the  SD-OCT  software  allows  many operations with  3D  data relative  to the  traditional TD-OCT. 

• The  great  number of scans done per unit time allows SD-OCT systems to generate 3D reconstructions, which can be further manipulated. Visualization of this data in 3D demonstrates subtle pathology that is not evident with traditional 2D images. 

Physical Principles of OCT:

• Tissue Contrast in OCT

Optical coherence tomography displays cross-sectional images of biological tissue. Near infrared light is moderately focused into the tissue. The light penetrates into the tissue and scatters back from structures like cell nuclei, mitochondria, membranes or collagen fibrils. These structures have a slightly different index of refraction than their surrounding and therefore form inhomogenities in the refractive index of the tissue which give rise to scattering. The backscattered light is analysed in the OCT system.

• Interferometry as the depth measuring principle of OCT

OCT utilizes the wave properties of light for the depth measurement, namely the interference caused by superposition of the electrical field amplitude. The backscattered light is superimposed with a reference beam. Usually this is accomplished by an interferometer setup such as the Michelson type interferometer. The superimposed beams show interference if the light of the two beams is coherent. Coherence depends on a fixed phase difference of the waves in the two arms of the interferometer. 

Working pattern of Michelson Interferometer.

If a perfectly monochromatic laser  is used, the coherence will be independent of the light path difference of the two arms. The main idea of OCT is to use light sources with low coherence. In this case there is only an appearant interference if the two arms of the interferometer are equally long. The difference in pathway has to be within the coherence length of the light. In this way the depth of the scatterers in the sample is encoded in the coherence with the reference light. The axial resolution of the OCT, as defined by the axial width of a reflecting surface, is equal to the coherence length. The coherence length is inversely proportional to the spectrum of the light. Therefore OCT uses spectrally broad light sources. The broader the spectrum of the light source, the better the axial resolution.

• Amplitude Scan and Brightness Mode Image

The envelope scan of the interference signal detected is called amplitude scan or A-scan. This terminology is the same as in ultrasound sonography. The A-scan is a depth profile of scatters along the beam. The first surface reflection of the tissue mostly forms the highest peak and then different layers in the tissue might be responsible for different amplitudes or interfaces might form additional peaks in the signal. Because the sensitivity for the reflectivity of the light is very high the amplitude scans usually cover a dynamic range of more than 50dB. Therefore the amplitude is logarithmized and converted to dezibel scale.

As in ultrasound, an image is formed by placing a row of A-Scans next to each other. In OCT this is usually done by scanning the beam transversely with scanning mirrors and optics. The amplitude of the A-scans is converted into grey values. The brightness of one pixel is related to the scattering amplitude at this position. Because of interferometric measurement there is a phenomena called speckle - a granulation of the image. The bachscattering in the sample usually does not create a perfect wave front. There are different scatterers in the volume formed by the focussing and the coherence gate. Some scattered photons might even find their way, to the detector after bouncing around several times in the tissue. The waves interfere differently than it would be the case with a perfect mirror. There is a mixture of constructive and destructive interference resulting in one pixel value. In some cases speckles are not just noise but may contain some information in their statistics.

Working Principles of OCT:

Diagram of the working Principle.

During the past two decades, optical coherence tomography (OCT) has become an essential tool in ophthalmology. Its ability to image detailed ocular structures noninvasively in vivo with high resolution has revolutionized patient care. 

OCT technology is based on the principle of low-coherence interferometry, where a low-coherence (high-bandwidth) light beam is directed on to the target tissue and the scattered back-reflected light is combined with a second beam (reference beam), which was split off from the original light beam. The resulting interference patterns are used to reconstruct an axial A-scan, which represents the scattering properties of the tissue along the beam path. Moving the beam of light along the tissue in a line results in a compilation of A-scans with each A-scan having a different incidence point. From all these A-scans, a two-dimensional cross-sectional image of the target tissue can be reconstructed and this is known as a B-scan.

Pathway of the Reflection of the light.

Typically OCT instruments use an infrared light source centered at a wavelength of about 840 nm. For a given wavelength, the axial resolution is dictated by the bandwidth of the light source. The latest commercial instruments typically have an axial resolution of approximately 5 µm, while research instruments have been built with a resolution as high as approximately 2 µm. The lateral resolution is limited by the diffraction caused by the pupil and it is normally about 20 µm. For clinical purposes, the image acquisition time is limited by the patient's ability to avoid eye movements, i.e., less than 2 seconds in the typical patient. The instrument's scanning speed (number of A-scans acquired per second) is then the crucial parameter determining the amount of data available for a single OCT dataset.

TD-OCT Diagram.

The early OCT instruments, known as time domain OCT (TD-OCT), used a single photo detector, and an A-scan was created by moving a mirror to change the optical path of the reference beam in order to match different axial depths in the target tissue. This setup limited the scanning speed to a few thousand A-scans per second. A newer technique, known as spectral domain OCT (SD-OCT), Fourier domain OCT (FD-OCT), or high-definition OCT (HD-OCT), is able to acquire an entire A-scan by using an array of detectors instead of using multiple reference beams from a moving mirror. Scanning speeds with SD-OCT instruments can exceed 100 000 A-scans per second, about 200 times faster than TD-OCT. Currently available SD-OCT commercial systems operate at a scanning rate of approximately 27 000 A-scans per second.

SD: OCT Diagram.

The scanning pattern with the commercial TD-OCT instrument (Stratus OCT, Carl Zeiss Meditec, Dublin, CA) incorporated six radial, concentric, 6-mm-long B-scans centered on the fovea. With the recent development of high-speed SD-OCT systems, several novel and important imaging strategies have been introduced based on acquiring three-dimensional datasets and B-scan averaging.

Applications in Ophthalmology:

OCT is particularly suitable for ophthalmology due to the optical properties of the eye and the accessibility of the retina to transpupillary examination. OCT imaging is performed in conjunction with slit-lamp biomicroscopic viewing, which directs the OCT scan to the area of interest. OCT has characterized a wide variety of retinal pathology and the images correspond with histopathologic features of these disorders. OCT may establish a diagnosis, evaluate the clinical course, monitor treatment efficacy, and play a role in determining the pathogenesis of some ocular disorders. In some disorders, e.g., macular edema, OCT may be able to replace more invasive tests such as fluorescein angiography for diagnosis and monitoring.

• OCT has been mainly used in retinal imaging. OCT can perform “optical biopsy”, providing visualization directly and in real time, to guide microsurgical procedures minimally invasively in the eye and beyond.  

• Also, OCT can monitor normal retinal function, the progression of retinal disease and other tissues characteristics. 

• Doppler OCT methods can provide informations about retinal blood flow (Wang et al 2011). Kagemann and colleagues (Kagemann et al 2007) used the spectral data of SD-OCT to assess blood oxygenation in retinal arteries and veins.  

• The  emergence  of  ultrabroad  bandwidth  femtosecond  laser  technology  has allowed  the  development  of  an ultra-high resolution OCT.  

• Ultrahigh-resolution OCT  was used in “optophysiology”, to identify changes in the reflectance of certain layers of the in vivo  retina  following  exposure  to  light  and  to  improve  the  early  diagnosis  of  various ophthalmic pathologies (Bizheva et al 2006).

Interpretation:

Assessment of retinal anatomy requires knowledge of the OCT features of a normal eye.

A scan of the layers of the retina.

The cross-sectional contour of the fovea and optic disk and the layered structure of the retina are evident. Central fovea thickness was calculated at 147 ± 17 µm in normal eyes. A highly reflective red layer measuring 70 µm in thickness corresponds to the retinal pigment epithelium and choriocapillaris. The contrast between this red reflective layer and the nerve fiber layer creates a boundary for reproducible measurements of neurosensory retinal thickness. The dark layer immediately anterior to the retinal pigment epithelium/choriocapillaris represents the photoreceptor outer segments. The middle retinal layers exhibit moderate backscattering. The nerve fiber layer corresponds to a bright red reflective layer at the inner retinal margin. The vitreoretinal interface is well defined due to the contrast between the nonreflective vitreous and the backscattering retina. Reflections from the deep choroid and sclera are weak due to signal attenuation after passing through the retina. 

There is a strong correlation between retinal morphology and cross-sectional macular OCT images. Layers of high reflectivity correspond to horizontally aligned retinal components (nerve fiber layer, plexiform layers, retinal pigment epithelium, and choroid), whereas the nuclear and photoreceptor inner and outer segments show low reflectivity.

Vitreomacular Traction and Epiretinal Membranes:

• OCT can reveal macular traction produced by a partial posterior vitreous detachment in vitreomacular traction syndrome. A detached posterior vitreous is seen as a patchy, thin, reflective band anterior to the retinal surface. 

• OCT may reveal associated intraretinal thickening. Serial examinations may determine when intervention is required and direct the surgical approach.

• OCT can classify epiretinal membranes as either globally adherent to the retina or separated from the retina with focal points of underlying attachment. 

• Additional findings that may contribute to visual loss such as membrane opacity, neurosensory retinal disruption, and intraretinal edema can be identified by OCT. Membrane opacity is indicated by increased thickness and reflectivity. 

• The mean central macular thickness measured by OCT in eyes with epiretinal membrane correlates with visual acuity. 

• OCT information regarding membrane position, thickness, and adherence may have prognostic value for visual recovery. 

• In some cases, visualization of the edge of the epiretinal membrane may guide surgical removal.

Macular Holes:

• OCT can stage macular holes, assess the vitreoretinal interface, measure hole diameter, and quantify surrounding intraretinal fluid. 

• The normal fovea depression is absent on OCT in a stage I macular hole. 

• OCT is useful to evaluate stage 1 and 2 holes, which may be more difficult to differentiate biomicroscopically. As stage 2 holes often progress to stage 3 with some visual loss, accurate staging can help determine when surgery is indicated.

• Vitreofoveal traction plays a role in the pathogenesis of macular holes and can be identified by OCT. 

• The risk of macular hole formation in the fellow eye of unilaterally affected patients can be evaluated by OCT. 

• Vitreous detachments as small as 150 µm can be detected by OCT. The size of the macular hole and postoperative resolution can be evaluated. 

• OCT images distinguish full-thickness macular holes from lamellar holes, pseudoholes, and cysts. 

Retinoschisis and Retinal Detachment:

• The cross-sectional OCT image effectively distinguishes retinoschisis from retinal detachment. 

• OCT in retinoschisis demonstrates splitting of the neurosensory retina, which is consistent with known histopathologic separation at the outer plexiform layer.

• In contrast, OCT of a retinal detachment shows separation of full-thickness neurosensory retina from the underlying retinal pigment epithelium. 

• Although OCT visualization of the peripheral retina is limited, it can distinguish between components that extend posterior to the equator when clinical examination is equivocal.

Macular Edema:

• OCT has been used to assess macular edema secondary to diabetes, retinal vein occlusion, uveitis, epiretinal membrane, and cataract extraction. It is useful for diagnosing macular edema and demonstrating resolution after treatment.

• Macular edema is characterized by retinal thickening and intraretinal areas of decreased reflectivity on OCT. 

• Advantages of OCT assessment include high accuracy and reproducibility. 

• OCT may be more sensitive than slit-lamp biomicroscopy for detecting small changes in retinal thickness. 

• Although fluorescein angiography demonstrates vascular leakage, it does not provide quantitative information about retinal thickness. 

• Increased retinal thickness and vision loss may occur in the absence of detectable fluorescein leakage. 

• Central foveal thickness measured by OCT correlates with visual acuity in eyes with diabetic macular edema. This result was similar to that noted in eyes with epiretinal membranes.

Central Serous Chorioretinopathy:

• Central serous chorioretinopathy (CSCR) is characterized by detachments of the neurosensory retina and retinal pigment epithelium secondary to leakage at the level of the retinal pigment epithelium. 

• In CSCR, OCT demonstrates an area of decreased reflectivity between the neurosensory retina and the highly reflective retinal pigment epithelium/choriocapillaris layer, corresponding to a neurosensory detachment. 

• OCT may reveal small neurosensory detachments that are clinically undetectable by slit-lamp biomicroscopy. 

• Longitudinal OCT examinations accurately detect changes in the height of the neurosensory detachment and thus can detect resolution of CSCR. 

• In questionable cases of CSCR, OCT may be able to exclude subretinal neovascularization (SRNV) and other macular disorders.

Age-related Macular Degeneration:

• OCT has been used to characterize drusen, retinal pigment epithelium atrophy, neurosensory and retinal pigment epithelium detachments, and SRNV associated with age-related macular degeneration.

• OCT of isolated argon laser lesions in the primate retina demonstrates early outer retinal high reflectivity with subsequent surrounding low reflectivity that corresponds to the histopathologic findings.

• OCT has been used to confirm resolution of SRNV after photodynamic therapy with the photosensitizer tin ethyl etiopurpurin. 

• Potential uses for OCT in age-related macular degeneration include visualization of SRNV that is obscured by a thin layer of fluid or hemorrhage, demarcation of the boundaries of SRNV, and evaluation for resolution of SRNV after treatment.

Optic Nerve Head Pit:

• OCT of optic nerve head pits associated with macular pathology reveals prominent cystic outer retinal edema that mimics a schisis cavity. The schisis-like cavity or edematous retina communicates with the optic disk. 

• These images support the hypothesis that fluid from the optic nerve pit enters into the neurosensory retina and that macular detachment develops secondary to a preexisting schisis-like lesion comprised of severe outer retinal edema. 

Optic Nerve Head Drusen:

• Nerve fiber layer thinning has been noted on OCT in some eyes with optic nerve head drusen. 

• OCT revealed significant nerve fiber layer thinning in two eyes with optic nerve head drusen and glaucoma and may provide a quantitative means of managing eyes with anomalous optic nerves. 

• OCT may evaluate the cross-sectional appearance of optic nerve drusen as well as disorders such as papilledema or glaucoma.

Glaucoma:

• Circular OCT scans around the optic nerve permit quantitative measurement of nerve fiber layer thickness in different regions around the disk. 

• The cross-sectional nerve fiber layer thickness is determined by computer and correlates with the extent of this red, highly reflective layer at the vitreoretinal interface. 

• OCT measurement of nerve fiber layer thickness correlates with functional status of the optic nerve, as measured by visual field examination, and it appears promising as a tool for early diagnosis of glaucoma.

Anterior Segment abnormalities:

• OCT of anterior segment structures has not been widely evaluated. Cross-sectional images may evaluate abnormalities in the cornea, angle, and iris. 

• Potential OCT applications include accurate non-contact biometry, grading of cataracts, and evaluation of anterior segment masses.

Problems and Solutions in OCT Techniques:

Many factors, including the patient’s eye movements can affect the quality and resolution of OCT images. Consequently, the speed of image acquisition plays a vital role in the resolution of the images obtained.

• The original OCT technology commercially available in 2002 was time-domain OCT. It used an infrared light source with a wavelength of approximately 840 nm. The speed of imaging with TD-OCT was limited by the required movement of a mirror to change the optical path of the reference beam. This requirement limited the overall sampling density and resolution of images with only a few thousand A-scans acquired per second. The newer technology of spectral-domain OCT or Fourier-domain OCT does not use a moving mirror but instead uses an array of detectors (ie, a spectrometer), so scanning speeds are now 50 to 200 times faster than with TD-OCT. This increase in speed has allowed SD-OCT to have greater resolution with significantly shorter acquisition times. In addition, SD-OCT helps minimize motion artifact and provides volumetric analysis with three-dimensional imaging capabilities.

• The most recent technologies to become available is swept source (SS)-OCT. SS-OCT uses a 1,050- to 1,060-nm light source and is able to obtain 100,000 to 400,000 A-scans/second. The advantages of SS-OCT over SD-OCT include a further reduction in motion artifact and a reduction in the impact of cataracts and other ocular opacities. The increased wavelength of SS-OCT allows for imaging at increased depth with improved visualization of choroidal details.

• Currently, the majority of available OCT machines provide 6 – 9 mm B-scan lengths, and the current software is also primarily geared toward macular imaging. While this technique can provide more detailed information over a greater area, it takes more time and significant effort to create montaged images. The mounting of an OCT machine to a slit lamp has been shown to successfully expand the potential area of retina that can be imaged, compared to the range of commercially available OCT devices.

• OCT is used extensively for clinical decision making and monitoring of many posterior segment diseases based on macular, optic nerve and RNFL imaging, until recently, the choroid was not able to be clearly imaged with this technique. New innovations in SD-OCT hardware and software now allow for accurate choroidal thickness measurements. 

• Apart from the commercially available systems, prototype OCT systems have contributed to an ever-growing body of research studies in this field. These include, but are not limited to, the ultra high-resolution OCT (UHR-OCT), SD-OCT systems employing a longer-wavelength light source permitting deeper tissue penetration, and swept-source OCT (SS-OCT) systems. UHR-OCT uses broadband light sources to achieve 3 μm resolutions in tissue. SS-OCT uses another form of Fourier domain detection to measure light echoes. It employs a tunable frequency swept laser light source, which sequentially emits various frequencies in time, and the interference spectrum is measured by photodetectors instead of a spectrometer. This increases the signal quality in deep tissue by elimination of the sensitivity of a spectrometer to higher frequency modulation as with SD-OCT, thereby improving the visualization of the choroid.

• The potential application of OCT in the sub-RPE space and choroid is limited by its shallow penetration: approximately 1–3 mm. The degree of choroidal penetration is determined by several factors: the proportion of scattered photons, the absorption spectrum of water, the scatter by the ocular media, the absorption by melanin. Photon scattering is a phenomenon that influences the image formation in OCT: photons that are singly scattered add to the OCT signal, whereas photons that are scattered multiple times contribute to the background noise. The large amount of water within the eye limits the light wavelengths that can be used. The absorption spectrum of water has two regions where the light absorption is low: at approximately 950 nm and between 1,000–1,100 nm. The devices with wavelengths in the range of 1,000–1,100 nm can be used for the enhanced sub-RPE imaging, with ultrahigh-speed image acquisition and axial resolution in the range of 8 μm. This is useful in the managements of sub-RPE space diseases, particularly in AMD.

• Commercial widefield OCT has become available with the Optovue (Fremont, CA) Avanti RTVue-XR. This widefield system uses SD technology and can obtain 70,000 A-scans/second. The Avanti RTVue-XR can create 12 mm x 9 mm B-scans, and its active eye tracking enhances image stability. The Optovue system offers 3-µm digital resolution, providing detailed imaging of both the choroid and the retina.

• Enhanced depth imaging OCT (EDI-OCT) relies on placing the objective lens of the SD-OCT device closer to the eye, which allows the deeper structures to be localized closer to the zero delay. Subsequently, the choroid is better visualized. EDI-OCT is combined with other technological improvements: high-speed scanning, eye-tracking system, image-averaging technology, reduced noise, and greater coverage of the macula. As a consequence, the choroid is visualized as high resolution, repeatable, and reliable images.

• In contrast to ICG and fluorescein angiography, which are two-dimensional investigations for blood flow analysis, Doppler OCT is a promising technology, in that it is depth resolved, such that precise location of vascular abnormalities can be localized using cross-sectional imaging. Doppler OCT can evaluate blood flow and volume of retinal and choroidal vasculature, highlight vessels where the flow is present and evaluate abnormalities in retinal and choroidal vasculature. Given the evidence of choroidal angiopathy in various retinal diseases, this technology promises to help with monitoring of chorioretinal diseases, in particular wet AMD. It is also expected to aid in the differentiation among diseases such as wet AMD, CSCR and PCV.

• Software modifications, improvements and efficient processing of data are important for effective evaluation of changes in retina and choroid in posterior segment diseases. One of the advancements, known as en-face imaging, allows the clinician to visualize three-dimensional data in a fundus projection. Using this technique, particular retinal and/or choroidal layers at a given depth are projected onto an en-face view. Although cross-sectional images (B-scans) have helped delineate pathological features in retinal diseases, as such, microstructural changes and morphology of the retinal and choroidal vasculature are hard to evaluate using B-scans. This is expected to improve as en-face imaging provides further detail about the subtle pathological features in the retina and choroid in diseased states. In addition, the involvement of the specific vascular layers of the choroid in different diseases such as AMD, CSCR, diabetic retinopathy and inherited retinal dystrophies is expected to delineate in further detail using this technique.

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