Atlas of Anterior Segment Optical Coherence Tomography

Part of the Essentials in Ophthalmology series, this atlas is designed to comprehensively cover optical coherence tomography of the anterior segment of the eye. The aim is to improve knowledge of the fundamentals of OCT technology for anterior segment, clarify the differences with posterior segment OCT and emphasize the immense relevance and usefulness that anterior segment OCT study has for diagnosis, therapeutic orientation, surgical guidance, and improvement in patient management.

 

Atlas of Anterior Segment Optical Coherence Tomography is organized into comprehensive chapters on the following topics: fundamentals, technologies and technological differences among platforms, application of OCT, corneal OCT angiography, as well as case-based chapters. Numerous highly-detailed figures, illustrations and photographs make this an ideal resource for the corneal specialist seeking further instruction on this cutting-edge technology. The case-based chapters include such conditions as bowman dystrophies, trauma, cataract, glaucoma, sclera, refractive surgery, ocular infections, and are structured to facilitate the consultant surgeon by providing practical information applicable to practical cases in their practice.

• Language: English
• Publisher: Springer
• Release date: Oct 31, 2020
• ISBN: 9783030533748

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© Springer Nature Switzerland AG 2021

J. L. Alió, J. L. A. del Barrio (eds.)Atlas of Anterior Segment Optical Coherence TomographyEssentials in Ophthalmologyhttps://doi.org/10.1007/978-3-030-53374-8_1

1. Anterior Segment OCT: An Overview

Shinichi Fukuda¹, ², Yoshiaki Yasuno², ³   and Tetsuro Oshika¹, ²  

(1)

Department of Ophthalmology, Faculty of Medicine, University of Tsukuba, Ibaraki, Japan

(2)

Computational Optics and Ophthalmology Group, University of Tsukuba, Ibaraki, Japan

(3)

Computational Optics Group, University of Tsukuba, Ibaraki, Japan

Yoshiaki Yasuno

Email: [e-mail]

Tetsuro Oshika (Corresponding author)

Email: [e-mail]

Keywords

Time-domainFourier-domainSpectral-domainSwept-source OCTOCT angiographyOCT-APolarization-sensitive OCT

Optical coherence tomography (OCT) is a modality using low-coherence interferometry that was initially developed for retinal imaging by Huang et al. in 1991 [1]. It is a noncontact, in vivo imaging technology to produce cross-sectional images of ocular tissues. The first anterior segment OCT (AS-OCT) was proposed in 1994 by Izatt et al. [2]. AS-OCT imaging allows for visualization and assessment of anterior segment ocular features, including the tear film, cornea, conjunctiva, sclera, rectus muscles, anterior chamber angle, crystalline lens, and anterior hyaloid membrane. Subconjunctival space such as intra-bleb structures after glaucoma filtering surgery can also be depicted noninvasively by AS-OCT. It is faster, less invasive, more patient friendly, and far less cumbersome to perform than other anterior segment imaging devices such as ultrasonic biomicroscopy (UBM).

In its development history, the early studies for anterior segment were done with the custom-built systems of the commercially available retinal OCT adapted for anterior segment imaging [3]. After a while, a laboratory-based OCT specially designed for anterior investigation appeared [2]. This first generation of AS-OCT was based on a time-domain low-coherence interferometer, which generates depth-resolved interference signal by mechanically scanning one of the mirrors in the interferometer. In 2005, a commercially available time-domain anterior segment OCT system, Visante™ (Carl Zeiss Meditec, CA, USA), was approved by the United States Food and Drug Administration (USFDA). Visante, which was based on time-domain OCT technology, provided two-dimensional cross-sectional images of the anterior segment of the eye with a 1,310-nm wavelength probe beam. Although this wavelength is suboptimal to image the retina due to high absorption by the aqueous and vitreous humor, it deeply penetrates through the conjunctiva and sclera, so it is suitable for anterior segment imaging. In addition, another commercially available anterior OCT, slit-lamp OCT (SL-OCT , Heidelberg Engineering GmbH, Heidelberg, Germany), was approved by USFDA in 2006. Although these systems provide cross-sectional images, three-dimensional OCT investigation was not available owing to the limitation of measurement speed.

The cornea anterior module of RTVue Fourier-domain OCT (Optovue, Fremont, CA, USA) enabled three-dimensional investigation of the cornea. RTVue was based on a spectral-domain OCT technology, which is a sub-type of Fourier-domain OCT technology, which offers 10 times higher acquisition speed than the time-domain systems. However, the field of view was considerably limited with RTVue. The Cirrus HD-OCT (Carl Zeiss Meditec, Dublin, CA, USA) had a built-in anterior segment-imaging module, which scanned only a 3 × 1 mm area. Since both systems used a wavelength of 840 nm, the penetration was quite shallower than the system using 1,300-nm wavelength.

The three-dimensional AS-OCT (CASIA SS-1000 OCT, Tomey, Nagoya, Japan) using a wavelength of 1,300 nm was developed on the basis of the swept-source OCT technology, which is another sub-type of the Fourier-domain OCT [4]. It enables 16-mm horizontal scan and possesses an axial resolution of 10 μm. With dramatic improvement of measurement speed and resolution, AS-OCT imaging has become an important part of clinical evaluation of the cornea, anterior chamber, and chamber angle.

Ultrahigh-resolution OCT is capable of axial resolution of 1 to 4 μm [5] and enables detailed imaging of the corneal and conjunctival layers. In 2015, the deep range AS-OCT (CASIA2, Tomey Corporation, Nagoya, Japan) has improved the scan speed to 50,000 A-scans/s and a scan depth of 13 mm, making it possible to perform imaging from the cornea through the posterior surface of the crystalline lens in a single session [6]. Automated quantitative analysis of the angle parameters was also provided.

Tritontm SS-OCT (Topcon, Tokyo, Japan) is another ophthalmic swept-source OCT which uses a 1,050-nm wavelength probe beam and possesses a scan speed of 100,000 A-scans per second and a scan depth of 3 mm. Although this OCT is mainly designed for posterior segment investigation, an external add-on lens enables anterior segment imaging.

The MS-39 (Costruzione Strumenti Oftalmici, Florence, Italy) is a stand-alone device that combines spectral-domain OCT and Placido disk corneal topography to obtain measurements of the anterior segment of the eye [7]. After an autocalibration, the scanning process acquires (approximately in 1 s) 1 keratoscopy, 1 iris front image (used for the pupil detection), and a series of 25 OCT radial scans. The device uses a SLED light source at 845 nm and provides an axial resolution of 3.6 μm (in tissue) and transversal resolution of 35 μm (in air). Each section measures 16 × 7.5 mm and includes 1024 A-scans. The ring edges are detected on the keratoscopy so that elevations, slope, and curvature data can be derived by the arc-step with conic curves algorithm. Profiles of the anterior cornea, posterior cornea, anterior lens, and iris are derived from the OCT scans. Data for the anterior surface from the Placido image and OCT scans are merged using a proprietary method. All other measurements for internal structures (posterior cornea, anterior lens, and iris) are derived solely from spectral-domain OCT data. Its unique application includes automated measurements of the corneal epithelium, which has been shown to be accurate and useful in clinical situations.

OCT angiography (OCT-A) delineates blood vessels by analyzing temporal variation of OCT signal, such as signal decorrelation and signal variance [8]. It emerged as a noninvasive technique for imaging the microvasculature of the retina and the choroid. The main advantages of OCT-A over the conventional angiography include shorter acquisition time and enhanced safety. Injection of fluorescein and indocyanine green dyes is not required, which is associated with the risk of systemic adverse effects and even anaphylactic reactions. Current commercially available OCT-A systems are not specifically designed for the anterior segment, but may be adapted to assess the conjunctival, corneal, iridial, and scleral vessels [9]. Although anterior segment OCT-A has received little attention to date, imaging of corneal neovascularization is among the most obvious applications of this technique. In contrast to evaluation with slit lamp, OCT-A allows for objective measures of the extent and depth of angiogenesis.

The AS-OCT has a potential as research and clinical tools in the field of anterior segment diseases. These include detailed assessment of ocular surface, anterior chamber angle, cornea, sclera, limbus, extraocular muscle, conjunctiva, and subconjunctival space (filtering bleb). AS-OCT is also useful in ocular injuries and trauma. Ultrahigh-resolution OCT can differentiate various corneal and ocular surface pathologies, including ocular surface squamous neoplasia, lymphoma, pterygium, melanosis, and corneal degeneration [10]. In cataract surgery, AS-OCT is utilized in intraocular lens power calculation as well as in preoperative evaluation of crystalline lens, anterior chamber, and angle structures [11]. It is also valuable in corneal transplantation surgery, particularly in lamellar transplantation, and is used for preoperative evaluation of graft donor tissue for thickness and preservation. AS-OCT enables precise monitoring of laser in situ keratomileusis (LASIK) flap [12]. Intraoperative use of AS-OCT has been described for in vivo assessment of clear cornea wound architecture and OCT-guided femtosecond laser-assisted cataract surgery. LenSx (Alcon LenSx Lasers Inc., Aliso Viejo, CA, USA), Catalys (OptiMedica, Sunnyvale, CA, USA), and VICTUS (Technolas Perfect Vision GmbH, Munich, Germany) are commercially available laser cataract surgery systems that equip SD-OCT for three-dimensional and high-resolution reconstruction of the anterior segment structures to improve safety and accuracy.

Novel polarization-sensitive OCT (PS-OCT) and OCT elastography will expand functional potentials of AS-OCT [13, 14]. Further advancement of AS-OCT to ultrahigh resolution will sophisticate clinical evaluation of anterior ocular diseases. AS-OCT imaging, including anterior segment OCT-A, is a relatively new field, and there are still many areas that require fine-tuning. These modalities will provide novel insight into the pathophysiology of anterior segment diseases and will therefore remain an active area of research in the coming years.

Compliance with Ethical Requirements

Shinichi Fukuda declares that he has no conflict of interest.

Yoshiaki Yasuno received research grants from Tomey Corp., Topcon, Yokogawa Electric, Nikon, and Kao. Yoshiaki Yasuno licenses a patent to Tomey Corp.

Tetsuro Oshika has received research grants from Tomey Corp. Tetsuro Oshika has received a speaker honorarium from Tomey Corp., Topcon, and Carl Zeiss.

References

1.

Huang D, Swanson EA, Lin CP, Schuman JS, Stinson WG, Chang W, et al. Optical coherence tomography. Science (New York, NY). 1991;254(5035):1178–81.Crossref

2.

Izatt JA, Hee MR, Swanson EA, Lin CP, Huang D, Schuman JS, et al. Micrometer-scale resolution imaging of the anterior eye in vivo with optical coherence tomography. Arch Ophthalmol (Chicago, Ill: 1960). 1994;112(12):1584–9.Crossref

3.

Feng Y, Varikooty J, Simpson TL. Diurnal variation of corneal and corneal epithelial thickness measured using optical coherence tomography. Cornea. 2001;20(5):480–3.Crossref

4.

Fukuda S, Kawana K, Yasuno Y, Oshika T. Anterior ocular biometry using 3-dimensional optical coherence tomography. Ophthalmology. 2009;116(5):882–9.Crossref

5.

Wang J, Abou Shousha M, Perez VL, Karp CL, Yoo SH, Shen M, et al. Ultra-high resolution optical coherence tomography for imaging the anterior segment of the eye. Ophthalmic Surg Lasers Imaging. 2011;42 Suppl:S15–27.Crossref

6.

Chansangpetch S, Nguyen A, Mora M, Badr M, He M, Porco TC, et al. Agreement of anterior segment parameters obtained from swept-source Fourier-domain and time-domain anterior segment optical coherence tomography. Invest Ophthalmol Vis Sci. 2018;59(3):1554–61.Crossref

7.

Savini G, Schiano-Lomoriello D, Hoffer KJ. Repeatability of automatic measurements by a new anterior segment optical coherence tomographer combined with Placido topography and agreement with 2 Scheimpflug cameras. J Cataract Refract Surg. 2018;44(4):471–8.Crossref

8.

Hong YJ, Makita S, Jaillon F, Ju MJ, Min EJ, Lee BH, et al. High-penetration swept source Doppler optical coherence angiography by fully numerical phase stabilization. Opt Express. 2012;20(3):2740–60.Crossref

9.

Ang M, Sim DA, Keane PA, Sng CC, Egan CA, Tufail A, et al. Optical coherence tomography angiography for anterior segment vasculature imaging. Ophthalmology. 2015;122(9):1740–7.Crossref

10.

Shousha MA, Karp CL, Perez VL, Hoffmann R, Ventura R, Chang V, et al. Diagnosis and management of conjunctival and corneal intraepithelial neoplasia using ultra high-resolution optical coherence tomography. Ophthalmology. 2011;118(8):1531–7.Crossref

11.

Nguyen P, Chopra V. Applications of optical coherence tomography in cataract surgery. Curr Opin Ophthalmol. 2013;24(1):47–52.Crossref

12.

Li Y, Netto MV, Shekhar R, Krueger RR, Huang D. A longitudinal study of LASIK flap and stromal thickness with high-speed optical coherence tomography. Ophthalmology. 2007;114(6):1124–32.Crossref

13.

Fukuda S, Yamanari M, Lim Y, Hoshi S, Beheregaray S, Oshika T, et al. Keratoconus diagnosis using anterior segment polarization-sensitive optical coherence tomography. Invest Ophthalmol Vis Sci. 2013;54(2):1384–91.Crossref

14.

Ford MR, Dupps WJ Jr, Rollins AM, Sinha RA, Hu Z. Method for optical coherence elastography of the cornea. J Biomed Opt. 2011;16(1):016005.Crossref

© Springer Nature Switzerland AG 2021

J. L. Alió, J. L. A. del Barrio (eds.)Atlas of Anterior Segment Optical Coherence TomographyEssentials in Ophthalmologyhttps://doi.org/10.1007/978-3-030-53374-8_2

2. Anterior Segment OCT: Fundamentals and Technological Basis

Gabriele Vestri¹  , Claudio Macaluso² and Francesco Versaci¹  

(1)

CSO Costruzione Strumenti Oftalmici, Florence, Italy

(2)

Department of Medicine and Surgery, University of Parma, Parma, Italy

Gabriele Vestri (Corresponding author)

Email: [e-mail]

Francesco Versaci

Email: [e-mail]

Keywords

CorneaAS-OCTOCT working principlesAnterior segmentCorneal topographyAnterior segment tomographyPlacido discEpithelial thickness

Abbreviations

OCT

Optical coherence tomography

AS-OCT

Anterior segment optical coherence tomography

TD-OCT

Time domain optical coherence tomography

FD-OCT

Fourier domain optical coherence tomography

SD-OCT

Spectral domain optical coherence tomography

SS-OCT

Swept source optical coherence tomography

SLD

Superluminescent diode

Introduction

The last 30 years have seen a progressive evolution of diagnostic devices for the anterior ocular segment, which has accompanied the tremendous improvements in the fields of electronics, optics, and computer science.

The first breakthrough occurred with the invention of corneal topographers [1], based either on the reflection or the projection of a pattern of mires [2]. These devices provided an accurate measurement of a large area of the first corneal surface in one shot. They also provided ophthalmologists with an easy and reliable method to detect irregular astigmatism and keratoconus at mild stages, to measure the contribution of anterior corneal surface to ocular aberrations, and, in general, to better understand the morphology and the optics of the anterior corneal surface.

The second important development occurred with the release of optical scanning devices, which could capture the entire anterior segment. In this way, visual and quantitative information were added about posterior corneal surface and anterior chamber (iris, angles, and anterior portion of the crystalline lens). The first of these instruments was the Orbscan (Bausch & Lomb, Rochester, NY) based on a translating illuminating slit. This was followed by a series of machines based on a rotational illuminating scanning slit and on the Scheimpflug principle which allows the device to focus the illuminated section even if it is tilted with respect to the axis of the observation system: Pentacam (Oculus Optikgeräte, Wetzlar, Germany), Galilei (Ziemer, Switzerland), Sirius (CSO, Florence, Italy), and TMS4 (Tomey, Nagoya, Japan). With all these improvements, clinicians were able to get some further important information, mainly the elevation map of posterior corneal surface and the corneal thickness map. Consequently, some other achievements were obtained in the early detection of keratoconus and ectasia, in the measurement of posterior and total corneal astigmatism for toric intraocular lens (IOL) planning, and in the calculation of total corneal power for more accurate IOL calculation, particularly after corneal refractive surgery. Nevertheless, Scheimpflug imaging has some major limitations in the low resolution of the scans and the presence of artifacts caused by heavy tissue scattering.

The third milestone was the application of optical coherence tomography (OCT) to produce images with higher definition. The first OCT instrument completely dedicated to the anterior segment was Visante (Carl Zeiss Meditec, Dublin, CA). Despite its advanced design, some limitations of Visante soon became clear:

The relative reliability of the measurement made it necessary to combine with the corneal topographer Atlas (Carl Zeiss Meditec, Jena, Germany) for the measurement of the anterior corneal surface.

The relatively slow scanning speed and the consequent low number of scanned sections.

The quality of the image was inferior compared to that of retinal OCTs with a corneal adapter.

The next advancements were mainly achieved by retinal OCTs, which were equipped with corneal adapters, but the field of view in depth and/or width was limited to a few millimeters, and topographic maps were only partially available.

Finally, it was the turn of instruments completely dedicated to the anterior segment like Casia SS-1000, followed by Casia 2 (Tomey, Nagoya, Japan), and recently MS-39 (CSO, Florence, Italy) and Anterion (Heidelberg Engineering, Heidelberg, Germany). These devices allow for the acquisition of high-quality angle-to-angle pictures of the anterior segment and a quick scan of a large number of meridians in order to produce accurate, detailed, and extended topographic maps.

OCT Systems: How Do They Work?

Until a few years ago, optical coherence tomography found a very successful application in ophthalmology only for the study of the retina and for the measurement of intraocular distances, firstly the axial length. Only recently have manufacturers turned their attention to topography and tomography of the anterior segment of the eye. The reason behind this delay is due to the complexity of this technology, the reliability, the difficulty of obtaining images that contain the complete anterior segment, and the difficulty in obtaining accurate measurements similar to those of simpler techniques.

OCT is an imaging technique based on the interference of two beams of a broadband radiation (typically infrared) from a reference arm and a sample arm [3–6].

In its simplest implementation (Fig. 2.1), it requires a broadband radiation source, an interferometer with at least four arms, a detector for collecting the interference signal, and a processing unit that transforms the interference signal into intelligible data. The four arms of the interferometer are used for the following purposes:

1.

One is for the source of the radiation (source arm).

2.

One is used for creating a reference in distance and for generating one of the interfering beams (reference arm).

3.

One is for imaging the sample, i.e., for generating the interfering beam coming from the object to be imaged (sample arm).

4.

One is for collecting the interference from the reference and the sample arm (detection arm).

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Fig. 2.1

Basic OCT schematic

The broadband source – usually an infrared superluminescent diode (SLD)  – emits the radiation into the interferometer, which partially sends it to the reference arm and partially to the sample arm. At the end of the reference arm, a mirror reflects the beam back toward the detection arm where the photodetector collects it. Similarly, the radiation sent to the sample arm is backscattered by the ocular tissues toward the detection arm, where it interferes with the back-reflected beam from the reference arm. If the reference mirror translates axially, thereby altering the length of the reference arm, the interference due to the ocular structures encountered by the incident beam of the sample arm can be sampled at various depths. As a result, the photodiode reveals a peak signal for each backscattering element encountered by the beam incident on the sample at a position corresponding to that of the moving mirror of the reference arm. This design (Fig. 2.2), called time domain OCT (TD-OCT), was the first to be applied in the field of ophthalmology for the measurement of the inter-distances between the various ocular interfaces, in particular the axial length of the eye, the corneal thickness, the anterior chamber depth, and the crystalline lens thickness. The weak point of this technique is the need to move the reference mirror for obtaining a response from the structures at various depths. Thus, it suffers a speed limit when it is necessary to scan hundreds or thousands of contiguous lines (A-scans) to create an image of an ocular section (B-scan). Eye movements during a slow scan lead to artifacts in the scanned image which cannot be corrected.

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Fig. 2.2

Time domain OCT (TD-OCT)

A more complex implementation is the one which goes under the name of Fourier domain OCT (FD-OCT) [7], which eliminates the need for a moving reference mirror to have a measurement at various depths. This offers the possibility of acquiring axial scans very quickly, thereby reducing artifacts due to eye movements. The FD-OCT, in addition to decreasing the time of acquisition, also presents advantages in terms of signal-to-noise ratio compared to the time domain technique. The basic idea is to measure the spectral interference between the radiation returning from the reference arm and the sample arm in a certain range of wavelengths emitted by the source. This means that, in a single A-scan, for each wavelength, the detection arm collects the interference value of the radiation coming back from both the sample and reference arms.

The set of these values at various wavelengths is processed with more or less complex algorithms, basically, containing a Fourier transformation, to obtain the reflectivity profile of the sample along an axis.

The FD-OCT devices can be classified into the following two classes depending on how they get the interference values at various wavelengths: spectral domain OCT (SD-OCT in Fig. 2.3) and swept source OCT (SS-OCT in Fig. 2.4).

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Fig. 2.3

Spectral domain OCT (SD-OCT) schematic

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Fig. 2.4

Swept source OCT (SS-OCT) schematic

The first class of instruments, SD-OCT, use a broadband source that emits a certain range of wavelengths all at the same time. The detection arm contains a spectrometer to decompose the interference signal deriving from the return radiation of the sample and reference arms into its components at the various wavelengths. Therefore, in a single shot, the sensor of the spectrometer collects the spectrum necessary to determine the profile of reflectivity along one axis of the sample.

The second class of instruments, SS-OCT, use a light source that is a tunable laser, i.e., a laser whose wavelength can be varied very quickly [8]. The light source is driven to emit the various wavelengths in sequence and synchronizes with a photodiode, which replaces the spectrometer on the detection arm. A processing unit associates the wavelengths emitted by the source with the values measured by the photodiode and reconstructs the interference spectrum between the beams coming back from the reference and sample arms.

In order to illustrate the working principle of this technology in detail, let us imagine an object able to backscatter the incident radiation and positioned within the field of view of the instrument. The incident radiation containing all the wavelengths of the source (all together in the SD-OCT case, one at a time as a fast sequence in the SS-OCT case) is partially backscattered toward the instrument and will recombine in the detection arm with the radiation coming from the reference arm which at its turn will contain the same wavelengths.

The combination of both the radiations will be collected in the spectrometer if the instrument implements the SD technique and in a photodiode if the instrument adopts the SS technique. In both cases, for each wavelength available in the source, the sensor collects the intensity of a beam generated by the constructive, destructive, or partially constructive interference of two beams coming from the reference and measuring arms.

Let us suppose that the backscattering object is placed in a position, which is very near the one corresponding to the reference mirror, i.e., at the same distance from the origin of the interferometer. In this case, the spectrum measured by the device will be a sequence of peaks and valleys superimposed on the spectrum of the source with a relatively wide period, or in other words a low-frequency sinusoid, which modulates in amplitude the source spectrum (Fig. 2.5a). The reconstructed intelligible signal, obtained through a Fourier transform of the measured spectrum, will contain a peak near the zero position (Fig. 2.6a). The peak signal is then converted to gray levels to be associated with the pixels of a column of an image.

../images/489439_1_En_2_Chapter/489439_1_En_2_Fig5_HTML.png

Fig. 2.5

Spectral interference due to (a) a near object, (b) a far object, (c) both near and far objects

../images/489439_1_En_2_Chapter/489439_1_En_2_Fig6_HTML.png

Fig. 2.6

Reconstructed signal from the examples of Fig. 2.5 in case of (a) a near object, (b) a far object, (c) both a near and a far object

Let us suppose now that the backscattering object is placed relatively far from the position corresponding to the reference mirror. In this case (Fig. 2.5b), the spectrum measured by the device will be a sequence of peaks and valleys very close to each other superimposed on the spectrum of the source, or in other words a high-frequency sinusoid, which modulates in amplitude the source spectrum. The reconstructed intelligible signal, again obtained through a Fourier transform of the spectral intensity, will contain a peak far from the zero position (Fig. 2.6b). The peak signal is newly converted to gray levels to be associated with the