Role of Optical Coherence Tomography in Anterior ...

15 Role of Optical Coherence Tomography in Anterior Segment Imaging

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Tin Aung Tun, Sze-Yee Lee, Rachel Nge, and Louis Tong Contents 15.1 Introduction......................................................................................................................... 271 15.2 Corneal Imaging................................................................................................................. 272 15.2.1 Corneal Thickness Measurement......................................................................... 274 15.2.2 Corneal Diseases..................................................................................................... 275 15.2.2.1 Corneal Ectasia......................................................................................... 275 15.2.2.2 Descemet’s Membrane Detachment...................................................... 277 15.2.3 Ocular Surgeries..................................................................................................... 279 15.2.3.1 Corneal Refractive Surgeries.................................................................. 279 15.2.3.2 Corneal Grafting...................................................................................... 280 15.2.3.3 Cataract Extraction Surgery................................................................... 282 15.3 Glaucoma Imaging............................................................................................................. 283 15.3.1 Anterior Chamber Angle.......................................................................................284 15.3.2 Anterior Chamber Biometry................................................................................. 286 15.3.3 Iris............................................................................................................................. 287 15.3.4 Ciliary Body............................................................................................................. 287 15.3.5 Glaucoma Surgeries................................................................................................ 288 15.3.5.1 Laser Peripheral Iridotomy and Iridoplasty........................................ 288 15.3.5.2 Trabeculectomy........................................................................................ 288 15.3.5.3 Tube Shunt Surgery................................................................................. 289 15.4 Miscellaneous...................................................................................................................... 290 15.4.1 Crystalline Lens...................................................................................................... 290 15.4.2 Tear Film.................................................................................................................. 290 15.4.3 Conjunctivochalasis................................................................................................ 292 15.4.4 Ocular Injuries........................................................................................................ 292 15.5 Future Directions and Summary..................................................................................... 294 Acknowledgment......................................................................................................................... 294 References...................................................................................................................................... 294

15.1  Introduction Optical coherence tomography (OCT) is a noncontact and noninvasive investigation for in vivo imaging of the ocular tissues at a microscopic level. Measurements at micrometer scale and visualization of structures in the anterior eye are required in the study of the 271

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pathophysiology of anterior eye diseases and therapies. OCT is widely used nowadays and has evolved from time domain to Fourier domain recently. Soon after the introduction of the use of OCT in retina imaging in 1991, Izatt et al. reported that OCT is capable of the imaging of the cornea and anterior segment (anterior segment OCT [AS-OCT]). OCT has become a powerful tool to assist in the diagnosis of various ocular diseases [26,29]. In order to produce 2D images of biological tissues in a way that is analogous to ultrasound biomicroscopy, OCT uses low-coherence interferometry that compares the echo time delay and intensity of the reflected and backscattering light from a sample and a reference mirror to determine the longitudinal depth of the sample. OCT uses two infrared wavelengths—840 and 1310 nm, mainly—but studies at 1050 nm were demonstrated recently [60]. For anterior segment imaging, a wavelength of 840 nm provides higher resolution, while 1310 nm gives better penetration due to lesser backscattering effect of light at longer wavelength. Therefore, 840 nm produces better results for corneal imaging with higher resolution, while 1310 nm is used for anterior chamber biometry and angle assessment with higher depth imaging. In the time domain system, an optical signal is produced from a superluminescent diode to a sample in one interferometer arm and a reference mirror that is varying in the other arm. Due to the mechanical movement of the reference mirror, time domain systems have a limited speed (maximum 2000 A-scans per second). Visante AS-OCT (Carl Zeiss Meditec, Dublin, CA) and slit lamp-mounted AS-OCT (Heidelberg Engineering, GmbH, Dossenheim, Germany) are commercially available time domain OCTs. The basic principle of Fourier domain systems is similar to time domain. Fourier domain system uses a fixed reference plane, whereas time domain system does not. The spectrometer detects the signals from the sample and reference mirror. The axial depth of tissue is measured by Fourier transformation of the interference spectrum data and Fourier domains have a faster scan speed (≥20,000 A-scans per second) compared to time domains. A few Fourier/spectral domain OCTs in the market are available to image the anterior segment with the aid of an external adaptor. RTVue OCT (Optovue, Fremont, CA) and Cirrus HD-OCT (Carl Zeiss Meditec, Dublin, CA) are widely used for anterior segment imaging. Another form of Fourier domain OCT is the swept source OCT that uses a monochromatic tunable fast-scanning laser source and a photodetector for the receiving of wavelengthresolved interference signals. The CASIA (Tomey, Nagoya, Japan) is a commercially available swept source OCT used for anterior segment imaging [40]. By increasing the signal/noise ratio combined with high-speed scanning, Fourier domains are able to reduce motion artifacts and produce high-quality images. The specifications of commercially available AS-OCTs are mentioned in Table 15.1. In this section, we will introduce the clinical and research applications of OCT to understand anatomical structures and function of an anterior segment of the eye. We will cover the usage of AS-OCT in corneal and glaucoma imaging by explaining the pathophysiology of the diseases or conditions and the usefulness of AS-OCT in these conditions.

15.2  Corneal Imaging The cornea is the transparent tissue at the anterior eye that provides two-thirds of the refractive power required for clear vision. Its transparency is crucial in maintaining vision; therefore, the cornea possesses several features to ensure it retains its function [33].

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Table 15.1 Comparison of Commercially Available AS-OCTs Time Domain OCTs Visante OCT Manufacturer Wavelength of light source Axial resolution Scan length Image acquisition speed

SL-OCT

Fourier Domain OCTs Cirrus HD-OCT

RTVue FD-OCT

CASIA SS-OCT

Carl Zeiss Meditec 1310 nm

Heidelberg Engineering 1310 nm

Carl Zeiss Meditec 840 nm

Optovue

Tomey

840 nm

1310 nm

18 µm 16 mm (LD) 8 mm (HD) 2000 A-scans per second