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Ординатура / Офтальмология / Английские материалы / Handbook of Optical Coherence Tomography_Bouma, Tearney_2002

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OCT Posterior Segment Disorders

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(a)

(b)

Figure 8 (a) Optical coherence tomographic image through the fovea of a patient with diabetic macular edema. There is distortion of the retinal contour and retinal thickening.

(b) OCT image of a patient with cystoid macular edema. Note the intraretinal cysts represented by optically empty areas within the retina. (See color plate.)

Figure 9 Optical coherence tomographic image of a patient with severe vitreomacular traction and intraretinal edema. The vitreous is displayed attached to the macula. (See color plate.)

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ing schisis from retinal detachment, although lesions anterior to the equator cannot be imaged unless there is a posterior component [16]. The appearance of retinoschisis is consistent with the known histopathology. The images display splitting of the neurosensory retina at the outer plexiform layer (Fig. 10) (see color plate). Retinal detachment appears as a detachment of the entire neurosensory retina from the RPE.

17.5OPTIC NERVE DISEASE

Optical coherence tomography has provided a better understanding of optic nerve disorders, particularly optic nerve pits. Additionally, OCT may yet prove useful for the early diagnosis and monitoring of patients with glaucoma.

17.5.1 Optic Nerve Pits

Congenital pits of the optic nerve head occur in approximately one in 11,000 patients [17]. They consist of round or oval depressions in the optic disc, usually temporally. Serous macular retinal detachment complicates 25–75% of optic disc pits [18]. OCT images of eyes with optic nerve pits and serous macular retinal detachments show a retinoschisis type of cavity that appears to communicate with the optic pit (Fig. 11) (see color plate). OCT images of eyes with resolved serous detachments show deep excavations corresponding to the optic pit and frequently display cystoid and schisis types of retinal changes. Lincoff and coworkers [19] suggested that fluid emanated from the optic disc through a communication with the subarachnoid space, causing the formation of a retinoschisis-like cavity and the secondary development of a serous retinal detachment in the macular. OCT has been used to image eyes with optic nerve pits with and without serous macular detachments [20,21]. The images obtained suggest the development of cystoidand schisis-like changes preceding serous macular detachment.

17.5.2 Glaucoma

The early diagnosis and early detection of glaucomatous progression are challenges that the ophthalmologist must face. Significant axon loss may precede the develop-

Figure 10 Optical coherence tomographic image of a patient with retinoschisis. Splitting of the retina as the outer plexiform layer is evident. (See color plate.)

OCT Posterior Segment Disorders

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Figure 11 Optical coherence tomographic image of a patient with an optic nerve pit with a retinoschisis-like cavity within the retina. The schisis cavity communicates with the optic nerve pit. (See color plate.)

ment of visual field defects and identifiable cupping [21]. Optical coherence tomography is a noninvasive modality that is capable of obtaining high resolution crosssectional images of the optic nerve head (Fig. 12) (see color plate). Because of its high resolution, OCT is capable of detecting nerve fiber layer thinning before the onset of visual changes [4]. Reproducibility results for nerve fiber layer measurements are good, with a standard deviation of 10–20 m [23]. Nerve fiber layer thickness measurements have been shown to have good correlation to histology and also to correspond to visual function [4]. Longitudinal use of nerve fiber layer measurements is an objective method of detecting early nerve fiber layer changes before they are evident by examination or visual field study.

Figure 12 Optical coherence tomographic image of a patient with severe glaucoma and thinning of the nerve fiber layer. Compare the numerical values to those of Fig. 1c. (See color plate.)

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Figure 13 Optical coherence tomographic image of a patient with papilledema. Note the increase in nerve fiber layer thickness. (See color plate.)

17.5.3 Papilledema

Optical coherence tomography has utility both in the evaluation of papilledema and in monitoring patients with papilledema for improvement. OCT images of papilledema show loss of the optic cup and thickening of the nerve fiber layer (Fig. 13) (see color plate).

17.6SUMMARY

The use of optical coherence tomography in the posterior segment has contributed to our understanding of the pathology of a number of disorders, particularly macular holes, vitreomacular traction, and optic nerve pits. OCT has found use in the evaluation of patients with ARMD, CSCR, epiretinal membranes, macular edema, retinoschisis, and glaucoma. Furthermore, OCT is helpful in the longitudinal management of patients with many disorders of the posterior segment.

REFERENCES

1.Puliafito CA, Hee MR, Schuman JS, et al. Optical coherence Tomography of Ocular Diseases. Thorofare, NJ: Slack, 1996.

2.Toth CA, Narayan DG, Bappart SA, et al. A comparison of retinal morphology viewed by optical coherence tomography and light microscopy. Arch Ophthalmol 115:1425– 2428, 1997.

3.Hee MR, Puliafito CA, Duker JS, et al. Topography of diabetic macular edema with optical coherence tomography. Ophthalmology 105:360–370, 1998.

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4.Schuman JS, Hee MR, Puliafito CA, et al. Quantification of nerve fiber layer thickness in normal and glaucomatous eyes using optical coherence tomography. Arch Ophthalmol 113:586–596, 1995.

5.Gass JDM. Idiopathic senile macular hole: Its early stages and pathogenesis. Arch Ophthalmol 106:629–639, 1988.

6.Gass JDB, Jooneph BC. Observations concerning patients with suspected impending macular holes. Am J Ophthalmol 109:638–646, 1990.

7.Hee MR, Puliafito CA, Wong C, et al. Optical coherence tomography of macular holes. Ophthalmology 102:748–756, 1995.

8.Gass JDM. Pathogenesis of disciform detachment of the neuroepithelium, II: idiopathic central serous choroidopathy. Am J Ophthalmol 63:587–615, 1967.

9.Hee MR, Puliafito CA, Wong C, et al. Optical coherence tomography of central serous chorioretinopathy. Am J Ophthalmol 120:65–74, 1995.

10.Johnson MW. Epiretinal membrane. In: M. Yanoff, JS Duker, eds. Ophthalmology. London: Mosby, 1999: 8.32.1

11.Wilkins JR, Puliafito CA, Hee MR, et al. Characterization of epiretinal membranes using optical coherence tomography. Ophthalmology 103:2142–2151, 1995.

12.Hee MR. Baumal C, Puliafito CA, et al. Optical coherence tomography of age-related macular degeneration and choroidal neovascularization. Ophthalmology 103:1260–1270, 1996.

13.Puliafito CA, Hee MR, Lin CP, et al. Imaging of macular diseases with optical coherence tomography. Ophthalmology 102:217–229, 1995.

14.Nussenblatt R, Kaufman S, Palestine A, et al. Macular thickening and visual acuity. Ophthalmology 94:1134–1139, 1987.

15.Hee MR, Puliafito CA, Wong C, et al. Quantitative assessment of macular edema with optical coherence tomography. Arch Ophthalmol 113:1019–2029, 1995.

16.Ip M, Garza-Karren C, Duker JS, et al. Differentiation of retinoschisis from retinal detachment using optical coherence tomography. Ophthalmology 105:600–605, 1999.

17.Kranenburg EQ. Craterlike holes in the optic disc and central serous retinopathy. Arch Ophthalmol 64:912–924, 1960.

18.Brown GC, Shields JA, Goldberg RE. Congenital pits of the optic nerve head, II: Clinical studies in humans. Ophthalmology 87:51–65, 1980.

19.Lincoff H, Lopez, Kreissig I, et al. Retinoschisis associated with optic nerve pits. Arch Ophthalmol 109:61–67, 1988.

20.Krivoy D, Gentile R, Liebmann M, et al. Imaging congenital optic disc pits and associated maculopathy using optical coherence tomography. Arch Ophthalmol 114:165– 170, 1996.

21.Rutlege BK, Puliafito CA, Duker JS, et al. Optical coherence tomography of the macular lesions associated with optic nerve head pits. Ophthalmology 103:1047–1053, 1996.

22.Quigley HA, Addicks EM, Green WR. Optic nerve damage in human glaucoma, III: Quantitative correlation of nerve fiber loss and visual field defect in glaucoma, ischemic neuropathy, papilledema, and toxic neuropathy. Arch Ophthalmol 100:135–146, 1982.

23.Schuman JS, Pedut-Kloizman T, Hertzmark E, et al. Reproducibility of nerve fiber layer thickness measurements using optical coherence tomography. Ophthalmology 103:1889– 1898, 1996.

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18

Optical Coherence Tomography in the

Anterior Segment of the Eye

HANS HOERAUF

Medical University of Lu¨beck, Lu¨beck, Germany

REGINALD BIRNGRUBER

Medical Laser Center Lu¨beck, Lu¨beck, Germany

18.1INTRODUCTION

Since optical coherence tomography (OCT) was introduced as a new imaging method in ophthalmology [1], many studies have been published on OCT investigations of the posterior segment of the eye [2–7]. However, OCT can also be a useful tool in examining the anterior segment of the eye at microscopic resolution. It can be helpful to image and measure complex details of corneal pathologies and structural changes of the chamber angle and the iris. The ability to define the relationship of angle structures in cross section allows a new morphometric gonioscopy and imaging of these structures at high resolution, which may be potentially helpful in glaucoma research and treatment. Corneal thickness measurements by OCT can provide therapeutic control in refractive laser surgery. Because the commercially available OCT system is based on the fundus camera, it does not allow routine examinations of the anterior segment. Therefore only a few experimental studies of OCT measurements of the anterior segment have been published [8–10]. In these studies high two-dimen- sional resolution and excellent sensitivity have been demonstrated. To use examination techniques familiar to ophthalmologists, OCT was adapted to a slitlamp, which allows comfortable and rapid measurements in routine clinical use of the anterior segment [11] and with a handheld 78 dpt lens of the posterior segment as well.

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18.2METHODS

The images presented in this chapter were generated by a newly developed slitlampadapted OCT system. They demonstrate the potential and limitations of this technique as a diagnostic and biometric tool for measurements of the anterior segment in healthy subjects and in patients with pathological changes. The slitlamp-adapted OCT system used a scanning module with a lateral scan range of 7 mm, attached to a normal slitlamp (see Figs. 1 and 2). The source was a superluminescent diode with a power of 1 mW and a center wavelength of 830 nm. The axial resolution achieved with this source was 13:5 m. Images were acquired using 100–400 A-scans with an axial scan frequency of 100 Hz. The total axial depth in the OCT images was 1.5 mm.

18.2.1 Cornea

The slitlamp-adapted OCT system is capable of differentiating three corneal layers. The highest reflectivity is found at the epithelial-Bowman layer and at the Descemetendothelial layer, whereas lower reflectivity is observed in the corneal stroma. An OCT image of a section of normal human cornea is shown in Fig. 3. It cannot be further differentiated between the epithelial layer and the Bowman’s membrane or between the Descemet’s membrane and the endothelial layer. In the corneoscleral junction the different arrangements of the collagen fibers in the cornea and sclera are responsible for the different optical properties in the two adjacent tissues, leading to a dramatic change in reflectivity. Conjunctiva, tenon, and sclera appear as only one highly reflective complex due to the highly scattering sclera, which limits OCT imaging of deeper structures.

Figure 1 Photograph of a prototype of the slitlamp-adapted OCT system. The scanning module is integrated in a Haag-Streit (BQ 900) slitlamp (arrow).

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Figure 2 Schematic diagram of the slitlamp-adapted OCT. The sample arm of the interferometer is coupled into the slitlamp illumination.

Figure 3 OCT image of a healthy human cornea in vivo with higher reflectivity of the epithelium and endothelium and lower reflectivity in the corneal stroma. The higher signal in the central area is caused by the perpendicular angle of the OCT beam to the arrangement of the collagen fibers in the cornea (100 Hz scanning rate, 200 axial scans, 5:5 mm 2 mm).

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18.2.2 Pachymetric Analysis and Photorefractive Laser Surgery

Photorefractive laser surgery has proven successful correcting myopia, hyperopia, and astigmatism in recent years. An exact corneal thickness measurement prior to refractive ablation is essential to avoid overor undercorrection; the corneal thickness measurement is called pachymetric analysis. OCT offers the possibility to perform two-dimensional morphometric measurements of the cornea. An example is given in Fig. 4. Scanning the incident beam perpendicular to the surface of the cornea results in a central specular reflection in the OCT image. On the other hand, off-axis reflectivity drops rapidly. To measure the corneal thickness precisely, the OCT beam was directed perpendicularly to the corneal surface. Calibration was performed with a glass plate of defined thickness (148 m). The achievable precision of central pachymetry is less than 3 m.

The optical path through the cornea is defined as the distance between the endothelial and epithelial maxima of the axial profile. The geometric thickness of the cornea equals the optical pathlengths divided by the group refractive index of the cornea, which was assumed to be 1.376 [12]. Measurements were performed in the center of the cornea, and the diameter of the scanning beam waist was 20 m. The center of the pupil was taken as reference. Preliminary results of corneal thickness OCT measurements showed good agreement with ultrasonic pachymetry. The median central corneal thickness measured by OCT and by ultrasonic pachymetry was 540 m and 546 m, respectively. The median difference was only 6 m, which corresponds to a systematic underestimation with the corneal OCT of 1.2%. The double standard deviation of corneal thickness was 22 m, which resulted in an error (2 SD=xmean) of 4.1%.

Figure 4 Pachymetric measurement of a healthy human cornea in vivo showed an optical path of 776 m, which results in a corneal thickness of 562 m considering a refractive index of 1.38 (100 Hz, 200 scans, 5:5 mm 2 mm).

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