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Techniques of Fundus Imaging Amit B Recent advance Techniques of Fundus Imaging Amit B. Jain, Vadivelu Jaya Prakash and Muna Bhende Shri Bhagwan Mahavir Introduction Discussion in this review will be limited to Vitreoretinal Services, Ophthalmology is a specialized field of medicine, non-contrast techniques of fundus imaging along Sankara Nethralaya where we can directly visualize the pathology and with recent advances. A detailed discussion on treat accordingly. Imaging of fundus helps in doc- FFA and ICG is beyond the scope of this review. Correspondence to: umenting the disease process which further helps The list of available machines mentioned is by no Vadivelu Jaya Prakash in classifying, treating and following up effect- means exhaustive and does not indicate any com- Associate Consultant, ively. Fundus imaging helps in diagnosing various mercial interest of the authors. Shri Bhagwan Mahavir ocular and systemic disorders like diabetic retino- Vitreoretinal Services, pathy, hypertensive retinopathy, age-related Sankara Nethralaya Red free fundus photography macular degeneration, glaucoma, subacute bacter- Email: [email protected] It is a technique of monochromatic retinal ial endocarditis, leukemia, systemic malignancy imaging which uses green contrast filters to with ocular metastasis etc. and thus helps in modify individual tones in monochrome images effectively managing the condition. This review and the increased scattering of light at shorter focuses on various fundus imaging modalities wavelengths. Various fundus structures can be fl such as digital fundus photography, auto uores- better visualized by limiting the spectral range of cence, infrared reflectance, etc. making a note of the illuminating source.3 recent advances in fundus imaging technology. Green light enables excellent contrast and view The history of fundus imaging dates back to early of fundus as the peak spectral sensitivity of the 20th century, but it has advanced as time has pro- human eye falls in the green–yellow portion of gressed. With the advent of digital imaging, the the spectrum. Less scatter compared to shorter fi lm-based imaging is almost an obsolete tech- wavelengths, results in a better view even in nique now. Table 1 summarizes the major differ- media opacities. Retinal vasculature and nerve fi ences in lm-based and digital imaging. fiber layer (RNFL) are best seen in red free photo- fi Fundus imaging can be de ned as a two- graphy, thus helping in better identification of dimensional image of a three-dimensional retinal hemorrhages, drusen, microaneurysms, RNFL fl tissue captured using re ected light. Fundus defects and exudates. For this reason, green filter imaging can be categorized into contrast and non- ‘red-free’ photos are routinely taken as baseline contrast modalities as shown in Table 2. images before fluorescein angiography.4 Table 1 Comparison of digital fundus and film-based fundus imaging1 Digital Film-based Resolution 1000 pixels 10000 pixels (in ideal conditions) Stereoscopic viewing Available Available Image manipulation Easy Difficult Processing and duplication time Seconds Hours Lesion measurement Irregular tracing made easy Best fit circles Transmission Worldwide via internet (instantaneous) Manual delivery of copies (slow) Cost Installation cost high Installation cost less Table 2 Various imaging modalities based on use of contrast Contrast Non-contrast Fluorescein angiography (FFA) 1. Red-free photography Indocyanine angiography (ICG) 2. Color fundus photography a Stereo fundus photography b Digital fundus photography c Confocal scanning laser ophthalmoscopy (cSLO) 3. Fundus autofluorescence 4. Infrared reflectance 5. Hyperspectral retinal imaging 6. Adaptive optics SLO 100 Sci J Med & Vis Res Foun May 2015 | volume XXXIII | number 2 | Recent advance Color fundus imaging A. Stereo-imaging Diagnostic information can be greatly improved by creating a visual sense of depth by laterally shifting the fundus camera a few millimeters between sequential photographs. The illuminating beam of the fundus camera falls on opposite slopes of the cornea creating a cornea-induced parallax.4 This results in pseudo three-dimensional image formation from an ordinary two- dimensional fundus photographs. Dedicated stereo display software is available for accurate align- ment of digital stereo images.4 B. Digital fundus photography Figure 2. Montage of FFA of the right eye of a Carl Zeiss Company in 1926 introduced the first patient with vasculitis showing commercially available fundus camera which pro- neovascularisation at the disc along with vided a 20° field of view of the retina.5 extensive capillary nonperfusion areas and laser Traditionally used fundus cameras (Fig. 1) provide marks superiorly. Note the enlarged view 30° to 45° field of view which can subsequently obtained by montage technique (approximately be increased by mosaic or montage patterns. 75°). Imaging beyond 50° is called wide-field pho- tography and the recent development came with C. Confocal scanning laser ophthalmoscopy imaging the introduction of ‘ultrawidefield’ photography. Some of the newer imaging techniques use the These include the Pomerantzeff camera,6 the principle of confocal scanning laser ophthalmos- Retcam (Clarity Medical Systems, Inc., Pleasanton, copy (cSLO). Instead of a bright flash of white CA, USA), the Panoret-1000™ camera (Medibell light as in traditional photography, cSLO uses Medical Vision Technologies, Haifa, Israel), the laser light to illuminate the retina. Confocal Optos® camera (Optos PLC, Dunfermline, UK), and imaging acts by a detailed point-by-point scan- the Staurenghi lens7 (Ocular Staurenghi 230 SLO ning of the entire field by a focused laser beam Retina Lens; Ocular Instruments Inc, Bellevue, and then capturing the reflected light through a WA, USA) etc. allowing 100° to 200° view of small confocal pinhole, which in turn suppresses fundus. This allows for simultaneous evaluation of any scattered or reflected light outside the focal the peripheral and central retina without causing plane that could blur the image. This result in a any patient discomfort. sharp, high-contrast image of an object layer located within the focal plane.8,9 The advantages of using cSLO over traditional fundus photography include improved image quality, suppression of scattered light, and patient comfort through less bright light, three- dimensional imaging capability, video capability, and effective imaging of patients who do not dilate well. Examples of cSLO-based ultrawidefield imaging (UWFI) systems include the Optos® camera (Optos PLC, Dunfermline, UK), and Spectralis® (Heidelberg Retina Angiograph (HRA 2), Heidelberg Engineering, Germany). The main limitation in wide-field imaging is the difficulty in projecting a curved surface on a flat two-dimensional plane (Greenland effect) and software are being developed to address this. Research is also going on to determine whether retinal surface area can be precisely measured in Figure 1. Zeiss FF 450 plus IR colour fundus square millimeters.10 camera which provides good quality colour fundus images with optical zoom of three magnification levels (20°, 30° and 50°) and FFA Fundus autofluorescence along with the option of adding ICG and FAF to Fluorescein angiography uses fluorescein dye to its software. delineate retinal vascular and associated Sci J Med & Vis Res Foun May 2015 | volume XXXIII | number 2 | 101 Recent advance pathology, the basic nature of dye to cause fluor- are longer than the other colored wavelengths of escence helps in the diagnostic ability. The human the visible spectrum making them less prone to eye itself has few components which fluoresce absorption by blood and melanin. This results in without any external addition of dye. Imaging of greater amount of light (reflectance) being avail- such auto-fluorescing substances has given an able to form an image as compared to shorter altogether new dimension to diagnostic imaging. wavelengths in the visible spectrum, and hence Fundus autofluorescence imaging takes advantage constitute better tool for imaging structures of the intrinsic autofluorescence of lipofuscin, deeper to the RPE.18,19 normally found in RPE cells accumulated as a IR is a non-invasive en face imaging technique result of phagocytosis of the constantly shed which helps in visualizing sub-retinal pathology.18 photoreceptor outer segments. RPE dysfunction IR imaging has several advantages over flash leads to an imbalance between lipofuscin forma- fundus photography. The invisibility of infrared tion and clearance resulting in excessive accumu- light makes it better acceptable for imaging chil- lation of lipofuscin.11 Higher the lipofuscin dren and light-sensitive patients. As the penetra- content of the RPE cell, the more autofluorescent tion of infrared light is better, it acts as a useful it will appear on FAF. Atrophic (dead) RPE cells imaging tool in patient with media opacities like appear severely hypoautofluorescent as they do dense cataract. IR imaging may offer better visual- not contain lipofuscin, whereas viable cells have a ization of epiretinal membranes, cystoid macular varying content of lipofuscin and accordingly, a edema and deeper structures compared to fundus varying degree of hyperautofluorescence. photography and red-free imaging. IR was found Modified fundus cameras use a single flash to be superior to standard color fundus photog- with excitation spectrum of 535–585 nm to record raphy in screening for neovascular AMD.19 a single image whereas confocal-based systems use continuous scanning with an excitation
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