anterior segment OCT

Maximize the Use of Anterior Segment OCT

Optical coherence tomography has become an important tool for evaluating the cornea and the anterior segment.

KATHRYN RICHDALE, O.D., M.S., F.A.A.O.
Columbus, Ohio

Developed almost two decades ago for retinal imaging, optical coherence tomography (OCT) has successfully crossed over from the posterior segment to the cornea and anterior segment and made its way into forward-thinking optometric practices. In this article, I describe the basic principles of OCT technology and discuss how optometrists can use anterior segment OCT to its fullest potential for diagnosing and managing conditions of the cornea and anterior chamber.

OCT overview

Based on time-delay principles similar to those of ultrasound, OCT uses reflected light from superluminescent diodes to acquire high-resolution, cross-sectional images without contact with the eye.^1-4 Because the speed of light is faster than the speed of sound, OCT relies on interferometry – interference between a reference and a sampling beam of light – to detect minute differences in tissue depth. To construct a 2-dimensional OCT image, multiple single axial scans (A-scans) are combined, analogous to how multiple ultrasound A-scans are combined to create a B-scan ultrasound image.²

Building on these basic principles, scientists have developed two types of OCT instrumentation: time domain and Fourier domain (also known as spectral domain).^1-4 Time domain systems vary the position of a reference mirror when acquiring scans. The reference mirror must move for each A-scan to determine the depth of ocular structures, which limits the acquisition speed. Fourier domain systems use a fixed reference mirror and a Fourier transform of the spectral interferogram for spatial localization. This system allows not only faster acquisition speeds, but also a higher signal-to-noise ratio and, thus, better image quality.³

The resolution of an anterior segment OCT (AS-OCT) system is related to wavelength and bandwidth.^2,3 Axial resolution is usually 3 to 5 times better than transverse resolution. In general, a shorter wavelength or a wider bandwidth allows higher resolution. The trade-off for higher resolution, however, is often a smaller field of view and poorer penetration through opaque tissue.

AS-OCT systems

Three OCT systems capable of anterior segment imaging are available in the United States. The Visante (Carl Zeiss Meditec, Dublin Calif.) is a time domain system that uses a 1,310 nm wavelength. Axial resolution is less than 20 μm, and transverse resolution is 60 μm. The system acquires 2,000 A-scans per second. The standard imaging mode allows a wider view (6 mm deep x 16 mm wide) but uses fewer A-scans (256) compared with the high-resolution mode (3 mm x 10 mm and 512 A-scans).

Since its entry into the market, the Visante system has undergone several improvements and software updates. The latest is the integration of data acquired from an Atlas corneal topography system (Carl Zeiss Meditec, Dublin Calif.) to yield anterior and posterior topography.

The other two systems combine posterior and anterior OCT with the option of an additional lens to focus the anterior segment. The RTVue (Optovue, Inc., Fremont, Calif.) has an optional corneal adaptor module, and the Spectral OCT-SLO (OPKO Health, Inc., Miami, Fla.) offers a cornea lens. Both are Fourier domain systems and use an 830 nm wavelength. These systems can acquire more than 25,000 A-scans per second with a resolution of approximately 5 μm axial (20 μm transverse). The viewable area is approximately 2 mm to 3 mm deep and 8 mm to 12 mm wide. Both systems offer corneal topography, and the Spectral OCT-SLO combines OCT with a scanning laser ophthalmoscope for microperimetry.

Clinical applications

Anterior segment OCT has widespread application in optometric practice.^4-9 All systems allow imaging of the cornea, the anterior chamber, the crystalline lens and the iris, which can be valuable in managing anterior segment disease, glaucoma and refractive surgery patients (Figure 1).

► Cornea. Corneal dystrophies and degenerations, such as keratoconus, can be imaged using AS-OCT to document the extent of corneal thinning and scarring. The superior resolution of AS-OCT versus standard slit-lamp photography is also helpful when documenting the depth and extent of corneal ulcers, scars and areas of inflammation. The Fourier domain systems have demonstrated their ability to image tiny Acanthamoeba cysts.³ In addition, because the systems use infrared light, patients with foreign bodies, who are often photophobic, may be more comfortable during evaluation with OCT. The total area and depth of corneal involvement can be evaluated and documented.

Refractive surgery patients can be imaged prior to surgery to ensure they have a healthy cornea of sufficient thickness. After surgery, practitioners can inspect LASIK flaps and check for epithelial ingrowths or other abnormalities. For patients considering an enhancement, the caliper tools can be used to accurately measure flap and residual bed thickness. Likewise, doctors can examine the health of corneas after penetrating keratoplasty or implantation of intrastromal corneal rings or corneal inlays.

The AS-OCT systems have varying options for pachymetry maps. The scanned area ranges from 4 mm to 8 mm in diameter, and some systems display segmented zones with minimum, average and maximum thickness (Figure 2). Unlike traditional ultrasound pachymetry, OCT pachymetry maps are free of compression and less sensitive to decentration artifacts and, thus, may provide more accurate values for calculating correction factors for glaucoma patients.^3,4 Nomograms have been published that use AS-OCT pachymetry maps to allow practitioners to determine if a patient is a suspect for keratoconus.

Figure 2. Central pachymetry map (OCT-SLO, OPKO Health).

► Anterior chamber. Of the many applications for anterior chamber imaging, the technique most commonly cited is the anterior chamber angle evaluation for narrow-angle glaucoma (Figure 3).^3,7,9 Each system has software tools to measure the anterior chamber angle in degrees or the area in microns. Angle analysis with AS-OCT is accurate and repeatable and has been shown to correlate with traditional gonioscopy and ultrasound biomicroscopy.^7,9 The measurement may be superior to traditional methods because it is free from mechanical compression artifacts, and the ambient light level in the examination room can be dimmed when capturing images to better evaluate risk for angle closure. Clinical guidelines have been published regarding the risk of closure by angle degrees or area.⁹

Figure 3. Top: Chamber depth and angle-to-angle distance. Bottom: Narrow angle (Visante, Carl Zeiss Meditec, Inc.).

Software tools in some of the systems also allow calculation of anterior chamber distance, angle-to-angle distance and other biometric measurements.^3,7 Some clinicians also use AS-OCT to document flare and synechiae in the anterior chamber.

► Other uses. Anterior segment OCT is also helpful for examining the crystalline lens, the sclera and the ciliary body (Figure 4).^3,5,8 Using AS-OCT, practitioners can measure the density of nuclear sclerosis or the area and extent of subcapsular cataracts. Using the internal optometer, one can stimulate accommodation and calculate central lens thickness and curvature, pupil diameter and anterior chamber depth.^6,8 Crystalline lens thickness measurements made with OCT have been shown to be more repeatable than A-scan ultrasonography.⁸ Clinicians may also use AS-OCT to evaluate phakic and pseudophakic IOLs for centration, stability and posterior capsular fibrosis.

Figure 4. Ciliary body (left) and fluid-filled cysts in sclera (right). (Visante, Carl Zeiss Meditec)

Tumors, nevi, cysts and other lesions on the iris and sclera can be photodocumented for changes in size, depth and pigmentation, or exported and e-mailed for consultation. The ciliary body can be imaged with the time domain system as the longer wavelength of light allows better penetration through the sclera.

Benefits and limitations

The benefits of AS-OCT include:

► It is noninvasive and does not require the use of anesthetic. The patient is imaged in a comfortable, seated position. This makes it especially useful in children, the elderly, and nervous or uncooperative patients.

► The systems are straightforward. A technician or staff member can easily be trained to take the necessary images.

► Anterior segment OCT is an excellent instrument for objectively measuring and following ocular disease, a growing trend in the medical field.

► It is an exceptional tool for patient education and may help to improve a patient's comprehension of ocular disease and compliance with treatment.

Unfortunately, none of the AS-OCT systems can image structures obscured by the iris, including the peripheral lens, zonules or posterior chamber IOL haptics. In more darkly pigmented eyes, imaging the ciliary body becomes challenging. As was the case for retinal OCT, until the use of AS-OCT becomes more widespread, billing options remain limited.

Each OCT system has its own benefits and limitations. The OCT-SLO and RTVue systems combine anterior and posterior scanning and provide higher resolution cross-sectional images (Figures 1 and 2). The Visante system images the entire anterior chamber at once and allows better penetration of opaque tissues like the sclera (Figures 3 and 4).

Mainstay of practice

Anterior segment OCT is a relatively new technology, and hardware and software updates continue to be unveiled. As clinicians and scientists explore new and expanded applications for this technology, it will likely become a mainstay of clinical practice. In the future, we will see faster acquisition speeds, 3-D imaging and even higher resolution. OM

References are available upon request.

Dr. Richdale is a Senior Research Associate and Clinical Attending at The Ohio State University College of Optometry in Columbus. Her research focuses on accommodation and presbyopia. She has no financial interest in any of the products mentioned in this article. You can e-mail her at KRichdale@optometry.osu.edu.