OCT Imaging for the Rest of Us

Optical coherence tomography (OCT) is rapidly becoming a must-have technology for all ophthalmology practices, not only those that focus on retinal pathology. OCT has progressed from fuzzy, grainy images of the macula to near histologic quality images of ocular structures. With the development of Fourier-domain OCT, which utilizes a faster scanning rate than time-domain OCT systems, ocular tissue can be examined as never before. In addition to better images of the macula, some OCT technologies can be used to manage glaucoma and to image the cornea.

OCT is a noncontact imaging technology that obtains images by measuring the time delay of reflected near-infrared light. Each individual axial reflection is combined over a transverse dimension, yielding the image of the structure. Similarly, information can be obtained about the optic nerve head and ganglion cells for evaluating glaucoma or about the macula for evaluating macular disease. Some OCT devices also have anterior segment imaging capabilities, enabling them to image the cornea, the crystalline lens, and other anterior structures.

This article provides a brief overview of new ways in which OCT can benefit general ophthalmologists and anterior segment subspecialists.

GLAUCOMA EVALUATION AND MANAGEMENT

Perhaps the single most valuable use of OCT in a nonretinal practice is in the area of glaucoma evaluation and management. Glaucoma management requires a combination of clinical evaluation and objective testing to follow the health of the optic nerve. Our clinical evaluation of the optic nerve is ultimately limited by what we can view with the slit lamp. We determine optic nerve health by following trends such as optic nerve cupping, standard automated perimetry or visual fields, intraocular pressure (IOP), and corneal thickness.

The optic nerve head can be monitored for cupping with optic nerve head drawings, but there can be drastic variability from observer to observer.¹ Optic nerve head photographs are useful but can be limited in their ability to pick up subtle details. Glaucoma patients are typically older and are not always able to accurately perform the current gold standard visual field test. Nothing can be more frustrating than trying to make long-term clinical decisions in glaucoma management with visual field data that have large numbers of fixation losses and false positives and negatives. We then must base the patient’s IOP target on these potentially limited clinical findings of optic nerve health and visual fields.

The RTVue (Optovue Inc., Fremont, California), an ultrahigh-speed, high-resolution OCT scanner, enhances ophthalmologists’ ability to care for the eye. With an axial resolution of approximately 5 μm and producing 26,000 A-scans per second, the RTVue offers better resolution than other OCT instruments.

OCT with the RTVue adds a new set of data to help diagnose and manage glaucoma. With this instrument, we can quickly and accurately measure optic nerve head thickness (ONHT) at all points circumferentially and the peripapillary retinal nerve fiber layer (RNFL). Because the sensitivity and specificity of these measurements are independent of the patient’s ability to respond to standard automated perimetry testing, we have a more standardized way to document the health of the optic nerve and aid in choosing a desired IOP. This modality is also less affected by cataract.

Reduced macular thickness in glaucoma has been reported by Ziemer et al.² Measuring the inner RNFL, the ganglion cell layer, and the inner plexiform layer, referred to collectively as the ganglion cell complex (GCC), has been shown to increase diagnostic accuracy for glaucoma.^3,4 RNFL measurements combined with changes in the ONHT provide more data to follow glaucoma progression.^5,6 OCT can provide information about changes in the health of the optic nerve and the GCC earlier than we as clinicians can detect them at the slit lamp (Figure 1). Changes in the RNFL and ONHT precede visual field loss. OCT can provide earlier detection of RNFL and OHNT changes, which should ultimately help to decrease visual field loss and provide better glaucoma management.

ANTERIOR SEGMENT AND CORNEAL IMAGING

Anterior segment and corneal imaging using OCT has obvious applications in evaluating corneal opacities, LASIK flaps, Descemet stripping endothelial keratoplasty buttons, and in measuring the anterior chamber angle. Other uses include evaluation of corneas for keratoconus and measurement of corneas for true corneal power after LASIK to aid in post-LASIK IOL calculations.

OCT devices with anterior-segment imaging capabilities can be used to detect abnormal corneal thinning in keratoconus (Figure 2).⁷ Keratoconus evaluation can be performed by comparing thickness relationships between the superonasal (SN) octant and the inferotemporal (IT) octant, and between the superior (S) and inferior (I) octant, and by identifying the thinnest corneal thickness and the magnitude of corneal thickness differences. Using these diagnostic parameters and suggested cutoff values (Table 1), the results of a corneal scan can be used to determine the likelihood of keratoconus.

As shown in Table 2, the SN-IT value (124 μm) is greater than the cutoff (52 μm), which is consistent with keratoconus. The S-I value (210 μm) is much greater than the minimum thickness value (54 μm). The minimum corneal thickness (336 μm) is less than the cutoff value (464 μm), and the minimum-median value (-137 μm) is less than the cutoff value (-33 μm). The location on the y axis of the thinnest point of the cornea is 1.47 mm, which is less than the -1.1 mm cutoff value. This cornea is abnormal in five of five parameters, a condition highly suggestive of keratoconus.

A growing problem facing ophthalmologists is calculating IOL powers for cataract surgery in post-LASIK patients. Many formulas are used to estimate corneal power in these patients, but using anterior-segment OCT to measure corneal pachymetry and curvature, true corneal power can be calculated. Using a vergence formula, the corneal power as measured by OCT can be used to accurately predict the IOL power that should be used in post-LASIK eyes. This is a promising technique that is currently being studied and has shown great accuracy.⁸

CONCLUSION

OCT has always been a great way to image the macula, and it continues to be a useful technology to quickly and easily evaluate patients for retinal thickening related to an epiretinal membrane, central serous retinopathy, cystoid macular edema, or diabetes. But now OCT is a technology that can also significantly benefit anterior segment surgeons and general ophthalmologists.

For patients requiring glaucoma evaluation and management, I typically perform one OCT per year and one visual field per year, each separated by 6 months. Once patients experience the passive OCT scan, many complain about the rigorous nature of visual field testing. With straightforward training of technicians, the OCT scans can be obtained quickly and with great accuracy. The ability to acquire digitized, repeatable optic nerve parameters along with ganglion cell thickness maps aids in clinical diagnosis of glaucoma and helps to identify glaucoma progression. In cases in which the patient cannot perform standard visual field testing, OCT is invaluable.

The ability to image the cornea with OCT provides one more technique to diagnose and manage corneal disease. Measuring true corneal power will better allow surgeons to predict accurate IOL powers in post-LASIK patients before cataract surgery.

Robert Brass, MD, is the lead physician and surgeon at the Brass Eye Center, Latham, New York, and an Associate Clinical Professor at Albany Medical College, Albany, New York. Dr. Brass states that he has no financial interest in the products or companies mentioned. He may be reached at e-mail: drbrass@brasseyecenter.com.

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2. Zeimer R,Asrani S,Zou S,Quigley H,Jampel H.Quantitative detection of glaucomatous damage at the posterior pole by retinal thickness mapping:a pilot study.Ophthalmology.1998;105(2):224-231.
3. Ishikawa H,et al.Macular segmentation with optical coherence tomography.Invest Ophthalmol Vis Sci. 2005;46(6):2012-2017.
4. Tan O,Li G,Lu AT,Varma R,Huang D.Mapping of macular substructures with optical coherence tomography for glaucoma diagnosis.Ophthalmology.2008;115(6):949-956.
5. Garas A,et al.Reproducibility of retinal nerve fiber layer and macular thickness measurement with the RTVue-100 optical coherence tomograph.Ophthalmology.2010;117:4:738-746.
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8. Tang M,Chen A,Li Y,Huang D.Corneal power measurement with Fourier-Domain optical coherence tomography. J Cataract Refract Surg.2010:36(12):2115-2122.

TAKE-HOME MESSAGE

• In glaucoma management and evaluation, the RTVue can measure optic nerve head thickness at all points circumferentially and the peripapillary retinal nerve fiber layer.
• OCT can be used to evaluate corneal opacities, LASIK flaps, Descemet stripping endothelial keratoplasty buttons, and to measure the anterior chamber angle.