Today, ophthalmologists have a vast array of imaging tools to choose from—sophisticated devices that would have astonished our forebears in their day—but sometimes this abundance of options can be nearly as much a curse as a blessing. It requires the practice to invest in many costly technologies and to ensure that both physicians and technical staff are well trained in their use. Appropriately weighing the impact of imaging results on patient care decisions can also pose a challenge.
If you are looking for more details, kindly visit our website.
The intention of this article is not to provide an all-encompassing review of current technology, but rather to focus on the most common modalities, their inherent advantages and disadvantages, and how to optimize their use in clinical practice. We also will take a look at devices that prove best for imaging a few common pathologies.
A brief review of the history of ocular imaging permits a better understanding of the technology of today. Hermann von Helmholtz's invention of the direct ophthalmoscope in 1851 gave birth to the field of ocular imaging.1 For the first time, physicians were able to examine the retina in vivo rather than as a post-mortem specimen. Ocular imaging took another great step forward in 1961 when Novotny and Davis used fluorescein dye to image the retinal circulation. This technique, known as fluorescein angiography (FA), advanced ophthalmologists' understanding of vascular disease of the retina.2
Use of light filters necessary to obtain the images with fluorescein dye inadvertently led to the discovery of another retinal imaging technique that is commonplace today—fundus autofluorescence (FAF).3 Prior to injection of the dye, the natural fluorophores of lipofuscin granules hyperfluorescence in the retinal pigment epithelium. Fundus images that capture FAF are now used to study RPE changes in diseases such as macular degeneration and pattern dystrophies.
The next major breakthrough came about in the 1990s, when optical coherence tomography was invented and provided a non-invasive, radiation-free technique to precisely visualize the layers of the retina—something heretofore done only on pathology slides.4 Needless to say, OCT has become an indispensable tool in virtually all retina practices.
Fundus photography still remains an important imaging tool. There are two methods for capturing fundus photographs: digital fundus photography and confocal scanning laser ophthalmoscopy (cSLO). A digital camera captures a single image of the fundus, whereas cSLO uses laser light projected through a pinhole aperture that isolates a single wavelength to capture an image of the retina point by point. Adjusting the distance of the pinhole aperture captures different depths, and scans are obtained in a raster pattern. cSLO uses lasers of different wavelengths to add false color to the fundus image; however, digital fundus photography captures a truer color image. cSLO can make it more difficult for patients to stay still during image capture due to the laser light intensity, but fast acquisition speeds and after-image processing help correct movement artifacts. cSLOs can use longer wavelengths, which make them ideal to capture images for FAF and indocyanine green (ICG) angiograms.5
The two techniques for imaging vasculature are FA and ICG. A combination of phthalic acid with the plant resin resorcinol is used as the dye for FA, which fluoresces at a peak around 530 nm.6 Fluorescein leaks from damaged retinal vessels and can be used for evaluation of macular edema, venous occlusions and diabetic retinopathy. It delineates areas of choroidal nonperfusion in diabetic retinopathy and vascular leakage in choroidal neovascular membranes. ICG uses a tricarbocyanine dye that has a fluorescence peak around 800 nm. It is highly protein-bound and thus stays within choroidal vasculature better than fluorescein, which permits much-improved evaluation of the choroid. This advantage has made ICG the gold standard for diagnosis of the choroidopathies, including central serous retinopathy. ICG has milder side effects than FA for the most part; however, since it is iodine based, it is contraindicated in patients with iodine allergy or seafood allergy due to cross-reaction.7
Optical coherence tomography has quickly become a mainstay of retinal imaging for ophthalmologists. OCT systems are based on the principle of interferometry, which is similar to ultrasound except that near-infrared light (810 nm) replaces sound as the medium that is reflected as backscatter to create an image.8 Initially, OCT images were captured with time-domain OCTs (TD-OCT) but the next iteration came soon after with spectral-domain devices (SD-OCT). They provide sharper resolution of retinal layers and faster image-capture speeds than time-domain, and have found increased adoption in clinical practice. Recent research has lead to the advent of the novel frequency-domain OCT (FD-OCT) devices, which provide yet another significant improvement in resolution and speed.
SD-OCT differs from its predecessor because of its use of the Fourier mathematical transformation, which can sum a single periodic function into a series of sinusoidal functions. Once applied to the OCT, the Fourier transformation allows the simultaneous measurement of light reflection in the SD-OCT, compared to the sequential measurement in TD-OCT. The net result is an improvement in resolution of images from 10 µm to 1 µm, which has allowed detection of small cystic changes such as those seen early in the pathogenesis of wet AMD.9 The FD-OCT, though not yet available for widespread use, may well have the broadest application in clinical practice of the three OCT systems. Whereas SD-OCT uses a broadband light source and a spectrometer, the FD-OCT utilizes a wavelength-swept laser source. This change allows an improvement from the 40 kHz readout rates of the SD-OCTs to repetition rates as high as 370 kHz in the FD-OCTs. Clinically, the potential of the FD-OCTs is vast because of their wide fields of view, and they have found such far-reaching uses as imaging of coronary arteries and esophageal epithelium.10
Modifications of cSLO technology have led to innovative devices including the Optos Optomap SLO and the Nidek F10 SLO. The Optomap device uses confocal imaging in combination with an angled mirror to capture an ultra-widefield imaging of the retinal periphery with an almost 200° angle and 1984x1984 pixel resolution through a non-mydriatic pupil. Most Optomap devices have two laser wavelengths: red (633 nm) and green (532 nm). However, some devices have added blue-light laser to capture FA images as well.11
The Nidek F10 is another SLO imaging device that uses confocal optical principles, but combines them with multiple wavelengths and various sized apertures to create unique fundus images. One feature of the F10 is the use of a pinhole aperture with a central stop that allows light to be collected off-axis, which creates an artificial three-dimensional-like image. The full application of this device is not known but currently is being studied in patients with diabetic cystoid macular edema and retinoschisis.12
Figure 1 provides a summary of the advantages and limitations of each of these modalities. Figure 2 presents four of the most common pathologies seen in practice, discussed in terms of optimizing usage of tools available in the clinic. Lastly, Table 1 includes a practical list of guidelines and pointers. We hope these materials will be of value to you in enhancing your use of imaging technology. RP
Advantages: Excellent contrast and detail; some cSLOs include OCT.
Limitations: Image quality more susceptible to media opacities, motion artifact and image processing; false color.
Optimal Uses: FAF, with ICG.
Advantages: Single image capture.
Limitations: Pupillary dilation usually needed.
Optimal Uses: Pigmented lesions, with FA.
Advantages: Evaluate proliferative diseases of the retina and choroid.
Limitations: Poor view of choroidal vasculature; invasive—requires injection.
Optimal Uses: Diabetic retinopathy, CNV, vein occlusion.
Advantages: Imaging of choroidal vessels.
Limitations: Iodine-based dye, contraindicated if allergy.
Optimal Uses: Polypoidal CNV, CSCR.
Advantages: User-friendly; less expensive.
Limitations: Lower resolution (10 µm); poor resolution of retinal layers; slower image capture; no eye tracking feature
Optimal Uses: Macular pathologies.
Advantages: Significantly improved resolution (1 µm); more images taken; eye tracking feature.
Limitations: More expensive; subject to inversion artifact.
Optimal Uses: Any cystic macular changes, ERM, VMT, following response to anti-VEGF agents.
Advantages: 3D-like image.
Limitations: No OCT modality; no normative database.
Optimal Uses: Minimally elevated lesions such as CME and macular schisis.
Advantages: Has OCT modality; good resolution of outer retina and choroid.
Limitations: Limited aspect ratio; limited normative database.
Optimal Uses: FAF and angiography (both FA and ICG).
Most Optimal Technique
Advantages: Allows for quantification of retinal thickness; easy and quick; non-invasive.
Limitations: Need SD-OCT to see small (<10 µm) lesions.
Alternative Technique
Advantages: Follow leakage; classify NV membranes as classic or occult.
Limitations: Longer test; invasive; requires more patient cooperation; may not see anti-VEGF response as clearly.
Most Optimal Technique
Advantages: See areas of NV and choroidal nonperfusion; used to target laser therapy.
Limitations: Longer test; invasive; requires more patient cooperation; may not see anti-VEGF response as clearly.
Alternative Technique
Advantages: Quantification of diabetic macular edema.
Limitations: Cannot visualize NV or non-perfusion; only captures macula, not the periphery.
weiqing supply professional and honest service.
Most Optimal Technique
Advantages: Shows classic gutter appearance for CSCR; fully captures RPE disruption.
Limitations: Does not show active source of leakage or “hot spot”; does not show extent of SRF; uses blue light.
Alternative Technique
Advantages: Technique is appropriate for imaging both acute and chronic CSR.
Limitations: Invasive; adverse reaction to dye.
Most Optimal Technique
Advantages: Comparative measurements of progression; better analysis of complications with macular pucker and macular hole.
Limitations: Cannot see entire extent of surface over which puckering may occur.
Alternative Technique
Advantages: Can see surface area over which puckering may occur.
Limitations: Cannot quantify membrane thickness and presence of edema.
1. von Helmholtz HLF. Beschreibung eines Augenspiegels. Berlin, Germany: A Förstner Sche Verlagsbuchhandlung; 1851.
2. Novotny HR, Alvis DL. A method of photographing fluorescence in circulating blood in the human retina. Circulation. 1961;24:82-86.
3. Machemer R, Norton EW, Gass JD, Choromokos E. Pseudofluorescence–a problem in interpretation of fluorescein angiograms. Am J Ophthalmol. 1970;70:1-10.
4. Huang D, Swanson EA, Lin CP, et al. Optical coherence tomography. Science 1991;254:1178-1181.
5. Yanuzzi LA, Ober MD, Slakter JS, et al. Ophthalmic fundus imaging: today and beyond. Am J Ophthalmol. 2004;137:511-524.
6. Blacharski, PA. 25 Years of fluorescein angiography. Arch Ophthalmol. 1985;103:1301-1302.
7. Stanga PE, Lim JI, Hamilton P. Indocyanine green angiography in chorioretinal diseases: indications and interpretation. Ophthalmology. 2003;110:15-21.
8. Bennet TJ, Barry CJ. Ophthalmic imaging today: an ophthalmic photographer's viewpoint—a review. Clin Exp Ophthalmol. 2009;37:2-13.
9. Kiernan DF, Mieler WF, Hariprasad SM. Spectral-domain optical coherence tomography: a comparison of modern high-resolution retinal imaging systems. Am J Ophthalmol. 2010;149:18-31.
10. Bouma BE, Youn SH, Vakoc BJ, et al. Fourier-domain optical coherence tomography: recent advances toward clinical utility. Curr Opin Biotechnol. 2009;20:111-118.
11. Meyer CH, Saxena S. Non-mydriatic imaging of a giant retinal tear with the optos optomap panorama 200MA. Clin Exp Ophthalmol. 2010;38:427-430.
12. Tanaka Y, Shimada N, Ohno-Matsui K, et al. Retromode retinal imaging of macular retinoschisis in highly myopic eyes. Am J Ophthalmol. 2010;149: 635-640.
• Although scanning laser ophthalmoscopy has advantages over traditional fundus photography for retinal imaging, it does not capture true color information.
• The Eidon confocal scanner obtains real color images with automated, dilation-free operation.
For many retinal conditions, such as diabetic retinopathy (DR) and age-related macular degeneration (AMD), early diagnosis and treatment can help delay or prevent vision loss, slow progression of the disease, and alleviate symptoms.1-7 Diagnosis of these diseases generally begins with visual acuity tests and a dilated eye exam, but many elderly patients are taking tamsulosin or other pharmacologic agents that cause pupil constriction. Others may have had cataract surgery that resulted in anterior capsular opacification, allowing limited visibility of the retina.
Traditional fundus cameras are red-saturated, which may cause images to appear homogeneous or washed out, distorting them and making measurement of the cup-to-disc ratio difficult. Consequently, diagnosing retinal conditions becomes problematic.8
Scanning laser ophthalmoscopy is in many ways superior to conventional fundus photography—it works through smaller pupils using confocal imaging, and it provides better contrast—but it is unable to capture true color information. The Eidon confocal scanner (CenterVue) addresses this deficiency and offers several welcome benefits, which are reviewed in this article.
The Eidon consists of a user-friendly software interface and its intuitive commands allow it to be used in fully automated or fully manual mode. Through the use of a high-resolution, multitouch, color display tablet, it operates as a standalone unit, with local storage of patient information and images. The device’s imaging and viewing features are described in more detail below.
Imaging Modalities
Whereas other confocal scanning systems such as the Spectral OCT/SLO device (Optos) use monochromatic lasers, the Eidon uses white light to provide true color imaging with three confocal imaging modalities: true color (obtained using white illumination), red-free (which can be used to enhance visibility of the retinal vasculature and retinal nerve fiber layer), and infrared (for choroid information).
High-resolution 60° images can be captured even in undilated eyes and without optic bleaching. Because the device can obtain images through a pupil as small as 2.5 mm, it allows capture of excellent images in almost any patient. The white light illumination allows increased perception of retinal pathologies and affords an enhanced view of the optic nerve, providing images that are on par with what would be observed directly. These true color images are essential in determining an accurate diagnosis.
Eidon is also capable of capturing multifield acquisitions, up to 110˚ automatically and 150˚ in manual mode. An option allows multiple images to be stitched together into mosaics to provide a widefield view that enables detection of pathology in the periphery of the retina (Figure).
Other Viewing Capabilities
The Eidon can be used to document pathology including macular degeneration, epiretinal membranes, macular holes, and peripheral lesions. In our general clinic, it doubles as the camera we use to take disc photos for glaucoma. We also use it as a screening tool in the clinic to take images of all patients coming through the door, dilated or not, because screening requires a widefield view. The camera is capable of capturing images in approximately 3 minutes, and this has sped up our patient flow.
Some patients do not require fluorescein angiography but need a fundus photo to document pathology. In such cases, patients can have a fundus image taken with the Eidon platform, which is more staff- and time-efficient and helps cut the cost of staff overhead.
Image Sharing Near and Far
Eidon’s touchscreen tablet interface is built to communicate with an office’s computer system so that images taken with the scanner can be accessed from any computer with an Internet connection. This is helpful in educating patients because one can bring up images on any Wi-Fi–capable tablet to better explain their pathology and what treatments will be necessary to combat the condition.
The remote viewing function also makes this technology ideal for telemedicine applications, as primary care physicians in rural areas can upload captured information for a retina specialist to review, enabling early disease detection.
Most cameras require operation by a highly skilled technician, but the Eidon will walk patients through the exam. It features automatic pupil alignment and retinal focusing, allowing any staff member to efficiently run tests. Specifically, it auto-aligns to a patient’s pupil; focuses the retina; and captures images, both infrared and color, using a soft light source to ensure patient comfort. If necessary, it is also possible to switch to manual mode and use a virtual joystick to focus and align with specific areas to capture particular pathologies in greater detail.
While similar devices on the market such as the Daytona (Optos) may provide comparable results, in my experience the Eidon is easier to use in terms of patient positioning and image capturing, and it is less than half the cost. For these reasons, this confocal scanner has been a welcome addition to our practice. n
Farrell C. Tyson, MD, is an ophthalmologist at Tyson Eye in Cape Coral, Fla., and a clinical investigator for numerous US Food and Drug Administration studies. He has no disclosures relevant to the content of this article. Dr. Tyson may be reached at tysonfc@hotmail.com.
If you are looking for more details, kindly visit Laser Retinal Imaging.