A Winning Combination
Advances in OCT systems and fundus photography give you the upper hand in optic nerve and retinal assessment.
By Robert Murphy
Just in the last few years, physicians have seen significant advances in the clinical capabilities of ocular coherence tomography (OCT) systems and digital fundus cameras.
On the OCT side, software upgrades have brought about more sophisticated disease-progression analysis with a greater range of imaging and data capture. Eye-tracking software makes it easier to scan patients whose fixation tends to drift.
Todays systems scan deeper into the retina than ever before, with three-dimensional analyisis of the macula and optic nerve. Screen displays facilitate comparisons from one visit to the next and of both eyes side by side. Mosaic images provide a panoramic view from the macula to the optic nerve. OCT has extended to the anterior segment for corneal curvature and angle assessment.
Advances in fundus cameras have likewise kept pace. Special filters allow for the evaluation of retinal autofluorescence (lipofuscin), an early sign of AMD. Full automation, touchscreen interfaces and intuitive menus and icons make fundus cameras easier than ever to operate.
In addition, lower flash intensity and sound dampening make imaging easier on the patient and reduce poor image capture due to blinking or movement. Wide-field imaging takes you from pole to periphery. A built-in computer in many models allows for stand-alone portability.
| Future Directions in OCT Scientists and biomedical engineers are looking at a new generation of OCT systems that aim to rectify current technical limitations. |
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| Even as today's OCT systems feature significant advances over previous-generation units, investigators are still trying to overcome several limitations. Among these are limited resolution due to infrared radiation absorbed by anterior-segment structures and ocular media, limited axial resolution owing to image scattering from ocular structures (speckle noise) and limited lateral resolution thanks to the optical system's restricted numerical aperture. Among the current efforts being made to redress these shortcomings are the following: • Swept-source OCT. SS-OCT technology uses a narrowband light source with a cavity-swept laser that can emit different light frequencies and that turns rapidly over a broad bandwidth. Using a high-speed complementary metal oxide semiconductor (CMOS) camera and two parallel photodetectors, the system achieves rates of 100,000-400,000 A-scans per second with 5-mm tissue axial resolution over a 4-mm imaging range. Compared with SD-OCT, SS-OCT produces reduced fringe washout, better sensitivity with imaging depth, longer imaging range and higher detection frequencies. These advantages allow for fewer patient-induced artifacts from movement and breathing, plus better penetration through cataracts or other opacities. Its long imaging range facilitates anterior-segment assessment. • Adaptive optics. Adaptive optics improve on current OCT techniques by correcting for higher-order ocular aberrations and allowing for near-cellular image resolution. Ultra-high-resolution adaptive optics uses a high-speed CMOS camera with a novel image-registration/dewarping algorithm to limit motion artifacts, increase lateral resolution, reduce speckle and enhance sensitivity. • Full-field OCT. Based on spatial coherence grating, this technology uses narrow-band illumination with high-numerical aperture objectives and a liquid crystal retarder to minimize defocusing and dispersion effects. This allows for three-dimensional imaging with ultra-high resolution. • Intraoperative OCT. OCT has revolutionized the detection of clinically significant disease — such as macular holes, epiretinal membranes, retinal detachments and traction — requiring surgical intervention. Intraoperative SD-OCT helps surgeons better delineate tissue structures, reducing surgical time, excessive illumination and the need for potentially toxic stains. (A separate sidebar describes two studies involving intraoperative OCT.) • Wide-field OCT. Wide-field OCT applies swept-source technology to evaluate larger swaths of the central retina. Image acquisition occurs at speeds of 684,000-1,368,700 A-scans per second. Multiple data sets are consolidated into a fourmegapixel, high-definition image. And that's not all. Good penetration of the choroid and choroidal-scleral interface is possible thanks to axial resolution of 6.7 to 19.0 μm. Combined with optical microangiography technology, it becomes feasible to map vascular perfusion down to the capillary level. • Doppler OCT. This technology allows the operator to assess blood-flow velocity by measuring the Doppler shift and relative angle between the OCT beam and a blood vessel. Among Doppler OCT's clinical applications, blood-flow measurements can be evaluated from the transection of all branch retinal arteries and veins using eight circular scans, each composed of 3,000 axial scans, with the total captured in about two seconds. The background axial inner retinal tissue boundary motion is compared to that of the vessel wall to obtain the Doppler shift produced by blood flow. Doppler OCT's potential clinical applications extend to any disease with a vascular component. Among these are such pathologies as proliferative diabetic retinopathy, ischemic optic neuropathy and glaucoma. • Functional OCT. Also known as polarization-sensitive (PS) OCT, this method assesses functional tissue by evaluating light polarization. It allows the operator to indentify individual retinal layers by measuring cross-sectional and volumetric birefringence, contrasting between the birefringent layers — the retinal nerve fiber layer, Henle's layer, sclera or fibrotic tissue — and other retina layers. The PS-OCT system yields multiple measurements at once. Among these are intensity (conventional OCT images), retardation, optic axis orientation to distinguish polarization-preserving tissue, birefringent tissue and polarization-scrambling tissue. |
These are just some of the features characteristic of today's OCTs and fundus cameras, which can expand a clinician's diagnostic scope. The same ingenuity that drove Hermann von Helmholtz's efforts in 1851, while developing the ophthalmoscope, motivates today's scientists and biomedical engineers to extend — or perhaps even transform — the realm of the ophthalmologist's perception.
The new technologies also offer sophisticated software with which to analyze and display your findings. The upshot amounts to earlier disease detection, which means patients can obtain treatment in a more timely fashion. Here's a selective rundown on the most clinically useful advances in today's OCTs and fundus cameras.
OCT Developments
First, understand that similar technical and clinical features of OCTs and fundus cameras may be found on perhaps just one or two, several, or all comparable systems. The present review highlights key advances in these products yet does not purport to include everything that can be mentioned. A full-blown comprehensive technical and clinical review of all OCT systems and fundus cameras on the market transcends the article's scope.
• Heidelberg Engineering. In March 2011, Heidelberg received FDA clearance for its Spectralis OCT anteriorsegment module. The upgrade reportedly provides highresolution images of the cornea, anterior-chamber angle and sclera. Clinicians can assess both angles at once using a 16-mm-wide angle-to-angle OCT scan.
Heidelberg's Spectralis integrates spectral-domain OCT with confocal scanning laser ophthalmoscopy. The two technologies enable new imaging capabilities, such as an eye-tracking function, as well as blue laser autofluorescence.
ASTIGMATISM IOL |
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| www.AcrySofIQTORIC.com CAUTION: Federal (USA) law restricts this device to the sale by or on the order of a physician. INDICATIONS: The AcrySof® IQ Toric posterior chamber intraocular lenses are intended for primary implantation in the capsular bag of the eye for visual correction of aphakia and pre-existing corneal astigmatism secondary to removal of a cataractous lens in adult patients with or without presbyopia, who desire improved uncorrected distance vision, reduction of residual refractive cylinder and increased spectacle independence for distance vision. WARNING/PRECAUTION: Careful preoperative evaluation and sound clinical judgment should be used by the surgeon to decide the risk/benefit ratio before implanting a lens in a patient with any of the conditions described in the Directions for Use labeling. Toric IOLs should not be implanted if the posterior capsule is ruptured, if the zonules are damaged, or if a primary posterior capsulotomy is planned. Rotation can reduce astigmatic correction; if necessary lens repositioning should occur as early as possible prior to lens encapsulation. All viscoelastics should be removed from both the anterior and posterior sides of the lens; residual viscoelastics may allow the lens to rotate. Optical theory suggests that high astigmatic patients (i.e. > 2.5 D) may experience spatial distortions. Possible toric IOL related factors may include residual cylindrical error or axis misalignments. Prior to surgery, physicians should provide prospective patients with a copy of the Patient Information Brochure available from Alcon for this product informing them of possible risks and benefits associated with the AcrySof® IQ Toric Cylinder Power IOLs. Studies have shown that color vision discrimination is not adversely affected in individuals with the AcrySof® Natural IOL and normal color vision. The effect on vision of the AcrySof® Natural IOL in subjects with hereditary color vision defects and acquired color vision defects secondary to ocular disease (e.g., glaucoma, diabetic retinopathy, chronic uveitis, and other retinal or optic nerve diseases) has not been studied. Do not resterilize; do not store over 45° C; use only sterile irrigating solutions such as BSS® or BSS PLUS® Sterile Intraocular Irrigating Solutions. ATTENTION: Reference the Directions for Use labeling for a complete listing of indications, warnings and precautions. |
• Optovue. One nice thing about Optovue's OCT systems is that you can pay for them on a per-use basis, minimizing the up-front cost. The new RTVue XR system features a 70,000 A-scan per second camera, reportedly nearly three times faster than some other systems. The RTVue's eye-tracking software yields precise image detail with a retinal scanning depth of nearly 3 mm.
Also worth mentioning is the RTVue's capacity to measure corneal powers for IOL calculations. The greatest benefit in this case is for postrefractive patients with altered corneal curvatures. The Rtvue measures both the cornea's front and back surfaces. The company's iVue OCT system, meanwhile, offers a lower-priced alternative often used for screening purposes.
• Topcon Medical Systems. Topcon's 3D OCT-2000 system — with its reportedly user-friendly and easily navigable color touch screen display and compact footprint — is said to be the first OCT to incorporate a high-resolution fundus camera. Topcon's software allows for dynamic viewing of two- and three-dimensional fundus images at one time.
Three-dimensional imaging is said to enhance a clinician's understanding of complex pathologies, such as vitreous traction, macular edema and retinoschisis, according to Topcon's website. The built-in fundus camera, meanwhile, allows the doctor to inspect conditions that might be missed on OCT such, as disc hemorrhages. Topcon's software is said to use 3D registration technology to reduce artifacts caused by eye movement.
The company's thickness-measurement function includes both overall retinal thickness and that of the retinal nerve fiber layer. A mosaic-type image display can create panoramic views from the macula to the optic disc.
• Carl Zeiss Meditec. The OCT pioneer and the patent-holder on the time-domain platform, Zeiss has long dominated the market. With the development of spectraldomain OCT without a patent — coupled with SD-OCT's near-universal preference among clinicians — Zeiss is now one among many players carving a share from a market it once owned.
Zeiss's new Stratus OCT software, version 6.0, provides cross-sectional images and maps showing details of retinal changes that are otherwise difficult to spot. Quantitative analysis of retinal features facilitates not only the initial diagnosis but also ongoing monitoring or treatment. Scans produce opportunities to analyze the retinal nerve fiber layer, optic nerve head and macular thickness, both at the present moment and serially from there.
• Bioptigen. While intended primarily for imaging retinal tissue, with a switch of focus, this company's SD-OCT system can be used to capture images of the cornea, sclera and conjunctiva, according to Bioptigen's website.
The system may be of interest both to researchers and clinicians, allowing both in their respective purposes to resolve the retina's microstructure in fine detail. Among the structural details that can be elucidated are the retinal nerve fiber layer, the inner and outer plexiform and nuclear layers, the retinal pigment epithelium and choroidal vessels.
Fundus Camera Developments
• Nidek. Nidek is one of several fundus camera makers whose products promote ease of use, patient comfort and practice efficiency. Full automation with auto-alignment on the x-, y- and z-axes means just about anyone in the office can learn to take accurate high-resolution photos. Sound dampening and a lower flash intensity remove the patient's apprehension and tendency to blink, move or panic.
The AFC-330 fundus camera's ease of use and automation also enhance practice efficiency by limiting the need for retakes. Clinicians may also discover benefits in its functional flexibility as a stand-alone portable unit.
• Optos. The 200Tx fundus camera captures 200° widefield retinal images through a small pupil using multiple — red, green and blue — wavelengths. Among its available options are color or red-free images, as well as fluorescein angiography and autofluorescence.
The 200Tx thus can be customized to fit a physician's clinical needs. It's also designed to enhance central posterior pole resolution, as well as provide a panoramic fundus view.
• Sonomed Escalon. OphthaVision imaging software is said to save time thanks to an intuitive and efficient interface. This in turn makes for a quick learning curve and streamlined patient flow.
Among its clinically useful features is the abililty to capture wide-field color-corrected panoramic images, according to the company's Web site. Clinicians can create stereoscopic pairs and side-by-side images with a click of a button. Autofluorescence and indocyanine green (ICG) angiography add to the OphthaVision system's utility.
| Intraoperative OCT: Ready for the Main Stage Studies show better visualization expedites multiple forms of keratoplasty. By René Luthe, Senior Associate Editor |
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| Two studies presented at this year's ARVO meeting indicate that intraoperative OCT can aid surgeons with enhanced visualization in various keratoplasty procedures. In one study, German researchers used the iOCT device from OptoMedical Technologies in nine consecutive patients with corneal endothelial dysfunction undergoing Descemet's membrane endothelial keratoplasty (DMEK). The procedure requires precise handling and orientation of delicate graft tissue within the anterior chamber to avoid graft dislocation. It has proved difficult for surgeons to unroll the thin tissue with sufficient exactitude. The investigators used intraoperative OCT to visualize the graft in the anterior chamber during orientation and attachment to the posterior cornea. Among their testing parameters were visualization of Descemet's membrane rolling behavior immediately after preparation, Descemet's membrane rolling inside the eye, and the graft's adhesion to the posterior stroma. All researchers said Descemet's membrane was readily visible in every case, thereby yielding a precise view of the membrane unfolding and orienting correctly. The iOCT system also helped surgeons verify complete graft attachment at the procedure's conclusion. In a second study, the same researchers evaluated the use of iOCT for lamellar preparation in deep anterior lamellar keratoplasty (DALK) as well as for graft alignment in both DALK and penetrating keratoplasty (PK). One patient receiving DALK and six undergoing PK were included in the study. Evaluation parameters incuded iOCT views of trephine depth, needle location, big-bubble effect, remaining stromal thickness, and surface steps in both DALK and PK. The iOCT system provided successful visualization in several respects. In DALK, trephine depth could be viewed and the injection needle located in a designated tissue plane. Clinicians could view big-bubble formation following posterior lamellar graft preparation. The remaining stromal tissue could be viewed and measured. Graft alignment was visualized as well, both in DALK and PK. |
• Topcon Medical Systems. Topcon offers a range of fundus camera models, many of which possess overlapping functional capacities. Its TRC-NW8F, for example, is said to be an easy-to-use mydriatic/non-mydriatic retinal camera featuring auto-focus and auto-capture.
Its software allows a user to obtain color, red-free and fluorescein angiographic images. Nine internal fixation points facilitate the creation of wide-angle retinal views, according to the company's website.
• Canon USA. Canon's CR-2 non-mydriatic and CX-1 mydriatic/non-mydriatic retinal cameras reportedly offer numerous clinical benefits. The CR-2's low flash intensity is said to minimize pupil constriction, thereby shortening the time required to take multiple pictures, such as stereo photos.
Its small-pupil mode features a 3.3-mm diameter, which allows for efficient screening, especially among patients who are difficult to dilate. The CX-1 hybrid mydriatic/non-mydriatic fundus camera incorporates autofluorescence as well.
• Kowa Optimed. In March, Kowa received 510(K) approval from the FDA for its VX-20 retina camera. Like its precursor, the VX-10, the new unit offers non-mydriatic, mydriatic and fluorescein angiographic modes with 50° and 30° views. An autofluorescence mode extends its diagnostic scope.
Capable of stand-alone use with a built-in computer, the VX-20's software allows for enhanced viewing and image manipulation, according to the company's Web site. Easy-to-use controls are designed to be intuitively operational. Further, its software can be used to converge images into a single file on a screen. Image organization and comparison are easy.
• Carl Zeiss Meditec. Zeiss's Visucam NM/FA system is said to be a two-in-one fundus camera designed to capture both high-resolution fluorescein angiography and color retinal photos. An autofluorescence option also allows for ICG angiography.
As with many comparable systems, the Visucam is said to use an efficient flash, especially for angiography. Its ease of use, thanks to an ergonomic design and auto-flash and auto-focus features, minimizes the need for corrective exposure actions by the operator. Its small-pupil mode enables photos with pupil diameters as small as 3.3 mm, according to the company's Web site.
Fruits of Progress
In addition to advancments in medical training and experience, new technologies have always been among the key parts of the driving force behind improving a physician's diagnostic skills.
In the case of OCT systems and digital fundus cameras, the last decade has witnessed an amazingly accelerated pace of technological development. Its widespread application in the clinic ultimately allows for earlier and more accurate diagnoses and, in turn, timely treatment when indicated. OM
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